Electrode potential–pH (Pourbaix) diagrams provide a phase map of the most stable compounds of a metal, its corrosion products, and associated ions in solution. The utility of these phase diagrams is that they enable the assessment of electrochemical stabilities, for example, of Ni metal and its derived oxides, hydroxides, and oxyhydroxides, against corrosion in aqueous environments. Remarkably, the Ni Pourbaix diagrams reported over the last 50 years are largely inconsistent with various electrochemical observations, which may be attributed to inaccurate experimental free energies of formation (ΔfG) for the complex Ni-based compounds used in producing the available diagrams. Here we show that state-of-the-art density-functional theory (DFT) can be used to obtain accurate ΔfG values, which lead to Ni Pourbaix diagrams that are more consistent with direct electrochemical experiments: Electrochemical impedance spectroscopy and surface-enhanced Raman spectroscopy are used to characterize the electrochemical stabilities of NiO and Ni(OH)2 formed on Ni, demonstrating the reliability in correction-free first-principles based Pourbaix diagrams. Our results show the importance in applying modern density functionals in combination with experimental advances in aqueous environment compound identification for assessing electrochemical phase stability of materials, which will be useful for the design, synthesis, and selection of corrosion-resistant metals, photoabsorbers, and photocatalytic materials.

The applications of transition metal oxides as photovoltaic and photocatalytic materials are mainly impeded by their poor visible light absorption, low photogenerated carrier mobility, and low valence band position, which originate from the generally large band gap (≥3 eV), narrow transition metal d states, and deep oxygen 2p states. Here, we conceive a design strategy to realize small band gap polar oxides with high carrier mobilities by combining small radii A cations with Bi3+/Bi5+ charge disproportion. We show that these cation sizes and chemical features shift the valence band edge to higher energies and therefore reduce the band gap, promoting the formation of highly dispersive Bi 6s states near the Fermi level as a byproduct. By means of advanced many-electron-based first-principles calculations, we predict a new family of thermodynamically stable/metastable polar oxides ABiO3 (A = Ca, Cd, Zn, and Mg), which adopt the Ni3TeO6-type (space group R3) structure and exhibit optical band gaps of ∼2.0 eV, as promising single phase photovoltaic and photocatalytic materials operating in the visible light spectrum.

One synthetic modality for materials discovery proceeds by forming mixtures of two or more compounds. In transition metal oxides (TMOs), chemical substitution often obeys Vegard’s principle, and the resulting structure and properties of the derived phase follow from its components. A change in the assembly of the components into a digital nanostructure, however, can stabilize new polymorphs and properties not observed in the constituents. Here we formulate and demonstrate a crystal-chemistry design approach for realizing digital TMOs without inversion symmetry by combining two centrosymmetric compounds, utilizing periodic anion-vacancy order to generate multiple polyhedra that together with cation order produce a polar structure. We next apply this strategy to two brownmillerite-structured TMOs known to display centrosymmetric crystal structures in their bulk, Ca2Fe2O5 and Sr2Fe2O5. We then realize epitaxial (SrFeO2.5)1/(CaFeO2.5)1 thin film superlattices possessing both anion-vacancy order and Sr and Ca chemical order at the subnanometer scale, confirmed through synchrotron-based diffraction and aberration corrected electron microscopy. Through a detailed symmetry analysis and density functional theory calculations, we show that A-site cation ordering lifts inversion symmetry in the superlattice and produces a polar compound. Our results demonstrate how control of anion and cation order at the nanoscale can be utilized to produce acentric structures markedly different than their constituents and open a path toward novel structure-based property design.

Owing to its ideal semiconducting band gap and good carrier-transport properties, the fully inorganic perovskite CsSnI3 has been proposed as a visible-light absorber for photovoltaic (PV) applications. However, compared to the organic–inorganic lead halide perovskite CH3NH3PbI3, CsSnI3 solar cells display very low energy conversion efficiency. In this work, we propose a potential route to improve the PV properties of CsSnI3. Using first-principles calculations, we examine the crystal structures and electronic properties of CsSnI3, including its structural polymorphs. Next, we purposefully order Cs and Rb cations on the A site to create the double perovskite (CsRb)Sn2I6. We find that a stable ferroelectric polarization arises from the nontrivial coupling between polar displacements and octahedral rotations of the SnI6 network. These ferroelectric double perovskites are predicted to have energy band gaps and carrier effective masses similar to those of CsSnI3. More importantly, unlike nonpolar CsSnI3, the electric polarization present in ferroelectric (CsRb)Sn2I6 can effectively separate the photoexcited carriers, leading to novel ferroelectric PV materials with potentially enhanced energy conversion efficiency.

On the basis of their short ultraviolet (UV) absorption edges, phosphates are ideal candidates for deep-UV nonlinear optical (NLO) applications. However, their often-weak second-harmonic generating (SHG) responses reduce their NLO applications. It has been demonstrated that the SHG response in polyphosphates or orthophosphates could be enhanced by highly polymerized P–O groups or aligned nonbonding O-2p orbitals of isolated PO4 units. Herein, we report on the design and synthesis of two pyrophosphates, K4Mg4(P2O7)3 and Rb4Mg4(P2O7)3, with potential NLO applications. Both materials exhibit relatively large SHG responses with 1064 nm radiation, 1.3× and 1.4× KH2PO4 (KDP) for K4Mg4(P2O7)3 and Rb4Mg4(P2O7)3, respectively. In addition, absorption edges below 200 nm were observed for both materials. For K4Mg4(P2O7)3, single crystal vacuum-UV transmission measurements revealed an absorption edge of 170 nm. First-principles electronic structure calculations identify that the NLO responses arise from the presence of the corner-connected [Mg4P6O21] double layers. We also investigated these compounds using hybrid density functionals, which are found to produce much better agreement with the experimental optical results. Finally, we detail the structural distortions giving rise to the NLO responses. Our results indicate that phosphates with low polymerized P–O groups, such as pyrophosphates, may exhibit large SHG responses if their structures are properly designed.

AbstractNonlinear optical (NLO) materials are of intense academic and technological interest attributable to their ability to generate coherent radiation over a range of different wavelengths. The requirements for a viable NLO material are rather strict, and their discovery has mainly been serendipitous. This study reports synthesis, characterization, and, most importantly, growth of large single crystals of a technologically viable NLO material—Rb3Ba3Li2Al4B6O20F. Through the judicious selection of cations, Rb3Ba3Li2Al4B6O20F exhibits a 3D structure that facilitates the growth of large single crystals along the optical axis direction. Measurements on these crystals indicate that Rb3Ba3Li2Al4B6O20F exhibits a moderate birefringence of 0.057 at 1064 nm enabling Type I phase-matching down to 243 nm. Theoretical calculations indicate the symmetry adapted mode displacement (SAMD) parameter scales with the second-harmonic generation intensity.

Noncentrosymmetric mixed-metal carbonate fluorides are promising materials for deep-ultraviolet (DUV) nonlinear optical (NLO) applications. We report on the synthesis, characterization, structure–property relationships, and electronic structure calculations on two new DUV NLO materials: KMgCO3F and Cs9Mg6(CO3)8F5. Both materials are noncentrosymmetric (NCS). KMgCO3F crystallizes in the achiral and nonpolar NCS space group P6̅2m, whereas Cs9Mg6(CO3)8F5 is found in the polar space group Pmn21. The compounds have three-dimensional structures built up from corner-shared magnesium oxyfluoride and magnesium oxide octahedra. KMgCO3F (Cs9Mg6(CO3)8F5) exhibits second-order harmonic generation (SHG) at both 1064 and 532 nm incident radiation with efficiencies of 120 (20) × α-SiO2 and 0.33 (0.10) × β-BaB2O4, respectively. In addition, short absorption edges of <200 and 208 nm for KMgCO3F and Cs9Mg6(CO3)8F5, respectively, are observed. We compute the electron localization function and density of states of these two compounds using first-principles density functional theory, and show that the different NLO responses arise from differences in the denticity and alignment of the anionic carbonate units. Finally, an examination of the known SHG active AMCO3F (A = alkali metal, M = alkaline earth metal, Zn, Cd, or Pb) materials indicates that, on average, smaller A cations and larger M cations result in increased SHG efficiencies.

A family of six nonlinear optical (NLO) materials, A3B3CD2O14 (A = Sr, Ba, or Pb; B = Mg or Zn; C = Te or W; and D = P or V), has been synthesized and characterized. In addition to the synthesis and crystal structures, comprehensive characterization of these compounds includes second harmonic generation (SHG) measurements, theoretical calculations, infrared and diffuse reflectance spectroscopies, and thermogravimetric measurements. We find that all of the reported materials are SHG-active at 1064 nm, with responses ranging from 2.8 to 13.5 × KDP, and exhibit absorption edges in the mid- to deep-ultraviolet regime. By systematically replacing the A, B, C, and D cations, we are able to tune these properties and investigate the role of different NLO-active structural units in producing the SHG responses. Specifically, our electronic structure calculations reveal that the presence of Pb2+ on the A-site and Te6+ on the C-site is critical for generating a large SHG response. The synthesis and structure–property relationships described in this family of materials will enable the design and discovery of new NLO materials.

Correction for ‘Design of noncentrosymmetric perovskites from centric and acentric basic building units’ by Joshua Young et al., J. Mater. Chem. C, 2016, 4, 4016–4027.

We present a detailed crystal-chemistry approach to lift inversion symmetry in inorganic crystals. By considering the ordering of centric and acentric basic building units in one and two dimensional systems, we review the structural “features” necessary to break spatial parity. We then extend this model to three dimensional materials, focusing on the family of ABX3 perovskites with corner-connected metal-anion BX6 octahedra and A-site cations filling the interstices (forming AX12 cuboctahedra). Although this extended BX6 network can tilt and rotate in space, these complex distortions preserve inversion symmetry; however, previous work has shown that certain rotational patterns of the octahedral units in combination with A-site cation ordering are able to lift inversion symmetry in perovskites. Herein, we extend this framework by comprehensively determining and describing the combinations of A- and B-site cation ordering schemes and BX6 octahedral rotation patterns that will produce noncentrosymmetric crystal structures independent of the chemical makeup. Although no combinations of simple B-site ordering lifts inversion, we find that a wide variety of polar, chiral, and second harmonic active structures can be realized with A-site and mixed A- and B-site cation ordering. We then show that the ability of such combinations to lift inversion symmetry depends on whether a given rotation pattern of the octahedral units distorts the A-site environment into centric or acentric polyhedra, as well as whether the cation ordering scheme aligns them in the proper orientation. Finally, we discuss the chemical factors stabilizing the various tilt patterns and ordering schemes, such as the tolerance factor and global instability index. The guidelines described here offer new insights into this vast family of materials, and detail a useful way to think about the design of noncentrosymmetric materials from basic building units.

Phase transitions in ABX3 perovskites are often accompanied by rigid rotations of the corner-connected BX6 octahedral network. Although the mechanisms for the preferred rotation patterns of perovskite oxides are fairly well recognized, the same cannot be said of halide variants (i.e., X = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (a0a0c+ in Glazer notation). To discern the chemical factors stabilizing this unique phase, we investigated a series of 12 perovskite bromides and iodides using density functional theory calculations and compared them with similar oxides. We find that in-phase tilting provides a better arrangement of the larger bromide and iodide anions, which minimizes the electrostatic interactions, improves the bond valence of the A-site cations, and enhances the covalency between the A-site metal and Br– or I– ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizing these factors.

Pb(II) has long been associated with lone pair activity and is often substituted in alkali earth metal borates to create new nonlinear optical (NLO) materials with enhanced second harmonic generation (SHG) capabilities. However, large enhancement in isomorphic Pb-free analogues is rare. Here we report a new NLO material Pb2Ba3(BO3)3Cl with a phase-matching SHG response approximately 3.2× that of KDP and 6× higher than its isomorphic compound Ba5(BO3)3Cl. We show that the enhanced SHG response originates from a unique edge-sharing connection between lead–oxygen polyhedra and boron–oxygen groups, making the dielectric susceptibility more easily affected by the external electric field of an incident photon. This understanding provides a route to identify systems that would benefit from SHG-active cation substitution in isomorphic structures that exhibit weak or null SHG responses.

A new deep-ultraviolet nonlinear optical material, RbMgCO3F, has been synthesized and characterized. The achiral nonpolar acentric material is second harmonic generation (SHG) active at both 1064 and 532 nm, with efficiencies of 160 × α-SiO2 and 0.6 × β-BaB2O4, respectively, and exhibits a short UV cutoff, below 190 nm. RbMgCO3F possesses a three-dimensional structure of corner-shared Mg(CO3)2F2 polyhedra. Unlike other acentric carbonate fluorides, in this example, the inclusion of Mg2+ creates pentagonal channels where the Rb+ resides. Our electronic structure calculations reveal that the denticity of the carbonate linkage, monodentate or bidendate, to the divalent cation is a useful parameter for tuning the transparency window and achieving the sizable SHG response.

A new ultraviolet nonlinear optical (NLO) material, Pb3Mg3TeP2O14 (PMTP), has been synthesized and characterized. The chiral material exhibits a large second harmonic generation (SHG) response of 13.5 × KDP (600 × α-SiO2), and the shortest absorption edge (250 nm) of reported materials with a strong SHG response (>10 × KDP). PMTP has a three-dimensional crystal structure of corner-shared MgO4, PO4, and TeO6 polyhedra, which form a [TeMg3P2O14]∞ framework. Electronic structure calculations revealed that the stereoactive lone pair on the Pb2+ cation is critical to producing the substantial NLO response and that the NLO activity is further enhanced by the presence of triply bidentate Te6+ cations found in Te–O–O–Pb rings.

Recent experimental and theoretical work has shown that the double perovskite NaLaMnWO6 exhibits antiferromagnetic ordering owing to the Mn d states, and computational studies further predict it to exhibit a spontaneous electric polarization due to an improper mechanism for ferroelectricity [King et al., Phys. Rev. B: Condens. Matter, 2009, 79, 224428; Fukushima et al., Phys. Chem. Chem. Phys., 2011, 13, 12186], which make it a candidate multiferroic material. Using first-principles density functional calculations, we investigate nine isostructural and isovalent AA′MnWO6 double perovskites (A = Na, K, and Rb; A′ = La, Nd, and Y) with the aim of articulating crystal-chemistry guidelines describing how to enhance the magnitude of the electric polarization through chemical substitution of the A-site while retaining long-range magnetic order. We find that the electric polarization can be enhanced by up to 150% in compounds which maximize the difference in the ionic size of the A and A′ cations. By examining the tolerance factors, bond valences, and structural distortions (described by symmetry-adapted modes) of the nine compounds, we identify the atomic scale features that are strongly correlated with the ionic and electronic contributions to the electric polarization. We also find that each compound exhibits a purely electronic remnant polarization, even in the absence of a displacive polar mode. The analysis and design strategies presented here can be further extended to additional members of this family (B = Fe, Co, etc.), and the improper ferroelectric nature of the mechanism allows for the decoupling of magnetic and ferroelectric properties and the targeted design of novel multiferroics.

We use a symmetry-based structural analysis combined with an electronic descriptor for bond covalency to explain the origin of the second-order nonlinear optical response (second harmonic generation, SHG) in noncentrosymmetric nonpolar ATeMoO6 compounds (where A = Mg, Zn, or Cd). We show that the SHG response has a complex dependence on the asymmetric geometry of the AO6 and AO4 functional units and the orbital character at the valence band edge, which we are able to distinguish using an A–O bond covalency descriptor. The degree of covalency between the divalent A-site cation and the oxygen ligands dominates over the geometric contributions to the SHG arising from the acentric polyhedra, and this can be understood from considerations of the local static charge density distribution. The use of a local dipole model for the polyhedral moieties (AO4/AO6, MoO4, and TeO4) can account for a nonzero SHG response, even though the materials exhibit nonpolar structures; however, it is insufficient to explain the change in the magnitude of the SHG response upon A-cation substitution. The atomic scale and electronic structure understanding of the macroscopic SHG behavior is then used to identify hypothetical HgTeMoO6 as a candidate telluromolybdate with an enhanced nonlinear optical response.

We present a detailed crystal-chemistry approach to lift inversion symmetry in inorganic crystals. By considering the ordering of centric and acentric basic building units in one and two dimensional systems, we review the structural “features” necessary to break spatial parity. We then extend this model to three dimensional materials, focusing on the family of ABX3 perovskites with corner-connected metal-anion BX6 octahedra and A-site cations filling the interstices (forming AX12 cuboctahedra). Although this extended BX6 network can tilt and rotate in space, these complex distortions preserve inversion symmetry; however, previous work has shown that certain rotational patterns of the octahedral units in combination with A-site cation ordering are able to lift inversion symmetry in perovskites. Herein, we extend this framework by comprehensively determining and describing the combinations of A- and B-site cation ordering schemes and BX6 octahedral rotation patterns that will produce noncentrosymmetric crystal structures independent of the chemical makeup. Although no combinations of simple B-site ordering lifts inversion, we find that a wide variety of polar, chiral, and second harmonic active structures can be realized with A-site and mixed A- and B-site cation ordering. We then show that the ability of such combinations to lift inversion symmetry depends on whether a given rotation pattern of the octahedral units distorts the A-site environment into centric or acentric polyhedra, as well as whether the cation ordering scheme aligns them in the proper orientation. Finally, we discuss the chemical factors stabilizing the various tilt patterns and ordering schemes, such as the tolerance factor and global instability index. The guidelines described here offer new insights into this vast family of materials, and detail a useful way to think about the design of noncentrosymmetric materials from basic building units.

Recent experimental and theoretical work has shown that the double perovskite NaLaMnWO6 exhibits antiferromagnetic ordering owing to the Mn d states, and computational studies further predict it to exhibit a spontaneous electric polarization due to an improper mechanism for ferroelectricity [King et al., Phys. Rev. B: Condens. Matter, 2009, 79, 224428; Fukushima et al., Phys. Chem. Chem. Phys., 2011, 13, 12186], which make it a candidate multiferroic material. Using first-principles density functional calculations, we investigate nine isostructural and isovalent AA′MnWO6 double perovskites (A = Na, K, and Rb; A′ = La, Nd, and Y) with the aim of articulating crystal-chemistry guidelines describing how to enhance the magnitude of the electric polarization through chemical substitution of the A-site while retaining long-range magnetic order. We find that the electric polarization can be enhanced by up to 150% in compounds which maximize the difference in the ionic size of the A and A′ cations. By examining the tolerance factors, bond valences, and structural distortions (described by symmetry-adapted modes) of the nine compounds, we identify the atomic scale features that are strongly correlated with the ionic and electronic contributions to the electric polarization. We also find that each compound exhibits a purely electronic remnant polarization, even in the absence of a displacive polar mode. The analysis and design strategies presented here can be further extended to additional members of this family (B = Fe, Co, etc.), and the improper ferroelectric nature of the mechanism allows for the decoupling of magnetic and ferroelectric properties and the targeted design of novel multiferroics.

Correction for ‘Design of noncentrosymmetric perovskites from centric and acentric basic building units’ by Joshua Young et al., J. Mater. Chem. C, 2016, 4, 4016–4027.