High pressure Mössbauer study of 197Au in Cs2Au2Cl6, Cs2AgAuCl6, and AuI

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Abstract
Mössbauer spectra of 197Au in Cs2Au2Cl6 were studied at 1 bar (4.2 and 45 K) and at 68 kbar (2.1, 4.2, 25, 40, and 45 K). Further, 197Au spectra in Cs2AgAuCl6 were studied at 1 bar and 31 kbar (4.2 K) and in AuI at 1 bar and 45 kbar (4.2 K). The pressure‐induced changes in the isomer shifts and quadrupole splittings of Cs2Au2Cl6, Cs2AgAuCl6, and AuJ suggest a positive sign of the electric field gradient at the Au+ nuclei and a negative sign at the Au+3 nuclei, i.e., contrary to previous predictions. For Au+, 5d6s rather than 6s6p hybridization is proposed. For Au3+ the positive contribution of the 5d8 free ion configuration to the electric field gradient overrides the negative contribution from 5d6s6p2 hybrids. On this basis, the chemical bonds in the cesium gold halides are concluded to be primarily ionic. The Mössbauer spectra shown that the Au+ and Au3+ ions are clearly distinguishable up to 45 K in the high pressure (68 kbar) state of Cs2Au2Cl6. Thus, the apparently equivalent Au positions and averaging of the Au valene states observed at 52 kbar with x‐ray diffraction is a dynamical process with a relatively long relaxation time due to the corner sharing topology of coordination octahedra. The significant increase in the electrical conductivity with increasing pressure appears to occur via Cl− ions, and not by direct Au+–Au3+ hopping.