Phase Relationships in the BaO–Ga₂O₃–Ta₂O₅ System and the Structure of Ba₆Ga₂₁TaO₄₀

Phase relationships in the BaO–Ga₂O₃–Ta₂O₅ ternary system at 1200 °C were determined. The A₆B₁₀O₃₀ tetragonal tungsten bronze (TTB) related solution in the BaO–Ta₂O₅ subsystem dissolved up to ∼11 mol % Ga₂O₃, forming a ternary trapezoid-shaped TTB-related solid solution region defined by the BaTa₂O₆, Ba_1.1Ta₅O_13.6, Ba_1.58Ga_0.92Ta_4.08O_13.16, and Ba₆GaTa₉O₃₀ compositions in the BaO–Ga₂O₃–Ta₂O₅ system. Two ternary phases Ba₆Ga₂₁TaO₄₀ and eight-layer twinned hexagonal perovskite solid solution Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄ were confirmed in the BaO–Ga₂O₃–Ta₂O₅ system. Ba₆Ga₂₁TaO₄₀ crystallized in a monoclinic cell of <i>a</i> = 15.9130(2) Å, <i>b</i> = 11.7309(1) Å, <i>c</i> = 5.13593(6) Å, β = 107.7893(9)°, and <i>Z</i> = 1 in space group <i>C</i>2/<i>m</i>. The structure of Ba₆Ga₂₁TaO₄₀ was solved by the charge flipping method, and it represents a three-dimensional (3D) mixed GaO₄ tetrahedral and GaO₆/TaO₆ octahedral framework, forming mixed 1D 5/6-fold tunnels that accommodate the Ba cations along the <i>c</i> axis. The electrical property of Ba₆Ga₂₁TaO₄₀ was characterized by using ac impedance spectroscopy

Phase Relationships in the BaO–Ga₂O₃–Ta₂O₅ System and the Structure of Ba₆Ga₂₁TaO₄₀

Phase relationships in the BaO–Ga₂O₃–Ta₂O₅ ternary system at 1200 °C were determined. The A₆B₁₀O₃₀ tetragonal tungsten bronze (TTB) related solution in the BaO–Ta₂O₅ subsystem dissolved up to ∼11 mol % Ga₂O₃, forming a ternary trapezoid-shaped TTB-related solid solution region defined by the BaTa₂O₆, Ba_1.1Ta₅O_13.6, Ba_1.58Ga_0.92Ta_4.08O_13.16, and Ba₆GaTa₉O₃₀ compositions in the BaO–Ga₂O₃–Ta₂O₅ system. Two ternary phases Ba₆Ga₂₁TaO₄₀ and eight-layer twinned hexagonal perovskite solid solution Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄ were confirmed in the BaO–Ga₂O₃–Ta₂O₅ system. Ba₆Ga₂₁TaO₄₀ crystallized in a monoclinic cell of <i>a</i> = 15.9130(2) Å, <i>b</i> = 11.7309(1) Å, <i>c</i> = 5.13593(6) Å, β = 107.7893(9)°, and <i>Z</i> = 1 in space group <i>C</i>2/<i>m</i>. The structure of Ba₆Ga₂₁TaO₄₀ was solved by the charge flipping method, and it represents a three-dimensional (3D) mixed GaO₄ tetrahedral and GaO₆/TaO₆ octahedral framework, forming mixed 1D 5/6-fold tunnels that accommodate the Ba cations along the <i>c</i> axis. The electrical property of Ba₆Ga₂₁TaO₄₀ was characterized by using ac impedance spectroscopy

Acceptor Doping and Oxygen Vacancy Migration in Layered Perovskite NdBaInO₄‑Based Mixed Conductors

The Ca²⁺ and Ba²⁺ solubility on Nd³⁺ sites in new layered perovskite NdBaInO₄ mixed oxide ionic and hole conductor and their effect on the oxide ion conductivity of NdBaInO₄ were investigated. Among the alkaline earth metal cations Ca²⁺, Sr²⁺, and Ba²⁺, Ca²⁺ was shown to be the optimum acceptor–dopant for Nd³⁺ in NdBaInO₄ showing the largest substitution for Nd³⁺ up to 20% and leading to oxide ion conductivities ∼3 × 10^–4–1.3 × 10^–3 s/cm within 600–800 °C on Nd_0.8Ca_0.2BaInO_3.9 composition, exceeding the most-conducting Nd_0.9Sr_0.1BaInO_3.95 in the Sr-doped NdBaInO₄. Energetics of defect formation and oxygen vacancy migration in NdBaInO₄ were computed through the atomistic static-lattice simulation. The solution energies of Ca²⁺/Sr²⁺/Ba²⁺ on the Nd³⁺ site in NdBaInO₄ for creating the oxygen vacancies confirm the predominance of Ca²⁺ on the substitution for Nd³⁺ and enhancement of the oxygen vacancy conductivity over the larger Sr²⁺ and Ba²⁺. The electronic defect formation energies indicate that the p-type conduction in a high partial oxygen pressure range of the NdBaInO₄-based materials is from the oxidation reaction forming the holes centered on O atoms. Both the static lattice and molecular dynamic simulations indicate two-dimensional oxygen vacancy migration within the perovskite slab boundaries for the acceptor-doped NdBaInO₄. Molecular dynamic simulations on the Ca-doped NdBaInO₄ specify two major vacancy migration events, respectively, via one intraslab path along the <i>b</i> axis and one interslab path along the <i>c</i> axis. These paths are composed by two terminal oxygen sites within the perovskite slab boundaries

La_1+<i>x</i>Ba_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>} Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites

Substitution of La³⁺ for Ba²⁺ in LaBaGa₃O₇ melilite yields a new interstitial-oxide-ion conducting La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} solid solution, which only extends up to <i>x</i> = 0.35, giving a maximum interstitial oxygen content allowed in La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} as about half of those allowed in La_1+<i>x</i>(Sr/Ca)_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>}. La_1.35Ba_0.65­Ga₃O_7.175 ceramic displays bulk conductivity ∼1.9 × 10^–3 S/cm at 600 °C, which is lower than those of La_1.35(Sr/Ca)_0.65­Ga₃O_7.175, showing the reduced mobility for the oxygen interstitials in La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} than in La_1+<i>x</i>(Sr/Ca)_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>}. Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in La_1.35Ba_0.65Ga₃O_7.175 are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the <i>c</i>-axis and stabilized via incorporating into the bonding environment of a three-linked GaO₄ among the five GaO₄ tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGa₃O₇ has the largest formation energy for oxygen interstitial defects among La_1+<i>x</i>M_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} (M = Ba, Sr, Ca), consistent with the large Ba²⁺ cations favoring interstitial oxygen defects in melilite less than the small cations Sr²⁺ and Ca²⁺. The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in La_1+<i>x</i>M_1–<i>x</i>Ga₃O_7+0.5x (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba²⁺ along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba²⁺-containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels

Structure Refinement and Two-Center Luminescence of Ca₃La₃(BO₃)₅:Ce³⁺ under VUV–UV Excitation

A series of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed using the powder X-ray diffraction (XRD) data, which shows occupation of Ce³⁺ on both Ca²⁺ and La³⁺ sites with a preferred location on the La³⁺ site over the Ca²⁺ site. The prepared samples contain minor second phase LaBO₃ with contents of ∼0.64–3.27 wt % from the Rietveld analysis. LaBO₃:1%Ce³⁺ was prepared as a single phase material and its excitation and emission bands were determined for identifying the influence of impurity LaBO₃:Ce³⁺ luminescence on the spectra of the Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples. The luminescence properties of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples under vacuum ultraviolet (VUV) and UV excitation were investigated, which exhibited two-center luminescence of Ce³⁺, assigned to the Ce(1)³⁺ center in the La³⁺ site and Ce(2)³⁺ center in the Ca²⁺ site, taking into account the spectroscopic properties and the Rietveld refinement results. The influences of the doping concentration and the excitation wavelength on the luminescence of Ce³⁺ in Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ are discussed together with the decay characteristics

Structure Refinement and Two-Center Luminescence of Ca₃La₃(BO₃)₅:Ce³⁺ under VUV–UV Excitation

A series of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed using the powder X-ray diffraction (XRD) data, which shows occupation of Ce³⁺ on both Ca²⁺ and La³⁺ sites with a preferred location on the La³⁺ site over the Ca²⁺ site. The prepared samples contain minor second phase LaBO₃ with contents of ∼0.64–3.27 wt % from the Rietveld analysis. LaBO₃:1%Ce³⁺ was prepared as a single phase material and its excitation and emission bands were determined for identifying the influence of impurity LaBO₃:Ce³⁺ luminescence on the spectra of the Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples. The luminescence properties of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples under vacuum ultraviolet (VUV) and UV excitation were investigated, which exhibited two-center luminescence of Ce³⁺, assigned to the Ce(1)³⁺ center in the La³⁺ site and Ce(2)³⁺ center in the Ca²⁺ site, taking into account the spectroscopic properties and the Rietveld refinement results. The influences of the doping concentration and the excitation wavelength on the luminescence of Ce³⁺ in Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ are discussed together with the decay characteristics

Thermodynamics and Kinetics Accounting for Antithermal Quenching of Luminescence in Sc₂(MoO₄)₃: Yb/Er: Perspective beyond Negative Thermal Expansion

Defects are common in inorganic materials and not static upon annealing of the heat effect. Antithermal quenching of luminescence in phosphors may be ascribed to the migration of defects and/or ions, which has not been well-studied. Herein, we investigate the antithermal quenching mechanism of upconversion luminescence in Sc2(MoO4)3: 9%Yb1%Er with negative thermal expansion via a fresh perspective on thermodynamics and kinetics, concerning the thermally activated movement of defects and/or ions. Our results reveal a second-order phase transition taking place at ∼573 K induced by oxide-ion migration. The resulting variation of the thermodynamics and kinetics of the host lattice owing to the thermally induced oxide-ion movement contributes to a more suppressed nonradiative decay rate. The dynamic defects no longer act as quenching centers with regard to the time scale during which they stay nearby the Yb3+/Er3+ site in our proposed model. This research opens an avenue for understanding the antithermal quenching mechanism of luminescence via thermodynamics and kinetics

Polymorphism and Oxide Ion Migration Pathways in Fluorite-Type Bismuth Vanadate, Bi₄₆V₈O₈₉

We report the synthesis, structural characterization, and ionic conductivity measurements for a new polymorph of bismuth vanadate Bi₄₆V₈O₈₉, and an <i>ab initio</i> molecular dynamics study of this oxide ion conductor. Structure determination was carried out using synchrotron powder X-ray and neutron diffraction data; it was found that β-Bi₄₆V₈O₈₉ crystallizes in space group <i>C</i>2/<i>m</i> and that the key differences between this and the previously reported α-form are the distribution of Bi and V cations and the arrangement of the VO₄ coordination polyhedra in structure. β-Bi₄₆V₈O₈₉ exhibits good oxide ion conductivity, with σ = 0.01–0.1 S/cm between 600 and 850 °C, which is about an order of magnitude higher than yttria stabilized zirconia. The <i>ab initio</i> molecular dynamics simulations suggest that the ion migration pathways include vacancy diffusion through the Bi–O sublattice, as well as the O^2– exchanges between the Bi–O and the V–O sublattices, facilitated by the variability of the vanadium coordination environment and the rotational freedom of the VO_<i>x</i> coordination polyhedra

Polymorphism and Oxide Ion Migration Pathways in Fluorite-Type Bismuth Vanadate, Bi₄₆V₈O₈₉

We report the synthesis, structural characterization, and ionic conductivity measurements for a new polymorph of bismuth vanadate Bi₄₆V₈O₈₉, and an <i>ab initio</i> molecular dynamics study of this oxide ion conductor. Structure determination was carried out using synchrotron powder X-ray and neutron diffraction data; it was found that β-Bi₄₆V₈O₈₉ crystallizes in space group <i>C</i>2/<i>m</i> and that the key differences between this and the previously reported α-form are the distribution of Bi and V cations and the arrangement of the VO₄ coordination polyhedra in structure. β-Bi₄₆V₈O₈₉ exhibits good oxide ion conductivity, with σ = 0.01–0.1 S/cm between 600 and 850 °C, which is about an order of magnitude higher than yttria stabilized zirconia. The <i>ab initio</i> molecular dynamics simulations suggest that the ion migration pathways include vacancy diffusion through the Bi–O sublattice, as well as the O^2– exchanges between the Bi–O and the V–O sublattices, facilitated by the variability of the vanadium coordination environment and the rotational freedom of the VO_<i>x</i> coordination polyhedra

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba_{0.5–<i>x</i>}TaO_3–<i>x</i>

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0.25–0.325) compositions are stabilized via the interpolation of Ba²⁺ cations and (TaO)³⁺ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the <i>c</i> axis. The slightly nonstoichiometric Ba_{0.5–<i>x</i>}­TaO_3–<i>x</i> (<i>x</i> = 0–0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< <i>x</i> < 0.25 nonstoichiometric range, resulting in two closely related (TaO)³⁺-containing and (TaO)³⁺-free TTB phases. The highly nonstoichiometric (TaO)³⁺-containing phase exhibits Ba²⁺ cationic migration. The incorporation of (TaO)³⁺ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)³⁺ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions