Exploring structural changes and distortions in Quaternary perovskites and defect pyrochlores using powder diffraction techniques.

摘要

Double perovskites (A₂MM′X₆) are an extensively studied structure type in solid state chemistry. Distortions from the ideal cubic symmetry are caused by electronic factors, M-cation displacement from the center of the MX₆ octahedra, and octahedral tilting. A major factor influencing the degree of tilting in a given compound is the nature of the cuboctahedral A-cation. Barium and calcium perovskites typically exhibit cubic and orthorhombic (or monoclinic) symmetry, respectively. These changes (or lack thereof) in symmetry are often easily seen using X-ray powder diffraction (XRPD). Yet, strontium compounds are prone to subtle tilting distortions that are not readily seen in XRPD, so many are mistakenly assigned to the incorrect space group. In this study, nine strontium perovskites with M ′5+ = Nb, Ta, Sb were examined using XRPD and neutron powder diffraction. The approach taken for determining possible space groups, the reliability of peak splitting seen in the XRPD data for determining space group symmetry, the extent of M-site cation ordering, and the degree of octahedral tilting seen in this family of compounds are discussed.; CaCu₃Ti₄O₁₂ has received significant attention recently due to its giant dielectric constant (∼1.0 × 104) that shows little temperature dependence between 100 and 600 K. In this work, evidence supporting the extrinsic internal boundary layer capacitance mechanism for the dielectric response is reported. CaCu₃ Ti₄O₁₂ and CdCu₃Ti₄O ₁₂ are analyzed by Rietveld analysis, and these results [along with ACu₃M₄O₁₂ compositions (A2+ = Ca, Cd; M4+ = Ru, Ge, Mn) and CaTiO₃] are compared to variable temperature stability and crystal structure calculations performed using the SPuDS structure prediction software.; The structural evolution of ANbWO₆ (A = NH₄ +, H+, Rb+, K+) defect pyrochlores have been studied as a function of pressure up to 7 GPa. In response to increasing hydrostatic pressure, NH₄NbWO₆ and RbNbWO ₆ initially contract, but then undergo a fairly abrupt increase in their unit cell volume above 3 GPa. Rietveld refinements reveal the expansion is driven by water insertion into the structural channels that interpenetrate the NbWO₆− framework, accompanied by displacement of the A-cations that triggers the pressure-induced expansion. Similar behavior is not observed for KNbWO₆·H₂O or HNbWO ₆·H₂O.