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

Abstract

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.

A novel approach for identifying and synthesizing highdielectric materials

“…This distortion is a response to the fact that the sodium ion is too small for a cubic MTeO 6 2− network. Unfortunately, the presence in each case of a minor (<3 wt%) unidentified impurity phase(s), whose peaks overlapped in some instances with the perovskite reflections, did not allow for accurate determination of the oxygen positions.…”

“…6 Shannon's polarizability table provides a method for identifying compositions that should have high dielectric constants. Because combinatorial synthesis is unlikely to be easily applicable to high-pressure experiments >1 GPa, theapproach we describe here requires some predictive modeling to increase the success rate.…”

“…6 A summary of these results is given in Table III and the observed, calculated and difference patterns from the Rietveld refinements using the data sets of the perovskite compounds are shown in Fig. Details of both the high pressure high temperature synthesis and diffraction analysis have been described elsewhere.…”

A-site cation ordering in AA′BB′O6 perovskites

“…The metastable form of BaLaMn 2 O 6 has already been mentioned. Another example is the partial ordering of Nd 3+ and Ag + , observed in NdAgTi 2 O 6 samples prepared under high-pressure conditions [22]. In 1984 Sekiya et al published the structure of NaLaMgWO 6 [24].…”

Structure and microwave dielectric properties of La(2−x)/3Nax(Mg1/2W1/2)O3

“…3 Park et al reported on the synthesis and structure of several new stoichiometric A-site systems. 4,5 Asite order-disorder reaction can play a critical role in affecting the properties of perovskites. Several studies have focused on the ionically conducting perovskites (RE 2/3−x Li 3x )TiO 3 (RE: rare earth).…”