The Fluorite-Like Phase Nd₅Mo₃O_16±δ in the MoO₃–Nd₂O₃ System: Synthesis, Crystal Structure, and Conducting Properties

This paper describes a study of the system MoO₃–Nd₂O₃ using a combination of X-ray powder diffraction (XRD), neutron powder diffraction (NPD), thermogravimetric analysis (TGA), and ac impedance spectroscopy (IS). A phase-pure material is observed at a composition of 45.5 mol % Nd₂O₃, which corresponds to an ideal stoichiometry of Nd₅Mo₃O_16.5. XRD and NPD show that the crystal structure is a superstructure of the fluorite arrangement, with long-range ordering of the two cation species leading to a doubled unit cell parameter. The sample is found to be significantly oxygen deficient, i.e. Nd₅Mo₃O_15.63(4), when it is prepared by a solid-state reaction at 1473 K in air. TGA measurements indicate that the sample loses only minimal mass on heating to 1273 K in O₂. IS studies of the mean conductivity under different atmospheres show that the sample is a mixed conductor between ambient temperature and 873 K, with a dominant electronic component at higher temperatures, as demonstrated by measurements under inert atmosphere. NPD measurements indicate that the anion vacancies are preferentially located on the O2 sites, while studies of the temperature dependence performed under an O₂ atmosphere to 1273 K show significantly anisotropic thermal parameters of the anions. Together with analysis of the total neutron scattering data, this supports a model of oxygen ions hopping between O2 positions, with a vacancy, rather than interstitial, mechanism for the anion diffusion

Sr₂GaScO₅, Sr₁₀Ga₆Sc₄O₂₅, and SrGa_0.75Sc_0.25O_2.5: a Play in the Octahedra to Tetrahedra Ratio in Oxygen-Deficient Perovskites

Three different perovskite-related phases were isolated in the SrGa_1–<i>x</i>Sc_<i>x</i>O_2.5 system: Sr₂GaScO₅, Sr₁₀Ga₆Sc₄O₂₅, and SrGa_0.75Sc_0.25O_2.5. Sr₂GaScO₅ (<i>x</i> = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) <i>Icmm</i>, <i>a</i> = 5.91048(5) Å, <i>b</i> = 15.1594(1) Å, and <i>c</i> = 5.70926(4) Å] with complete ordering of Sc³⁺ and Ga³⁺ over octahedral and tetrahedral positions, respectively. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ (<i>x</i> = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. <i>I</i>4₁/<i>a</i>, <i>a</i> = 17.517(1) Å, <i>c</i> = 32.830(3) Å]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr₁₀Ga₆Sc₄O₂₅ can be described as a stacking of eight perovskite layers along the <i>c</i> axis ...[−(Sc/Ga)­O_1.6–SrO_0.8–(Sc/Ga)­O_1.8–SrO_0.8−]₂.... Similar to Sr₂GaScO₅, this structure features a complete ordering of the Sc³⁺ and Ga³⁺ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr₁₀Ga₆Sc₄O₂₅ is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc³⁺ by Ga³⁺ leads to the formation of the cubic perovskite phase SrGa_0.75Sc_0.25O_2.5 (<i>x</i> = 0.25) with <i>a</i> = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa_0.75Sc_0.25O_2.5·<i>x</i>H₂O hydrate, which exhibits a proton conductivity of ∼2.0 × 10^–6 S/cm at 673 K