Predictive Modeling and High-Pressure-High-Temperature Synthesis of Perovskites Containing Monovalent Silver

The program POTATO, which was developed to model distorted perovskite structures, has been used to assess the stability of hypothetical compositions and guide the synthesis of new materials at high pressures and high temperatures. The initial result of this effort is the synthesis of two new perovskites, Ca₂NdAgTi₄O₁₂ and NdAgTi₂O₆, which were synthesized at 14-14.5 GPa and 1000 °C using the uniaxial split sphere anvil type press (USSA-2000). Both compounds are perovskites distorted from the ideal cubic structure by octahedral tilting distortions. Ca₂NdAgTi₄O₁₂ contains a random distribution of Ca²⁺/Nd³⁺/ Ag⁺ ions on the A-site, whereas NdAgTi₂O₆ has a partially ordered distribution of Nd³⁺/Ag⁺ ions. Ca₂NdAgTi₄O₁₂ adopts the GdFeO₃ structure (Glazer tilt system b^-a⁺b^-), space group Pnma, with a = 5.44883(4), b = 7.68915(6), and c = 5.42290(3) Å. NdAgTi₂O₆ belongs to the tetragonal space group P4/nbm, with a = 5.45337(3) and c = 7.72934(6) Å. This structure can be derived from the simple cubic perovskite structure by rotations of the octahedra about the c-axis (Glazer tilt system a⁰a⁰c^-) and partial ordering of Nd³⁺ and Ag⁺ ions into alternating layers, perpendicular to the c-axis.