High-performance infrared (IR) nonlinear optical (NLO) materials with large NLO response and suitable birefringence are urgently needed for various applications. A Hg-based IR NLO material KHg4Ga5Se12 with such desirable properties has been newly discovered. In the structure, obviously distorted HgSe4 and GaSe4 tetrahedra are connected to each other by vertex-sharing to form a three-dimensional framework with the counterion K+ residing in the cavities. Remarkably, all the NLO-active building units in the title compound are arranged in a completely parallel manner. Such a topological structure and the large susceptibility of the Hg–Se bonds enable the material to achieve good phase-matchability with a tremendous powder second harmonic generation (SHG) response at 2.09 μm that is about 20-times that of the benchmark material AgGaS2 (one of the largest responses among all the phase-matchable IR NLO chalcogenides reported to date). The optical band gap of KHg4Ga5Se12 was determined as 1.61 eV. Moreover, on the basis of the electronic band structure, the real-space atom-cutting analysis, the SHG-weighted electronic densities, and the local dipole moments calculations, the origin of the superior linear and nonlinear optical properties of the title compound is ascribed to the (Hg/Ga)Se4 group. The calculated values for the maximum coefficient d33 and birefringence (Δn) at 2.09 μm are −65.257 pm/V and 0.072, respectively. Such values agree well with experimental observations. Our study demonstrates that Hg-based metal chalcogenides are a class of IR NLO material with competitive features (good phase-matchability, very large SHG efficiency, wide transparency) desirable for practical applications.

A new Au-based sulfide BaAu2S2 was obtained through solid-state reaction. It crystallizes in the tetragonal space group I41/amd with unit cell parameters of a = 6.389 72(2) Å, b = 6.389 72(2) Å, c = 12.7872(1) Å, and Z = 4. Its structure features [AuS2/2]∞ zigzag chains composed of corner-sharing AuS2 linear units. With a direct band gap of 2.49 eV, BaAu2S2 is suitable for the visible-light harvesting. Moreover, it exhibits excellent visible-light photocatalytic activity, which is 1.3 times that of graphitic carbon nitride (g-C3N4) and also demonstrates excellent circulating stability. On the basis of the crystal and electronic structure analysis, the electrons are highly delocalized along the [AuS2/2] chains, and the electron effective mass of BaAu2S2 is only approximately one-fifth of that of g-C3N4, which may help the separation of the electron/hole pairs during the photocatalytic process. Additionally, the absorption coefficient of BaAu2S2 is extremely high, exceeding 1 × 104 cm–1 over the entire absorbable visible spectrum (hν > Eg), which is significantly higher than that of g-C3N4. Such factors may contribute to its outstanding photocatalytic performances. According to our best knowledge, BaAu2S2 is the first noble metal-based chalcogenide photocatalyst reported as intrinsic light-harvesting and electron/hole-generating centers. This study may provide valuable insights for further research on photocatalytic materials.

By combining different nonlinear optical-active structural chromophores with transition metal Mn into a crystal structure, two novel quaternary metal chalcogenides Pb0.65Mn2.85Ga3S8 (1) and Pb0.72Mn2.84Ga2.95Se8 (2) were successfully synthesized. Compounds 1 and 2 are isostructural, and they represent a new structure type that crystallizes in the space group P6̅ (No. 174) in the hexagonal system. Their structures feature an interesting three-dimensional open-tunnel framework composed of bridged infinite chains with Pb2+ cations filling in the biggest tunnels. Interestingly, both 1 and 2 demonstrate intense second harmonic generation responses at 2.09 μm that is about 1.5 and 4.4 times, respectively, of that of the benchmark material AgGaS2. However, 1 and 2 possess different optical diffuse reflectance spectra: 1 displays an evident multiband absorption characteristic with two distinguishing absorption edges of 738 and 551 nm, corresponding to two band gaps of 1.68 and 2.25 eV, respectively, while 2 exhibits only one sharp edge, and the corresponding band gap was estimated to be 1.65 eV. Moreover, apart from the considerable structural similarity between 1 and 2, the dc temperature dependent susceptibility measurements indicate that compound 1 is paramagnetic, while compound 2 exhibits spin-glass-like behavior.

A new mercury selenide BaHgSe2 was synthesized. This air-stable compound displays a large nonlinear optical (NLO) response and melts congruently. The structure contains chains of corner-sharing [HgSe3]4– anions in the form of trigonal planar units, which may serve as a new kind of basic functional group in IR NLO materials to confer large NLO susceptibilities and physicochemical stability. Such trigonal planar units may inspire a path to finding new classes of IR NLO materials of practical utility that are totally different from traditional chalcopyrite materials.

Visible-light-responsive photocatalytic materials have important applications. In this article, through inserting electropositive ion Ba2+ into the three-dimensional framework of ZnSe, a one-dimensional chalcogenide Ba2ZnSe3 has been obtained by traditional solid-state reaction. It crystallizes in orthorhombic centrosymmetric space group Pnma with unit cell parameters of a = 9.0744(2) Å, b = 4.4229(1) Å, c = 17.6308(4) Å, and Z = 4. Its structure features [ZnSe3]4– anionic straight chains parallel to the b direction, which are further separated by Ba2+ cations filling in the cavities. On the basis of the UV–vis–NIR diffuse reflectance spectroscopy, Ba2ZnSe3 possesses a typical direct band gap of 2.75 eV, which is in good agreement with the electronic structure calculation. Moreover, Ba2ZnSe3 shows good visible-light-responsive photocatalytic activity and excellent thermal stability and cyclability, which are favorable for its application.

Our earlier theoretical calculation and preliminary experiment highlighted LiZnPS4 as a good mid-infrared (mid-IR) nonlinear optical (NLO) material. However, this compound suffers from problems including corrosion of the silica tubes, a pungent smell, deliquescence, and incongruent-melting behavior in the further single crystal growth and applications. In order to overcome these problems, herein, we investigate the analogues of LiZnPS4 and propose that AgZnPS4 would be a good candidate. The combination of experimental and theoretical study demonstrates that AgZnPS4 exhibits a much stronger NLO effect than that of LiZnPS4 despite the relatively smaller energy band gap. More importantly, AgZnPS4 melts congruently with a melting point as low as 534 °C, much lower than those of traditional IR NLO crystals, and is nondeliquescent with enough stability in the air. Such a good crystal growth habit and chemical stability enable AgZnPS4 to possess much better overall performance for the practical mid-IR NLO applications.

Three new chalcogenides, namely NaGaGe3Se8, K3Ga3Ge7S20, and K3Ga3Ge7Se20, of an A–Ga–Ge–Q (A = Na, K; Q = S, Se) system were obtained for the first time. They crystallize into two different new structures, albeit both in the monoclinic space group P21/c. NaGaGe3Se8 has a layered structure consisting of two dimensional ∞2[M4Se8]− layers separated by Na+ cations, while the structures of K3Ga3Ge7Q20 (Q = S, Se) are constructed by incompletely isolated quasi-2D ∞2[M10Q21]5− layers, leading to large channels loosely occupied by K+ cations. Interestingly, thermal analysis indicates that the three title compounds are all congruent-melting compounds, which is uncommon for quaternary compounds, and makes bulk crystal growth using the Bridgman technique possible. UV-vis-NIR spectroscopy measurements reveal that the optical band gaps of the three compounds are 2.35, 3.25, and 2.23 eV. In addition, the electronic structure calculations on NaGaGe3Se8 show that the band gap is mainly determined by the GaSe4 and GeSe4 groups.

•A new double phosphate PbCd(PO3)4 has been synthesized and characterized for the first time.•The structure features [PO3]1∞ chains bridged by isolated [CdO6] triangular prisms with Pb2+ cations occupying the cavities.•PbCd(PO3)4 has a large band gap of 4.85 (2) eV and displays congruent-melting behavior with the melting point of 727 °C.•PbCd(PO3)4 exhibits a UV-photocatalytic activity to decompose RhB about 0.62 times that of P25.By combining Pb2+ with ds-block elements, a new double phosphate PbCd(PO3)4 has been synthesized for the first time. It crystallizes in space group P21/n of the monoclinic system with unit cell parameters of a = 7.1191 (14) Å, b = 9.0871 (18) Å, c = 14.681 (3) Å, β = 91.40 (3)° and Z   = 4. Its structure contains [PO3]1∞ chains running along the a direction, which are further bridged by isolated [CdO6] triangular prisms to generate a 3D framework with Pb2+ cations filling in the cavities. Based on UV-vis-NIR spectroscopy measurement, PbCd(PO3)4 has a large band gap of 4.85 (2) eV, which is consistent with the density functional theory (DFT) study. In addition, the DSC curve demonstrates that PbCd(PO3)4 melts congruently at 727 °C. Additionally, the title compound displays a UV-photocatalytic activity to decompose RhB under 500 W mercury lamp (λ = 256 nm) radiation about 0.62 times that of P25 (TiO2).A new double phosphate PbCd(PO3)4 has been synthesized. Its structure contains [PO3]1∞ chains bridged by isolated [CdO6] triangular prisms with Pb2+ cations occupying the cavities. It has a large band gap of 4.85 (2) eV and melts congruently at 727 °C. Interestingly, PbCd(PO3)4 displays a photocatalytic activity to decompose RhB about 0.62 times that of P25.

By introducing heavy Sn ions to a Ba–Ga–Q system, a new IR nonlinear optical material, BaGa2SnSe6, has been obtained. It crystallizes in space group R3 of the trigonal system with a = 10.1449(14) Å, c = 9.2490(18) Å and Z = 3. In the structure, three (Ga/Sn)Se4 tetrahedra are connected via corner-sharing to generate (Ga/Sn)3Se9 building groups, which are further joined to produce a three-dimensional network with Ba2+ lying in the cavities. Due to the contribution of the large and easily-polarizable Sn atom and the polar arrangements of the (Ga/Sn)Se4 tetrahedra, the compound exhibits a very strong NLO response, i.e., ∼5.2 times that of the benchmark AgGaS2 at a fundamental laser wavelength of 2.09 μm and shows type-I phase-matchable behavior. Furthermore, the calculated birefringence index is Δn = 0.1649 at 1.064 μm and the major SHG tensors are d11 = 62.91 pm V−1 and d33 = 50.26 pm V−1.

By combining the [GaS4] tetrahedra and the second-order Jahn–Teller distorted cation Pb2+, a new wide-gap nonlinear optical material PbGa4S7 has been obtained. In the structure, Pb2+ is coordinated to a distorted quadrangular pyramid of five S atoms, showing the stereo-chemical activity of the 6s2 lone pair electrons. The polar arrangement of the [GaS4] tetrahedra and the macroscopic arrangement of the [PbS5] distorted quadrangular pyramids result in a large IR nonlinear optical property for PbGa4S7, which is ∼1.2 times that of the benchmark AgGaS2 at a laser radiation of 2.09 μm. Moreover, PbGa4S7 has a large direct band gap of 3.08(2) eV, a desirable property for avoiding two-photon absorption of the conventional 1–2 μm pumping laser sources and improving the laser damage threshold. The DSC analysis indicates that the compound is thermally stable up to 1140 K. The electronic structure was also calculated to explain the optical properties.

The new zero-dimensional selenide Ba2AsGaSe5 was synthesized via a solid-state reaction at 900 °C. It belongs to the orthorhombic space group Pnma with a = 12.632(3) Å, b = 8.9726(18) Å, c = 9.2029(18) Å, and Z = 4. In the structure, the As atom adopts trigonal-pyramidal coordination owing to the stereochemically active 4s2 lone pair electrons and the Ga atom is tetrahedrally coordinated with four Se atoms. The AsSe3 trigonal pyramids share edges with GaSe4 tetrahedra to form novel [AsGaSe5]4– clusters, which are further separated from each other by Ba2+ cations. The optical band gap was determined as 1.39 eV according to UV–vis–NIR diffuse reflectance spectroscopy. Interestingly, the photocatalytic behavior investigated by decomposing rhodamine B indicates that the compound displays a 6.5 times higher photocatalytic activity than does P25.

A new sulfide, SnGa2GeS6, has been synthesized, which represents the first member in the quaternary Sn/M/M′/Q (M = Ga, In; M′ = Si, Ge; Q = S, Se, Te) system. It adopts a new structure type in the non-centrosymmetric space group Fdd2. In the structure, Sn2+ is coordinated to a distorted square-pyramid of five S atoms, demonstrating the stereochemical activity of the lone electron pair, while the Ge atom and Ga atom are both tetrahedrally coordinated to four S atoms. The SnS5 square-pyramids and the MS4 (M = Ga, Ge) tetrahedra are connected to each other via corner and edge-sharing to generate a three-dimensional framework. The compound exhibits a powder second harmonic generation signal at 2 μm whose strength is about one-fourth that of the benchmark material AgGaS2, which may be explained in view of the macroscopic arrangement of the SnS5 square-pyramids and the MS4 tetrahedra. Moreover, based on UV-vis-NIR spectroscopy measurements and the electronic structure calculations, SnGa2GeS6 has two optical transitions at about 1.12 eV and 2.04 eV respectively.

Two new quaternary selenides, namely Ba4Ga4SnSe12 and Ba6Ga2SnSe11, have been synthesized for the first time, representing the first two members in the A/M/Sn/Q (A = alkaline-earth metal; M = Al, Ga, In; Q = S, Se, Te) system. Ba4Ga4SnSe12 crystallizes in the non-centrosymmetric space group P21/c of the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe4 and SnSe4 tetrahedra with eight-coordinated Ba2+ cations residing in the cavities. Ba6Ga2SnSe11 crystallizes in a new structure type in the monoclinic centrosymmetric space group P21/c. The structure of Ba6Ga2SnSe11 features a zero-dimensional structure containing totally isolated distorted SnSe4 tetrahedra and a discrete Ga2Se7 unit with Ba2+ cations located between them. On the basis of the diffuse-reflectance spectra, the band gaps are 2.16 (2) eV and 1.99 (2) eV for Ba4Ga4SnSe12 and Ba6Ga2SnSe11 respectively. In addition, the electronic structure calculation of Ba4Ga4SnSe12 indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga4SnSe12]8− anionic framework.

•Sn2SiS4 has been synthesized for the first time.•The SnS6 and SiS4 polyhedra are connected to generate a three-dimensional framework.•Sn2SiS4 has an indirect band gap of 2.00 eV.•Sn2SiS4 exhibits a high visible-light-induced photocatalytic reactivity.The new ternary sulfide Sn2SiS4 has been synthesized via high-temperature solid state reaction. It crystallizes in the centrosymmetric space group P21/c of the monoclinic system. In the structure, the Sn2+ cations are coordinated to a heavily-distorted octahedron of six S atoms or a pentagonal pyramid of six S atoms, both geometries clearly demonstrating the effect of the stereo-chemically active electron lone pair on the Sn coordination environment. These SnS6 polyhedra and the SiS4 tetrahedra are connected to each other via corner and edge-sharing to generate a three-dimensional framework. Based on the diffuse reflectance measurement and the electronic structure calculation, Sn2SiS4 has an indirect band gap of 2.00 eV. Interestingly, Sn2SiS4 exhibits an efficient visible-light-driven photocatalytic activity pertaining to Rhodamine B (RhB) degradation, which is superior to the important photocatalyst C3N4. Moreover, the photocatalytic mechanism was also elucidated based on the active species trapping experiments.Sn2SiS4 contains a three-dimensional framework built by the SnS6 and SiS4 polyhedra and exhibits interesting photocatalytic property.

By introducing a highly electronegative halide anion into chalcogenides, a novel chalcohalide, NaBa4Ge3S10Cl, has been synthesized by a conventional high temperature solid state method. It crystallizes in a new structure type of space group P63, with a = 9.7653(2) Å, c = 12.0581(3) Å and Z = 2. The fundamental unit is the unique [Ge3S9] ring, comprised of three GeS4 tetrahedra sharing corner S atoms. The [Ge3S9] rings are arranged to form pseudo layers, which are stacked through Na–Cl–Ba chains to build up the structure. The macroscopic packing of these [Ge3S9] rings provides the material with a moderate nonlinear optical (NLO) response at 2090 nm fundamental light. Furthermore, UV-vis-IR spectroscopy shows that NaBa4Ge3S10Cl has a very large band gap of 3.49 eV, which is very beneficial for increasing the laser damage threshold and avoiding the two-photon absorption problem of conventional near IR laser pumping sources.

The first series of rare-earth chalcogenides with mixed-valence Sn atoms, namely the BaLnSn2Q6 (Ln = Ce, Pr, Nd, Q = S; Ln = Ce, Q = Se) compounds, were synthesized via stoichiometric solid-state reactions at 1100 °C. BaLnSn2Q6 belong to the polar space group Pmc21 of the orthorhombic system and contain mixed valent Sn atom in the ratio of Sn2+/Sn4+ = 1:3. In the structure, the Sn2+Q5 rectangular pyramids, Sn4+Q5 trigonal bipyramids, Sn4+Q6 octahedra, and LnQ8 bicapped trigonal prisms are connected with each other to form a three-dimensional framework with interspaces occupied by Ba2+ cations. As deduced from magnetic susceptibility measurements, BaPrSn2S6 and BaNdSn2S6 are paramagnetic and obey the Curie–Weiss law.

•Two new quaternary chalcogenides KBaMSe3 (M = As, Sb) were synthesized.•They are isostructural and crystallize in the centrosymmetric space group P21/c.•They adopt a zero-dimensional structure containing isolated MSe3 trigonal pyramids.•The optical band gaps are 2.26 (2) eV for KBaAsSe3 and 2.04 (2) eV for KBaSbSe3.•KBaMSe3 (M = As, Sb) are indirect band gap semiconductors.The first two members in the quaternary A/A′/M/Q (A = alkali metal; A′ = alkaline-earth metal; M = As, Sb, Bi; Q = S, Se, Te) system, namely the KBaMSe3 (M = As, Sb) selenides, have been synthesized by solid state reactions. They are isostructural and crystallize in the centrosymmetric space group P21/c of the monoclinic system. In the structure, the trivalent M atom (M = As, Sb) is coordinated to three Se atoms forming a trigonal pyramid with the Se atoms serving as the triangle base, showing the stereochemical activity of the ns2 lone pair electron. The MSe3 (M = As, Sb) trigonal pyramids are totally isolated from each other with K+ and Ba2+ cations located between them. The optical band gaps of 2.26 (2) eV for KBaAsSe3 and 2.04 (2) eV for KBaSbSe3, were deduced from the diffuse reflectance spectra. The first principles calculations were performed to study the electronic structures of KBaMSe3 (M = As, Sb) compounds and the results indicated that these two compounds are indirect band gap semiconductors.KBaMSe3 (M = As, Sb) are isostructural and crystallize in the centrosymmetric monoclinic space group P21/c with zero-dimensional structures containing totally isolated MSe3 (M = As, Sb) pyramids with K+ and Ba2+ cations lying between them.

•Three new chalcohalides: Ba4Ge2PbS8Br2, Ba4Ge2PbSe8Br2 and Ba4Ge2SnS8Br2 have been synthesized.•The MQ5Br octahedra and GeQ4 tetrahedra form a three-dimensional framework with Ba2+ in the channels.•Band Gaps and electronic structures of the three compounds were studied.Single crystals of three new chalcohalides: Ba4Ge2PbS8Br2, Ba4Ge2PbSe8Br2 and Ba4Ge2SnS8Br2 have been synthesized for the first time. These isostructural compounds crystallize in the orthorhombic space group Pnma. In the structure, the tetra-valent Ge atom is tetrahedrally coordinated with four Q (Q = S, Se) atoms, while the bi-valent M atom (M = Pb, Sn) is coordinated with an obviously distorted octahedron of five Q (Q = S, Se) atoms and one Br atom, showing the stereochemical activity of the ns2 lone pair electron. The MQ5Br (M = Sn, Pb; Q = S, Se) distorted octahedra and the GeQ4 (Q = S, Se) tetrahedra are connected to each other to form a three-dimensional framework with channels occupied by Ba2+ cations. Based on UV–vis–NIR spectroscopy measurements and the electronic structure calculations, Ba4Ge2PbS8Br2, Ba4Ge2PbSe8Br2 and Ba4Ge2SnS8Br2 have indirect band gaps of 2.054, 1.952, and 2.066 eV respectively, which are mainly determined by the orbitals from the Ge, M and Q atoms (M = Pb, Sn; Q = S, Se).Graphical abstract

A new iron selenide, K2FeGe3Se8, has been obtained by spontaneous crystallization. It adopts a new structure type in the noncentrosymmetric monoclinic space group P21. In the structure, FeSe4 and GeSe4 tetrahedra are connected alternately via corner-sharing to form one-dimensional (1D) ∞1[FeGeSe6]6– chains along the a-direction. These chains are further linked by sharing Ge2Se6 units to generate two-dimensional (2D) ∞2[FeGe3Se8]2– layers stacked parallel to the ac-plane and separated by K+ cations. Deduced from temperature-dependent susceptibility measurement and specific heat measurement under different magnetic fields, K2FeGe3Se8 exhibits an antiferromagnetic transition at ∼10 K. Furthermore, the magnetic property shows anisotropy between directions parallel and perpendicular to the plane of ∞2[FeGe3Se8]2– layer. The diffuse reflectance spectra measurement indicates that the band gap of K2FeGe3Se8 is ∼1.95(2) eV, consistent with the calculated values of 1.80 and 1.53 eV in the spin-up and spin-down directions, respectively. Based on electronic structure calculation, the spin of the Fe2+ cation is 1.85ℏ, which is comparable to the experimental value.

Five new chalcohalides, Ba3GaS4X (X = Cl, Br), Ba3MSe4Cl (M = Ga, In), and Ba7In2Se6F8, have been synthesized by conventional high-temperature solid-state method. These compounds crystallize in three different interesting structure types. Ba3GaQ4X (Q = S, X = Cl, Br; Q = Se, X = Cl) contain zigzag BaX pseudolayers and isolated GaQ4 tetrahedra, while Ba3InSe4Cl possesses one Ba–In–Se pseudolayer and one Ba–Cl pseudolayer, which are stacked alternately along the c-direction. Ba7In2Se6F8 is comprised of one-dimensional 1∞[InSe3]3– chains and unique [Ba7F8]6+ chains. In all those mixed anion compounds, the halide anions are only connected to alkaline-earth metal through strong ionic bonding, while the M (M = Ga, In) cations are only connected to chalcogenide anions through covalent bonding. UV–vis-NIR spectroscopy measurements indicate that Ba3GaQ4X (Q = S, X = Cl, Br; Q = Se, X = Cl) have band gaps of 2.14, 1.80, and 2.05 eV, respectively.

A new ternary selenostannate Ba6Sn6Se13 has been synthesized by a high temperature solid-state method. The compound crystallizes in the non-centrosymmetric orthorhombic space group P212121 and may be represented as Ba6Sn52+Sn4+Se13 with mixed valence Sn atoms. Sn4+ cations lie in a tetrahedral environment, while Sn2+ cations are found in two kinds of coordination environments: the trigonal pyramid and quadrangular pyramid. SnSen (n = 3, 4, 5) polyhedra are further connected to generate a three-dimensional framework with Ba2+ residing in cavities. Ba6Sn6Se13 shows moderate nonlinear optical response and is the first reported NLO compound in the Ba–Sn–Se system. In addition, diffuse reflectance spectroscopy measurement indicates that the band gap of Ba6Sn6Se13 is 1.52(2) eV and thermal analysis suggests that the compound melts incongruently. The theoretically calculated SHG response and band gap are in good agreement with experimental results.

A new noncentrosymmetric (NCS) rare earth borate crystal K21Yb8B45O90 has been grown using the K2O–CdO–B2O3 flux and its structure was determined by single crystal X-ray crystallographic study. The compound crystallizes in the NCS trigonal space group R32. In the structure, BO3 and BO4 polyhedra interconnect via sharing corners to form the basic structure unit B5O10 groups, which are bridged by YbO6 triangular prisms to build up a three-dimensional framework with one-dimensional channels occupied by K atoms. Second harmonic generation (SHG) measurement indicates that K21Yb8B45O90 possesses an SHG response close to KDP with a particle size of 50–61 μm. In addition, thermal analyses suggest that the compound melts at 955 °C incongruently. The band gap of K21Yb8B45O90 deduced from diffuse reflectance spectroscopy measurement is 5.90(2) eV.

•A new quaternary chalcogenide Ba7AgGa5Se15 was synthesized.•It adopts a new structure type in the space group P31c of the trigonal system.•The structure contains a three-dimensional framework built from GaSe4 and AgSe4 tetrahedra.•Ba7AgGa5Se15 is a direct semiconductor with the band gap of 2.60 (2) eV.•The electronic structure was calculated to explain the optical properties.A new quaternary chalcogenide Ba7AgGa5Se15 was synthesized by solid state reaction. It crystallizes in a new structure type in the noncentrosymmetric space group P31c of the trigonal system. In the structure, three Ga2Se4 tetrahedra and one Ga1Se4 tetrahedron are connected to each other by corner-sharing to form [Ga4Se10]8− anion clusters, which are further connected to AgSe4 tetrahedra by corner-sharing to form a three-dimensional framework with Ba, Se7, and isolated Ga3Se4 tetrahedra residing in the cavities. The optical band gap of 2.60 (2) eV for Ba7AgGa5Se15 was deduced from the diffuse reflectance spectrum. From a band structure calculation, Ba7AgGa5Se15 is a direct semiconductor and the transition between Se and Ba plays an important role in the band gap.

The six compounds Ba2GaMQ5 (M=Sb,Bi; Q=Se,Te) and Ba2InSbQ5 (Q=Se,Te) have been synthesized for the first time. Ba2GaMQ5 (M=Sb,Bi; Q=Se,Te) and Ba2InSbTe5 crystallize in the centrosymmetric space group Pnma, while Ba2InSbSe5 belongs to the noncentrosymmetric polar space group Cmc21. The structures of the six new compounds contain infinite [MM'Q5]4−∞1 anionic chains built by MQ4 (M′=Ga,In) tetrahedra and heavily distorted M′Q6 (M′=Sb,Bi) octahedra. Ba2InSbSe5 possesses a band gap of 1.92(5) eV and exhibits a weak powder second harmonic generation signal using the 2090 nm laser as fundamental wavelength.Graphical abstractBa2InSbSe5 contains infinite [InSbSe5]4−∞1 chains which are built by corner-shared InSe4 tetrahedra chains and edge-shared SbSe6 octahedra chains connected through edge sharing .Highlights► Ba2GaMQ5 (M=Sb,Bi; Q=Se,Te) and Ba2InSbQ5 (Q=Se,Te) have been synthesized. ► The structures contain infinite [MM'Q5]4−∞1 (M′=Ga,In) anionic chains. ► The chains are built by distorted MQ6 (M=Sb,Bi) octahedra and M′Q4 (M′=Ga,In) tetrahedra. ► Ba2InSbSe5 exhibits a weak powder second harmonic generation signal.

The new compound LiGaGe2Se6 has been synthesized. It crystallizes in the orthorhombic space group Fdd2 with a = 12.501(3) Å, b = 23.683(5) Å, c = 7.1196(14) Å, and Z = 8. The structure is a three-dimensional framework composed of corner-sharing LiSe4, GaSe4, and GeSe4 tetrahedra. The compound exhibits a powder second harmonic generation signal at 2 μm that is about half that of the benchmark material AgGaSe2 and possesses a wide band gap of about 2.64(2) eV. LiGaGe2Se6 melts congruently at a rather low temperature of 710 °C, which indicates that bulk crystals can be obtained by the Bridgman–Stockbarger technique. According to a first-principles calculation, there is strong hybridization of the 4s and 4p orbitals of Ga, Ge, and Se around the Fermi level. The calculated birefractive index is Δn = 0.04 for λ ≥ 1 μm, and the calculated major SHG tensor elements are d15 = 18.6 pm/V and d33 = 12.8 pm/V. This new material is promising for application in IR nonlinear optics.

The four isostructural compounds Li2In2MQ6 (M = Si, Ge; Q = S, Se) have been synthesized for the first time. They crystallize in the noncentrosymmetric monoclinic space group Cc with the three-dimensional framework composed of corner-sharing LiQ4, InQ4, and MQ4 tetrahedra. The second-harmonic-generation signal intensities of the two sulfides and two selenides were close to those of AgGaS2 and AgGaSe2, respectively, when probed with a laser with 2090 nm as the fundamental wavelength. They possess large band gaps of 3.61(2) eV for Li2In2SiS6, 3.45(2) eV for Li2In2GeS6, 2.54(2) eV for Li2In2SiSe6, and 2.30(2) eV for Li2In2GeSe6, respectively. Moreover, these four compounds all melt congruently at relatively low temperatures, which makes it feasible to grow bulk crystals needed for practical application by the Bridgman–Stockbarger method.

The twelve quaternary rare-earth selenides Ba2MLnSe5 (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er) have been synthesized for the first time. The compounds Ba2GaLnSe5 (Ln = Y, Nd, Sm, Gd, Dy, Er) are isostructural and crystallize in a new structure type in the centrosymmetric space group P1̅ of the triclinic system while the isostructural compounds Ba2InLnSe5 (Ln = Y, Nd, Sm, Gd, Dy, Er) belong to the Ba2BiInS5 structure type and crystallize in the noncentrosymmetric space group Cmc21 of the orthorhombic system. The structures contain infinite one-dimensional anionic chains 1∞[GaLnSe5]4– and 1∞[InLnSe5]4–, and both chains are built from LnSe6 octahedra and MSe4 (M = Ga, In) tetrahedra in the corresponding selenides. As deduced from the diffuse reflectance spectra, the band gaps of most Ba2MLnSe5 (M = Ga, In; Ln = Y, Nd, Sm, Gd, Dy, Er) compounds are around 2.2 eV. The magnetic susceptibility measurements on Ba2GaGdSe5 and Ba2InLnSe5 (Ln = Nd, Gd, Dy, Er) indicate that they are paramagnetic and obey the Curie–Weiss law, while the magnetic susceptibility of Ba2InSmSe5 deviates from the Curie–Weiss law as a result of the crystal field splitting. Furthermore, Ba2InYSe5 exhibits a strong second harmonic generation response close to that of AgGaSe2, when probed with the 2090 nm laser as fundamental wavelength.

The 12 new rare-earth tellurides Ba2MLnTe5 (M = Ga and Ln = Sm, Gd, Dy, Er, Y; M = In and Ln = Ce, Nd, Sm, Gd, Dy, Er, Y) have been synthesized by solid-state reactions. The two compounds Ba2GaLnTe5 (Ln = Sm, Gd) are isostructural and crystallize in the centrosymmetric space group P1̅, while the other 10 compounds belong to another structure type in the noncentrosymmetric space group Cmc21. In both structure types, there are one-dimensional anionic 1∞[MLnTe5]4– chains built from LnTe6 octahedra and MTe4 (M = Ga, In) tetrahedra, but the connectivity between the LnTe6 octahedra and MTe4 tetrahedra is different for the two structure types. On the basis of the diffuse-reflectance spectra, the band gaps are around 1.1–1.3 eV for these compounds. The Ba2MLnTe5 (M = Ga and Ln = Gd, Dy; M = In and Ln = Gd, Dy, Er) compounds are paramagnetic and obey the Curie–Weiss law, while the magnetic susceptibility of Ba2InSmTe5 deviates from the Curie–Weiss law. In addition, electronic structure calculation on Ba2MYTe5 (M = Ga, In) indicates that they are both direct-gap semiconductors with large nonlinear-optical coefficients.

Six new quaternary rare-earth sulfides Ba3LnInS6 (Ln = Pr, Sm, Gd, Yb) and Ba2LnGaS5 (Ln = Pr, Nd) have been synthesized for the first time. Ba3LnInS6 (Ln = Pr, Sm, Gd, Yb) belong to the centrosymmetric space group R3̅c of the trigonal system. The structures contain infinite one-dimensional anionic chains 1∞[LnInS6]6–, which are built from face-sharing LnS6 distorted triangular prisms and InS6 octahedra. Ba2LnGaS5 (Ln = Pr, Nd) crystallize in the centrosymmetric space group I4/mcm of the tetragonal system. Their structures consist of (BaLn)S layers built from (BaLn)S8 bicapped trigonal prisms. These layers are stacked perpendicular to the c axis and further connected by GaS4 tetrahedra to form a three-dimensional framework with channels occupied by Ba2+ cations. As deduced from magnetic susceptibility measurements on Ba2NdGaS5, it is paramagnetic and obeys the Curie–Weiss law. Besides, the band gap of Ba2NdGaS5 is determined to be about 2.12(2) eV.

A new ternary germanium phosphide, NaGe3P3, was obtained for the first time with the use of NaP as the reactive flux. It crystallizes in the orthorhombic space groupPmc21. The basic structural unit is an unprecedented [Ge3P7] ring built from one Ge(P)4 tetrahedron, one Ge(Ge)(P)3 tetrahedron and one Ge(Ge)(P)2 trigonal pyramid with Ge in mixed valences of 4+, 3+ and 1+. The bonding between a tetrahedrally coordinated Ge atom and a trigonal pyramidally coordinated Ge atom (with 4s2 lone pair of electrons) is observed for the first time in inorganic compounds. These [Ge3P7] rings are connected with each other to form two-dimensional 2∞[Ge3P3]− layers separated by Na+ cations. An optical band gap of 2.06(2) eV was deduced from the diffuse reflectance spectrum. Based on the electronic structure calculation, NaGe3P3 is an indirect gap semiconductor with the Ge4s, Ge4p and P3p orbitals strongly hybridizing around the Fermi level.

The four compounds BaGa2MQ6 (M = Si, Ge; Q = S, Se) have been identified as a new series of IR nonlinear optical (NLO) materials and are promising for practical applications. They are isostructural and crystallize in the noncentrosymmetric polar space group R3 of the trigonal system. Their three-dimensional framework is composed of corner-sharing (Ga/M)Q4 (M = Si, Ge; Q = S, Se) tetrahedra with Ba2+ cations in the cavities. The polar alignment of one (Ga/M)–Q2 bond for each (Ga/M)Q4 tetrahedra along the c direction is conducive to generating a large NLO response, which was confirmed by powder second-harmonic generation (SHG) using a 2090 nm laser as fundamental wavelength. The SHG signal intensities of the two sulfides were close to that of AgGaS2 and those for the two selenides were similar as that of AgGaSe2. The large band gaps of 3.75(2) eV, 3.23(2) eV, 2.88(2) eV, and 2.22 (2) eV for BaGa2SiS6, BaGa2GeS6, BaGa2SiSe6, and BaGa2GeSe6, respectively, will be very helpful to increase the laser damage threshold. Moreover, all the four BaGa2MQ6 (M = Si, Ge; Q = S, Se) compounds exhibit congruent-melting behavior, which indicates that bulk crystals needed for practical applications can be obtained by the Bridgman–Stockbarger method. The calculated birefringence indicates that these materials may be phase-matchable in the IR region and the calculated SHG coefficients agree with the experimental observations. According to our preliminary study, the BaGa2MQ6 compounds represent a new series of promising IR nonlinear optical (NLO) materials which do not belong to the traditional chalcopyrite-type materials such as AgGaQ2 (Q = S, Se) and ZnGeP2.

The first two members in alkaline-earth/group XI/group XIII/chalcogen system, namely Ba2AgInS4 and Ba4AgGa5Se12, were synthesized along with a Li analogue Ba4LiGa5Se12. Ba2AgInS4 crystallizes in space groupP21/c. It contains 2∞[AgInS4]4− layers built from AgS3 triangles and InS4 tetrahedra with Ba2+ cations inserted between the layers. Ba4AgGa5Se12 and Ba4LiGa5Se12 adopt two closely-related structure types in space group P21c with structural difference originating from the different positions of Ag and Li in them. The three-dimensional framework in Ba4AgGa5Se12 is composed of GaSe4 tetrahedra with the Ba and Ag atoms occupying the large and small channels respectively, whereas that in Ba4LiGa5Se12 is built from LiSe4 and GaSe4 tetrahedra with channels to accommodate the Ba atoms. As deduced from the diffuse reflectance spectra measurement, the optical band gaps were 2.32 (2) eV, 2.52 (2) eV, and 2.65 (2) eV for Ba2AgInS4, Ba4AgGa5Se12, and Ba4LiGa5Se12, respectively.

Red emission of monoclinic Zr(1−2x)EuxMxO2 (M = Nb, Ta; x = 0.05) was enhanced by the M addition, about five times higher than Zr0.95Eu0.05O2 excited by 395 nm or 469 nm at room temperature. The corresponding quantum yield improves 2–4 times, achieving ∼13%. The luminescence enhancement is attributed to that the codoped Eu3+ and M5+ ions result in the seven-coordinated Eu3+ activators with the low symmetry C1, compared with the eight-coordinated high symmetric (D2d) Eu3+ ions coexisted with oxygen vacancies in Zr(1−x)EuxO2. The thin film of Zr0.90Eu0.05Nb0.05O2, deposited by radio frequency magnetron sputtering method, also shows intensely red emission.Highlights► Red emission of ZrO2:Eu3+ is enhanced by five times through Nb5+ or Ta5+ addition. ► Quantum efficiency is improved by about three times. ► Emission enhancement is ascribed to monoclinic phase and defect elimination. ► Luminescence film shows high transmittance and intense red emission.

The ternary gallium selenide KGaSe2 has been synthesized by solid-state reactions and good quality crystal has been obtained. KGaSe2 crystallizes in the monoclinic space group C2/c with cell dimensions of a = 10.878(2) Å, b = 10.872(2) Å, c = 15.380(3) Å, and β = 100.18(3)°. In the structure, adamantane like [Ga4Se10]8− units are connected by common corners forming two-dimensional [GaSe2]− layers which are separated by K+ cations. KGaSe2 exhibits congruent-melting behavior at around 965 °C. It is transparent in the range of 0.47–20.0 μm and has a band gap of 2.60(2) eV. From a band structure calculation, KGaSe2 is a direct-gap semiconductor. The band gap is mainly determined by the [GaSe2]− layer.Graphical abstractHighlights► A single-crystal structural analysis of KGaSe2 was carried out for the first time. ► Adamantane-like [Ga4Se10]8− are connected to generate two-dimensional layers. ► KGaSe2 is transparent in the range of 0.47–20 μm and melts congruently at 965 °C. ► KGaSe2 is a direct-gap semiconductor according to the band structure calculation.

The new metal chalcogenide CsAgGa2Se4 has been synthesized by means of the reactive flux method. It crystallizes in the space group P21/c of the monoclinic system with cell dimensions of a=11.225(2) Å, b=7.9443(16) Å, c=21.303(4) Å, β=103.10(3), V=1850.3(6), and Z=8. The structure contains two-dimensional ∞2[AgGa2Se4]− layers separated by Cs+ cations. The ∞2[AgGa2Se4]− superlayer possesses a novel chain–sublayer–chain structure: a ∞2[Ag2GaSe6]7− sublayer, composed of ∞1[AgGaSe4]4− chains that are further connected by Ag+ ions, is sandwiched by parallel ∞1[Ga3Se8]7− chains to generate the ∞2[AgGa2Se4]− superlayer. From a band structure calculation, the orbitals of all atoms have contributions to the bottoms of conduction bands, but the band gap is mainly determined by the 4s, 4p orbitals of Ga and Se.Graphical AbstractCsAgGa2Se4 contains two-dimensional ∞2[AgGa2Se4]− layers with a novel chain–sublayer–chain structure.Highlights► New chalcogenide CsAgGa2Se4 has been synthesized. ► It possesses a new structure type with ∞2[AgGa2Se4]− layers separated by Cs+ cations. ► ∞2[AgGa2Se4]− consists of a ∞2[Ag2GaSe6]7− sublayer sandwiched by ∞1[Ga3Se8]7− chains. ► Band gap of CsAgGa2Se4 is mainly determined by the 4s, 4p orbitals of Ga and Se.

Two new quaternary chalcogenides, namely Ba4MInSe6 (M=Cu, Ag), were synthesized by solid state reactions. These two isostructural compounds adopt the Ba2MnS3 structure type in the orthorhombic space group Pnma. In the structure, the M and In atoms randomly occupy one crystallographic unique metal position with the molar ratio of 1:1 The (M/In)Se4 tetrahedra are connected to each other by corner-sharing to form one-dimensional chains along the b direction, which are separated by mono-capped trigonal prismatically coordinated Ba atoms. Based on the diffuse reflectance spectrum, the optical band gaps were determined to be 2.23(2) and 2.41(2) eV for Ba4CuInSe6 and Ba4AgInSe6, respectively.Graphical abstractIn the structure of Ba4MInSe6 (M=Cu, Ag), the (M/In)Se4 tetrahedra are connected by corner-sharing to form chains along the b direction, which are separated by Ba atoms.Highlights► Two new quaternary chalcogenides, Ba4MInSe6 (M=Cu, Ag), were synthesized. ► Ba4MInSe6 (M=Cu, Ag) are isostructural and crystallize in the Ba2MnS3 structure type ► The (M/In)Se4 tetrahedra are connected by corner-sharing to form chains along the b direction. ► The chains are separated by mono-capped trigonal prismatically coordinated Ba atoms. ► The optical band gaps are 2.23(2) and 2.41(2) eV for Ba4CuInSe6 and Ba4AgInSe6, respectively.

The new compound BaAl4Se7 has been synthesized by solid-state reaction. It crystallizes in the non-centrosymmetric space groupPc and adopts a three-dimensional framework built from AlSe4 tetrahedra and with Ba2+ cations in the cavities. The material has a large band gap of 3.40(2) eV. It melts congruently at 901 °C and exhibits a second harmonic generation (SHG) response at 1 μm that is about half that of AgGaS2. From a band structure calculation, BaAl4Se7 is a direct-gap semiconductor with strong hybridization of the Al 3s, Al 3p, and Se 4p orbitals near the Fermi level. The calculated birefractive index is about 0.05 for wavelength longer than 1 μm and major SHG tensor elements are: d15 = 5.2 pm V−1 and d13 = 4.2 pm V−1.

The new compound Ba5Ga4Se10 has been synthesized for the first time. It crystallizes in the tetragonal space groupI4/mcm with a = 8.752(2) Å, c = 13.971(9) Å, and Z = 2. The structure contains discrete [Ga4Se10]10− anions and charge-compensating Ba2+ cations. The novel highly anionic [Ga4Se10]10−cluster is composed of two Ga(Se)4 tetrahedra and two Ga(Ga)(Se)3 tetrahedra with Ga in the 2+/3+ valence states. It also exhibits an unusually long Ga–Se distance of 2.705(2) Å, which has only been observed under high pressure conditions before. A band gap of 2.20(2) eV was deduced from the UV/vis diffuse reflectance spectrum.

Two new barium selenides Ba5Al2Se8 and Ba5Ga2Se8 have been synthesized by solid-state reactions. The structures of Ba5Ga2Se8 and Ba5Al2Se8 were determined by single-crystal X-ray diffraction method and the Rietveld method, respectively. The two isostructural compounds crystallize in space group Cmca of the orthorhombic system with isolated MSe4 (M = Al, Ga) tetrahedra separated by Ba atoms. The optical band gap of 2.51(2) eV for Ba5Ga2Se8 was deduced from the diffuse reflectance spectrum. Band structure calculation indicates that Ba5Ga2Se8 is a direct-gap semiconductor. The valence band maximum is dominated by Se 4p orbitals, while the Ba 5d orbitals have the largest contribution to bottom of the conduction band.Graphical abstractResearch highlights▶ Two new compounds Ba5M2Se8 (M = Al, Ga) have been synthesized for the first time. ▶ The crystal structures contain isolated MSe4 tetrahedra separated by Ba atoms. ▶ The measured optical band gap is 2.51(2) eV for Ba5Ga2Se8. ▶ The Ba 5d orbitals were found to have significant contribution to the energy bands around Fermi level.

Double perovskite Ca2LaSbO6, successfully synthesized by solid state reaction method, was identified by Rietveld refinements to crystallize in the monoclinic space group P21/n, which is isostructural to Ca2LaMO6 (M=Nb, Ta). Excellent red luminescence of Eu-doped Ca2LaMO6 (M=Sb, Nb, Ta) can be obtained and no luminescence quenching effect was observed when Eu-doping level reached 40%. For Ca2La0.6NbO6:0.4Eu3+, quantum efficiencies of 20.9% and 27.7% were reached to show high light conversion and bright red emission excited at 465 nm (blue light) and 534 nm (green light), respectively, comparable to the commercial phosphors. Through systemic investigation for the series of double perovskite compounds, the excellent red emission in Ca2LaMO6 is attributed to highly distorted polyhedra of EuO8 (low tolerance factor of the pervoskite), and large bond distances of La−O (low crystal field effect of the activator).Graphical AbstractEu3+ doped double-perovskite compounds A2LnMO6 (A=Ca, Sr, Ba; Ln=La, Gd, Y; M=Sb, Nb, Ta) show the dependence of luminescence intensity on the crystal structure.Highlights► A series of double perovskite compounds were synthesized by solid state reaction. ► Eu3+ doped samples display intense red emission when excited by blue or green light. ► High quantum efficiency was obtained, comparable to the commercial phosphors. ► Luminescence properties were ascribed to crystal distortion and large Ln–O distance.

A new borate, Cs2Al2B2O7, was synthesized by solid-state reaction. It crystallizes in the monoclinic space group P21/c with a=6.719(1) Å, b=7.121(1) Å, c=9.626(3) Å, β=115.3(1)°, and Z=2. In the structure, two AlO4 tetrahedra and two BO3 planar triangles are connected alternately by corner-sharing to from nearly planar [Al2B2O10] rings, which are further linked via common O1 atom to generate layers in the bc plane. These layers then share the O3 atoms lying on a center of inversion to form a three-dimensional framework with Cs atoms residing in the channels. The IR spectrum confirms the presence of both BO3 and AlO4 groups and the UV–vis–IR diffuse reflectance spectrum indicates a band gap of about 4.13(2) eV.Graphical abstractA new borate, Cs2Al2B2O7, was synthesized. In the structure, BO3 triangles and AlO4 tetrahedra are connected to form a three-dimensional framework with Cs+ in the channels.Highlights► A new borate Cs2Al2B2O7 was synthesized and crystals were obtained by flux method. ► Cs2Al2B2O7 crystallizes in a new structure type. ► Two AlO4 and two BO3 generate a [Al2B2O10] ring in the structure. ► Rings are linked to form a three-dimensional framework with Cs+ in the channels. ► Optical band gap is about 4.13 eV.

Two new ternary bismuth chalcogenides, Bi3In4S10 and Bi14.7In11.3S38, were synthesized from the reactions of binary sulfides via a two-step flux technique. Single-crystal X-ray diffraction analyses indicate that Bi3In4S10 crystallizes in the non-centrosymmetric space group Pm and Bi14.7In11.3S38 crystallizes in the centrosymmetric space group P21/m. Both compounds adopt three-dimensional frameworks. A distinct structural feature in the two structures is the presence of chains of Bi atoms with alternating short Bi–Bi bonds of around 3.1 Å and longer distances of around 4.6 Å. The optical band gaps of 1.42(2) eV for Bi3In4S10 and 1.45(2) eV for Bi14.7In11.3S38 were deduced from the diffuse reflectance spectra.Graphical abstractTwo new bismuth sulfides Bi3In4S10 and Bi14.7In11.3S38 have been synthesized and characterized. The figure is the arrangement of Bi1 atoms along the b direction with alternating short and long distances (Å) in Bi3In4S10.

Two new bismuth sulfides KBiSiS4 and KBiGeS4 have been synthesized by means of the reactive flux method. They adopt the RbBiSiS4 structure type and crystallize in space group P21/c   of the monoclinic system. The structure consists of [BiMS4−]2∞ (M=Si, Ge) layers separated by bicapped trigonal-prismatically coordinated K atoms. The M atom is tetrahedrally coordinated to four S atoms and the Bi atom is coordinated to a distorted monocapped trigonal prism of seven S atoms. The optical band gap of 2.25(2) eV for KBiSiS4 was deduced from the diffuse reflectance spectrum. From a band structure calculation, the optical absorption for KBiSiS4 originates from the [BiSiS4−]2∞ layer. The Si 3p orbitals, Bi 6p orbitals, and S 3p orbitals are highly hybridized near the Fermi level. The orbitals of K have no contributions on both the upper of valence band and the bottom of conduction band.Graphical abstractTwo new bismuth sulfides KBiSiS4 and KBiGeS4 have been synthesized and characterized. The figure is the view down [0 1 0] of the crystal structure of KBiSiS4.

•Ca2SnS4 crystals have been synthesized and characterized for the first time.•The structure contains bi-layers with isolated polyhedra residing in the cavities.•Ca2SnS4 has a band gap of 2.32(3) eV, comparable to the calculated value 2.72 eV.single crystals of Ca2SnS4were obtained by traditional high temperature solid-state reaction. The compound crystallizes in space group Pnma of the orthorhombic system. Its structure contains bi-layers built from corner-sharing Ca1S6 octahedra and Ca2S6 octahedra with isolated SnS4 tetrahedra residing in the cavities along the b axis. UV–vis-NIR spectroscopy measurement indicates that Ca2SnS4 has a band gap of 2.32(3) eV, comparable to the calculated value 2.72 eV.

Two new quaternary selenides, namely Ba4Ga4SnSe12 and Ba6Ga2SnSe11, have been synthesized for the first time, representing the first two members in the A/M/Sn/Q (A = alkaline-earth metal; M = Al, Ga, In; Q = S, Se, Te) system. Ba4Ga4SnSe12 crystallizes in the non-centrosymmetric space group P21/c of the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe4 and SnSe4 tetrahedra with eight-coordinated Ba2+ cations residing in the cavities. Ba6Ga2SnSe11 crystallizes in a new structure type in the monoclinic centrosymmetric space group P21/c. The structure of Ba6Ga2SnSe11 features a zero-dimensional structure containing totally isolated distorted SnSe4 tetrahedra and a discrete Ga2Se7 unit with Ba2+ cations located between them. On the basis of the diffuse-reflectance spectra, the band gaps are 2.16 (2) eV and 1.99 (2) eV for Ba4Ga4SnSe12 and Ba6Ga2SnSe11 respectively. In addition, the electronic structure calculation of Ba4Ga4SnSe12 indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga4SnSe12]8− anionic framework.

A new ternary germanium phosphide, NaGe3P3, was obtained for the first time with the use of NaP as the reactive flux. It crystallizes in the orthorhombic space groupPmc21. The basic structural unit is an unprecedented [Ge3P7] ring built from one Ge(P)4 tetrahedron, one Ge(Ge)(P)3 tetrahedron and one Ge(Ge)(P)2 trigonal pyramid with Ge in mixed valences of 4+, 3+ and 1+. The bonding between a tetrahedrally coordinated Ge atom and a trigonal pyramidally coordinated Ge atom (with 4s2 lone pair of electrons) is observed for the first time in inorganic compounds. These [Ge3P7] rings are connected with each other to form two-dimensional 2∞[Ge3P3]− layers separated by Na+ cations. An optical band gap of 2.06(2) eV was deduced from the diffuse reflectance spectrum. Based on the electronic structure calculation, NaGe3P3 is an indirect gap semiconductor with the Ge4s, Ge4p and P3p orbitals strongly hybridizing around the Fermi level.

The first two members in alkaline-earth/group XI/group XIII/chalcogen system, namely Ba2AgInS4 and Ba4AgGa5Se12, were synthesized along with a Li analogue Ba4LiGa5Se12. Ba2AgInS4 crystallizes in space groupP21/c. It contains 2∞[AgInS4]4− layers built from AgS3 triangles and InS4 tetrahedra with Ba2+ cations inserted between the layers. Ba4AgGa5Se12 and Ba4LiGa5Se12 adopt two closely-related structure types in space group P21c with structural difference originating from the different positions of Ag and Li in them. The three-dimensional framework in Ba4AgGa5Se12 is composed of GaSe4 tetrahedra with the Ba and Ag atoms occupying the large and small channels respectively, whereas that in Ba4LiGa5Se12 is built from LiSe4 and GaSe4 tetrahedra with channels to accommodate the Ba atoms. As deduced from the diffuse reflectance spectra measurement, the optical band gaps were 2.32 (2) eV, 2.52 (2) eV, and 2.65 (2) eV for Ba2AgInS4, Ba4AgGa5Se12, and Ba4LiGa5Se12, respectively.

The four compounds BaGa2MQ6 (M = Si, Ge; Q = S, Se) have been identified as a new series of IR nonlinear optical (NLO) materials and are promising for practical applications. They are isostructural and crystallize in the noncentrosymmetric polar space group R3 of the trigonal system. Their three-dimensional framework is composed of corner-sharing (Ga/M)Q4 (M = Si, Ge; Q = S, Se) tetrahedra with Ba2+ cations in the cavities. The polar alignment of one (Ga/M)–Q2 bond for each (Ga/M)Q4 tetrahedra along the c direction is conducive to generating a large NLO response, which was confirmed by powder second-harmonic generation (SHG) using a 2090 nm laser as fundamental wavelength. The SHG signal intensities of the two sulfides were close to that of AgGaS2 and those for the two selenides were similar as that of AgGaSe2. The large band gaps of 3.75(2) eV, 3.23(2) eV, 2.88(2) eV, and 2.22 (2) eV for BaGa2SiS6, BaGa2GeS6, BaGa2SiSe6, and BaGa2GeSe6, respectively, will be very helpful to increase the laser damage threshold. Moreover, all the four BaGa2MQ6 (M = Si, Ge; Q = S, Se) compounds exhibit congruent-melting behavior, which indicates that bulk crystals needed for practical applications can be obtained by the Bridgman–Stockbarger method. The calculated birefringence indicates that these materials may be phase-matchable in the IR region and the calculated SHG coefficients agree with the experimental observations. According to our preliminary study, the BaGa2MQ6 compounds represent a new series of promising IR nonlinear optical (NLO) materials which do not belong to the traditional chalcopyrite-type materials such as AgGaQ2 (Q = S, Se) and ZnGeP2.

A new one-dimensional (1D) Mn-based chalcogenide Na2MnGe2Se6 has been obtained by a solid-state reaction. It crystallizes in the tetragonal space group I4/mcm with unit cell parameters a = 7.7718(16) Å, b = 7.7718(16) Å, c = 19.077(7) Å and Z = 2. In the structure, MnSe4 and GeSe4 tetrahedra are linked alternately via edge-sharing with a ratio of 1:2 to form 1D infinite [MnGe2Se6]2− anionic chains, which are further separated by Na+ cations. Such a spatial arrangement leads to large separations between magnetic ions, namely, 9.538(5) Å for the nearest intrachain Mn⋯Mn distance and 5.495(5) Å for the neighboring interchain Mn⋯Mn distance. Despite the large Mn⋯Mn separation, the temperature dependent susceptibility measurement and specific heat measurement still indicate an interesting antiferromagnetic interaction with the Néel temperature TN = 11 K for this compound. Such unusual magnetic properties have been seldom reported in other Mn chalcogenides. The magnetic interaction is investigated by the spin-polarized calculations. Besides, the UV-vis-NIR spectroscopy measurement indicates that Na2MnGe2Se6 has an indirect band gap of 1.93 eV.

By combining the [GaS4] tetrahedra and the second-order Jahn–Teller distorted cation Pb2+, a new wide-gap nonlinear optical material PbGa4S7 has been obtained. In the structure, Pb2+ is coordinated to a distorted quadrangular pyramid of five S atoms, showing the stereo-chemical activity of the 6s2 lone pair electrons. The polar arrangement of the [GaS4] tetrahedra and the macroscopic arrangement of the [PbS5] distorted quadrangular pyramids result in a large IR nonlinear optical property for PbGa4S7, which is ∼1.2 times that of the benchmark AgGaS2 at a laser radiation of 2.09 μm. Moreover, PbGa4S7 has a large direct band gap of 3.08(2) eV, a desirable property for avoiding two-photon absorption of the conventional 1–2 μm pumping laser sources and improving the laser damage threshold. The DSC analysis indicates that the compound is thermally stable up to 1140 K. The electronic structure was also calculated to explain the optical properties.

By introducing a highly electronegative halide anion into chalcogenides, a novel chalcohalide, NaBa4Ge3S10Cl, has been synthesized by a conventional high temperature solid state method. It crystallizes in a new structure type of space group P63, with a = 9.7653(2) Å, c = 12.0581(3) Å and Z = 2. The fundamental unit is the unique [Ge3S9] ring, comprised of three GeS4 tetrahedra sharing corner S atoms. The [Ge3S9] rings are arranged to form pseudo layers, which are stacked through Na–Cl–Ba chains to build up the structure. The macroscopic packing of these [Ge3S9] rings provides the material with a moderate nonlinear optical (NLO) response at 2090 nm fundamental light. Furthermore, UV-vis-IR spectroscopy shows that NaBa4Ge3S10Cl has a very large band gap of 3.49 eV, which is very beneficial for increasing the laser damage threshold and avoiding the two-photon absorption problem of conventional near IR laser pumping sources.

The new compound Ba5Ga4Se10 has been synthesized for the first time. It crystallizes in the tetragonal space groupI4/mcm with a = 8.752(2) Å, c = 13.971(9) Å, and Z = 2. The structure contains discrete [Ga4Se10]10− anions and charge-compensating Ba2+ cations. The novel highly anionic [Ga4Se10]10−cluster is composed of two Ga(Se)4 tetrahedra and two Ga(Ga)(Se)3 tetrahedra with Ga in the 2+/3+ valence states. It also exhibits an unusually long Ga–Se distance of 2.705(2) Å, which has only been observed under high pressure conditions before. A band gap of 2.20(2) eV was deduced from the UV/vis diffuse reflectance spectrum.

A new ternary selenostannate Ba6Sn6Se13 has been synthesized by a high temperature solid-state method. The compound crystallizes in the non-centrosymmetric orthorhombic space group P212121 and may be represented as Ba6Sn52+Sn4+Se13 with mixed valence Sn atoms. Sn4+ cations lie in a tetrahedral environment, while Sn2+ cations are found in two kinds of coordination environments: the trigonal pyramid and quadrangular pyramid. SnSen (n = 3, 4, 5) polyhedra are further connected to generate a three-dimensional framework with Ba2+ residing in cavities. Ba6Sn6Se13 shows moderate nonlinear optical response and is the first reported NLO compound in the Ba–Sn–Se system. In addition, diffuse reflectance spectroscopy measurement indicates that the band gap of Ba6Sn6Se13 is 1.52(2) eV and thermal analysis suggests that the compound melts incongruently. The theoretically calculated SHG response and band gap are in good agreement with experimental results.

A new sulfide, SnGa2GeS6, has been synthesized, which represents the first member in the quaternary Sn/M/M′/Q (M = Ga, In; M′ = Si, Ge; Q = S, Se, Te) system. It adopts a new structure type in the non-centrosymmetric space group Fdd2. In the structure, Sn2+ is coordinated to a distorted square-pyramid of five S atoms, demonstrating the stereochemical activity of the lone electron pair, while the Ge atom and Ga atom are both tetrahedrally coordinated to four S atoms. The SnS5 square-pyramids and the MS4 (M = Ga, Ge) tetrahedra are connected to each other via corner and edge-sharing to generate a three-dimensional framework. The compound exhibits a powder second harmonic generation signal at 2 μm whose strength is about one-fourth that of the benchmark material AgGaS2, which may be explained in view of the macroscopic arrangement of the SnS5 square-pyramids and the MS4 tetrahedra. Moreover, based on UV-vis-NIR spectroscopy measurements and the electronic structure calculations, SnGa2GeS6 has two optical transitions at about 1.12 eV and 2.04 eV respectively.

The new compound BaAl4Se7 has been synthesized by solid-state reaction. It crystallizes in the non-centrosymmetric space groupPc and adopts a three-dimensional framework built from AlSe4 tetrahedra and with Ba2+ cations in the cavities. The material has a large band gap of 3.40(2) eV. It melts congruently at 901 °C and exhibits a second harmonic generation (SHG) response at 1 μm that is about half that of AgGaS2. From a band structure calculation, BaAl4Se7 is a direct-gap semiconductor with strong hybridization of the Al 3s, Al 3p, and Se 4p orbitals near the Fermi level. The calculated birefractive index is about 0.05 for wavelength longer than 1 μm and major SHG tensor elements are: d15 = 5.2 pm V−1 and d13 = 4.2 pm V−1.

Three new chalcogenides, namely NaGaGe3Se8, K3Ga3Ge7S20, and K3Ga3Ge7Se20, of an A–Ga–Ge–Q (A = Na, K; Q = S, Se) system were obtained for the first time. They crystallize into two different new structures, albeit both in the monoclinic space group P21/c. NaGaGe3Se8 has a layered structure consisting of two dimensional ∞2[M4Se8]− layers separated by Na+ cations, while the structures of K3Ga3Ge7Q20 (Q = S, Se) are constructed by incompletely isolated quasi-2D ∞2[M10Q21]5− layers, leading to large channels loosely occupied by K+ cations. Interestingly, thermal analysis indicates that the three title compounds are all congruent-melting compounds, which is uncommon for quaternary compounds, and makes bulk crystal growth using the Bridgman technique possible. UV-vis-NIR spectroscopy measurements reveal that the optical band gaps of the three compounds are 2.35, 3.25, and 2.23 eV. In addition, the electronic structure calculations on NaGaGe3Se8 show that the band gap is mainly determined by the GaSe4 and GeSe4 groups.

By introducing heavy Sn ions to a Ba–Ga–Q system, a new IR nonlinear optical material, BaGa2SnSe6, has been obtained. It crystallizes in space group R3 of the trigonal system with a = 10.1449(14) Å, c = 9.2490(18) Å and Z = 3. In the structure, three (Ga/Sn)Se4 tetrahedra are connected via corner-sharing to generate (Ga/Sn)3Se9 building groups, which are further joined to produce a three-dimensional network with Ba2+ lying in the cavities. Due to the contribution of the large and easily-polarizable Sn atom and the polar arrangements of the (Ga/Sn)Se4 tetrahedra, the compound exhibits a very strong NLO response, i.e., ∼5.2 times that of the benchmark AgGaS2 at a fundamental laser wavelength of 2.09 μm and shows type-I phase-matchable behavior. Furthermore, the calculated birefringence index is Δn = 0.1649 at 1.064 μm and the major SHG tensors are d11 = 62.91 pm V−1 and d33 = 50.26 pm V−1.