Composition and structure of a new palladate Ca_4-1.5xAu_xPd₄O₈ (x to approximately 0.54) synthesized at high pressure

A new palladate was synthesized at 100 kbar. Electron microprobe (EM) measurement gave the average composition as: Ca3.20±0.15Au0.54±0.03Pd4.00±0.16O7.86±0.28, suggesting Pd and Au are di- and tri-valent, respectively, in the formula of Ca4-1.5xAuxPd4O8 (x to approximately 0.54). Rietveld refinement using powder X-ray diffraction data shows its tetragonal symmetry of space group P42/mmc (No. 131), a = 5.8484(2) angstroms, c = 5.5643(2) angstroms, V = 190.32(1) angstroms3, and Z = 1. This phase is closely related to the bronze, CaPd3O4, and the CaPt2O4 phase. Structurally, Pd(II) and Au(III) ions are coordinated with nearly square-planar oxygens, while Ca atoms are located in nearly cubic oxygen environments. Au and some Ca atoms partially occupy interstitial sites in the proto-CaPt2O4-type framework. This is a typical high-pressure effect which stabilizes the first alkaline-earth-gold-palladate oxide ever reported.