New quaternary arsenide oxides with square planar coordination of gold(i) – structure, 197Au Mössbauer spectroscopic, XANES and XPS characterization of Nd10Au3As8O10 and Sm10Au3As8O10

Abstract

The quaternary gold(I) arsenide oxides Nd₁₀Au₃As₈O₁₀ and Sm₁₀Au₃As₈O₁₀ were synthesized in sealed quartz ampoules from the rare earth (RE) elements, their appropriate sesquioxides, arsenic, arsenic(III) oxide and finely dispersed gold at maximum annealing temperatures of 1223 K. Both structures were refined from X-ray single crystal diffractometer data at room temperature and at 90 K. Nd₁₀Au₃As₈O₁₀ and Sm₁₀Au₃As₈O₁₀ crystallize with a new structure type that derives from the BaAl₄ structure through distortions and formation of ordered vacancies. The structures consist of stacked polycationic [RE₁₀O₁₀]¹⁰⁺ layers with oxygen in tetrahedral rare earth coordination and polyanionic [Au(I)₃(As₂)₄]¹⁰⁻ layers with gold in square planar or rectangular planar coordination of four arsenic dumbbells (255 pm As1–As2). In contrast to the well known ionic rare earth oxide layers, the gold arsenide layers rather show covalent bonding and account for the metallic nature of these two new arsenide oxides. This is confirmed by electronic structure calculations and resistivity measurements. The oxidation state of gold was investigated by ¹⁹⁷Au Mössbauer, X-ray absorption near edge structure (XANES) and photoelectron (XPS) spectroscopy. Due to missing comparative gold arsenide compounds, the monovalent gold phosphide oxides RE₂AuP₂O were measured for comparison. The XANES measurements additionally comprise monovalent gold arsenides REAuAs₂. The XPS study contains BaAuAs as reference compound instead. Combination of all data clearly indicates Au(I), which was not observed in square planar coordination up to now. Temperature dependent magnetic susceptibility data show Curie–Weiss paramagnetism for Nd₁₀Au₃As₈O₁₀ and no magnetic ordering down to 2.5 K. Sm₁₀Au₃As₈O₁₀ shows the typical Van Vleck type paramagnetism for samarium compounds along with a transition to an antiferromagnetically ordered state at T_N = 8.6 K.