•Eu3+-doped La0.8Gd1.2Hf2O7 transparent ceramics were first reported.•The crystal structure, transmittance, and luminescence properties with different Eu3+ content were investigated.•The annealing process affected the transmittance as well as the luminescence behavior of the ceramics.Eu3+-doped Lanthanum gadolinium hafnates (La0.8Gd1.2Hf2O7) transparent ceramics with different Eu3+ concentration were fabricated by vacuum sintering. XRD results showed all the ceramics are cubic pyrochlore structure. The effects of annealing process on in-line transmittance and luminescence behavior of the Eu3+-doped La0.8Gd1.2Hf2O7 transparent ceramics were investigated. Before annealing, the in-line transmittance of the ceramics was low and the luminescence intensity was weak. As Eu3+ doping content increased, the transmittance as well as the luminescence intensity decreased. This was ascribed to oxygen vacancy and other defects in the ceramics resulted from the vacuum sintering. After annealing, the transmittance and luminescence intensity were raised, indicating the elimination of oxygen vacancy. Moreover, with the increase of Eu3+ doping content from 1 at% to 10 at%, the luminescence intensity increased without concentration quenching.

•A new series of La2-xGdxHf2O7 transparent ceramics were fabricated by vacuum sintering using combustion-synthesized powders.•All the ceramics are transparent and the in-line transmittance can reach to 76.1% at 800 nm when x=1.2.•The Gd content has effects on the crystal structure, in-line transmittance, microstructures and densities of the ceramics.•With high density (7.91~8.88 g/cm3) and effective atomic number, some of the La2-xGdxHf2O7 transparent ceramics are promising candidates for scintillator hosts.La2-xGdxHf2O7 (x=0–2.0) transparent ceramics were fabricated through vacuum sintering from nano-powders synthesized by a simple combustion method. The phase composition of the powders and final ceramics, the in-line transmittance, microstructures and density of the ceramics were investigated. With the increasing of Gd content, the ceramics maintained the cubic pyrochlore structure, and the lattice parameters decreased, whilst the densities increased linearly. All the ceramics were transparent. The highest in-line transmittance was 76.1% at 800 nm (x=1.2). With high density (7.91–8.88 g/cm3) and effective atomic number, some of the La2-xGdxHf2O7 (x=0–2.0) transparent ceramics are promising candidates for scintillator hosts.

•Combustion method and vacuum sintering were used to fabricate La2-xGdxZr2O7.•The lattice parameters decreased with the increase of Gd3+ concent(x).•The absorption edge of the transmittance curves shifted to UV region from 0.4-2.0.• All the ceramics have high n (2.08), making them candidate for optical lens.•With the increase of x, the effective atomic number and density increased.Transparent La2−xGdxZr2O7 (x = 0–2.0) ceramics were prepared via vacuum sintering from nanometric powders synthesized by a simple combustion method. The changes of phase composition, morphology and in-line transmittance of the resulting ceramics with Gd3+ content’s variation were investigated. With the increase of Gd3+ content, the samples keep the pyrochlore structure, but the X-ray diffraction peaks of the powders and ceramics shift to higher angle as the lattice parameters become smaller. All the ceramics are transparent with high in-line transmittance and high refractive index (2.08 @ 632.8 nm, x = 0.4–1.6). These results indicate that La2−xGdxZr2O7 ceramics might be used as optical lens. Moreover, with the increase of Gd3+ content, the effective atomic number and density of the ceramics increase, therefore making them promising host candidates for scintillators.

A series of transparent ceramics with the composition of La2−xLuxZr2O7 (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La0.8Lu1.2Zr2O7 ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.

Highlights•The up-conversion emission intensity of NaYF4:Yb3+, Er3+ after silica coating was significantly enhanced.•The up-conversion intensity ratio between red emission and green emission was decreased after silica coating.•The mechanism of up-conversion luminescence and emission intensity change was illuminated.NaYF4:Yb3+, Er3+ nanoparticles were synthesized through co-precipitation using EDTA as the chelator. The monodisperse core–shell structure NaYF4:Yb3+, Er3+@sSiO2 were synthesized by sol–gel method using TEOS as silica source. The core–shell–shell structure NaYF4:Yb3+, Er3+@sSiO2@mSiO2 were prepared by coating mesoporous silica shell onto NaYF4:Yb3+, Er3+@sSiO2 through hydrolysis of TEOS using CTAB as mesoporous template. Effects of CTAB concentration, TEOS concentration and different addition ways of CTAB were studied. The results show that the NaYF4:Yb3+, Er3+@sSiO2@mSiO2 nanoparticles with uniform amorphous silica and mesoporous silica shell were successfully synthesized. Up-conversion spectra of these nanoparticles were recorded with 980 nm laser excitation under room temperature. There were no changes of the emission centers of nanoparticles before or after silica coating, but the emission intensities of nanoparticles after silica coating were significantly enhanced. Furthermore, the relative intensity ratio between red emission and green emission was decreased after silica coating.

A simple combustion method was used to synthesize LaGdZr2O7 powder and LaGdZr2O7 transparent ceramic was prepared by vacuum sintering at 1850 °C for 6 h. The final transparent ceramic, with a density of 6.46 g/cm3, has an in-line transmittance of 70.7% at 1000 nm and a refractive index of 2.08 at 632.8 nm.

•Gd3+ and/or Mn2+ doped NaYF4 possessed cubic structure was synthesized by a solvothermal route with PEI as the surfactant.•The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation.•The luminescence intensity of Mn2+ around 561 nm regions in NaYF4: Gd3+ was increased with the increase of Gd3+ doped concentration.•The strong green emission can be observed with naked eyes.•The energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process between Gd3+ and Mn2+.NaYF4 co-doped with Gd3+ and Mn2+ nanocrystals were synthesized by a solvothermal route with PEI as the surfactant. X-ray diffraction analyses evidenced that the phases of synthesized NaYF4: Gd3+, Mn2+ nanocrystals were all pure cubic structure. The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation. The photoluminescence emission intensity of NaYF4: Gd3+, Mn2+ at 561 nm was increased with the increase of Gd3+ doping concentration. The energy transfer mechanisms between Gd3+ and Mn2+ were discussed. The results showed that the energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process. Furthermore, the Mn2+ ions doped NaYF4: Gd3+ nanocrystals with a strong green emission and proper paramagnetism originated from Gd3+ and Mn2+ may find potential applications in bio-probes and magnetic resonance imaging.

Y2Hf2O7 transparent ceramics were successfully fabricated via vacuum sintering at 1900 °C for 6–12 h by the solid-state reaction using the corresponding nitrate and oxide powders without additives. The fabricated transparent ceramics showed a well-defined microstructure and no pores or other defects were observed. The linear optical transmittance reached to 60% in the visible wavelength region, which is about 76% of the corresponding single crystal. The refractive index of the Y2Hf2O7 transparent ceramics was above 2.01 in the region between 400 nm and 800 nm.Highlights► Y2Hf2O7 powder was synthesized by the solid-state reaction method without additives. ► Y2Hf2O7 transparent ceramics were fabricated via vacuum sintering. ► The linear light transmittance reached 60% in the visible wavelength region. ► The refractive index transparent ceramics was above 2.01 from 400 nm to 800 nm.

•Gd3+ and/or Mn2+ doped NaYF4 possessed cubic structure was synthesized by a solvothermal route with PEI as the surfactant.•The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation.•The luminescence intensity of Mn2+ around 561 nm regions in NaYF4: Gd3+ was increased with the increase of Gd3+ doped concentration.•The strong green emission can be observed with naked eyes.•The energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process between Gd3+ and Mn2+.NaYF4 co-doped with Gd3+ and Mn2+ nanocrystals were synthesized by a solvothermal route with PEI as the surfactant. X-ray diffraction analyses evidenced that the phases of synthesized NaYF4: Gd3+, Mn2+ nanocrystals were all pure cubic structure. The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation. The photoluminescence emission intensity of NaYF4: Gd3+, Mn2+ at 561 nm was increased with the increase of Gd3+ doping concentration. The energy transfer mechanisms between Gd3+ and Mn2+ were discussed. The results showed that the energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process. Furthermore, the Mn2+ ions doped NaYF4: Gd3+ nanocrystals with a strong green emission and proper paramagnetism originated from Gd3+ and Mn2+ may find potential applications in bio-probes and magnetic resonance imaging.