New quaternary arsenide oxides with square planar coordination of gold(i) – structure,¹⁹⁷Au Mössbauer spectroscopic, XANES and XPS characterization of Nd₁₀Au₃As₈O₁₀and 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 (197)Au 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 TN = 8.6 K.

Palladium pnictide oxides Nd10Pd3As8O10and Sm10Pd3As8O10– low temperature structural phase transition and physical properties

“…U eq is defined as one third of the trace of the orthogonalised U ij tensor These absolute values are remarkably higher than those we reported for the corresponding metallic gold compound Nd 10 Au 3 As 8 O 10 (r E 1350-2500 Â 10 À6 O cm (2.5-300 K)), which were measured with the same assembly. 19 Furthermore, an anomaly around 160 K is observed in the electrical resistivity of Nd 10 Pd 3 As 8 O 10 , which is in line with the structural phase transition described in the crystal chemistry part. To underline this step-like anomaly the derivative of the electrical resistivity is depicted in the inset of the corresponding figure.…”

“…19 Accordingly, the new structures consist of alternating polycationic [RE 10 O 10 ] 10+ layers with their well-known tetrahedral coordination and planar polyanionic [Pd 3 (As 2 ) 4 ] 10À layers (see Fig. 2), known from the corresponding iron arsenide compound Ca 10 (Fe 1Àx Pd x As) 10 Fig.…”

“…In comparison with the recently reported gold arsenide oxides RE 10 Au 3 As 8 O 10 , 19 syntheses of the appropriate palladium arsenide oxides comprises the problem of competing formation of PdAs 2 , 26 which prevents further reaction to the title compounds. However, syntheses succeeded by use of palladium sponge, obtained via dissolving pieces of palladium sheets (Agosi, 499.9%) in aqua regia, subsequent precipitation and dissolving of [Pd(NH 3 ) 4 ][PdCl 4 ] in ammonium hydroxide and final precipitation of [Pd(NH 3 ) 4 ]Cl 2 by use of hydrochloric acid.…”

“…The crystal chemical details of this structure type have been discussed extensively for both gold compounds 19 and will not be repeated herein. The present manuscript focuses mainly on the peculiarities of the superstructure formation for the palladium compounds.…”

“…15,16 Recent work on pnictide oxides with the coinage metals revealed interesting new structural motifs; (i) gold(I) in almost trigonal planar phosphorous coordination in RE 2 AuP 2 O 17,18 and (ii) gold(I) in square planar coordination in Nd 10 Au 3 As 8 O 10 and Sm 10 Au 3 As 8 O 10 . 19 The square planar layers present in the gold compounds are similar to the iron arsenide superconductors (CaFe 1Àx T x As) 10 T 4Ày As 8 (T c o 38 K, T = Pt, Pd, Ir). [20][21][22][23][24] In continuation of our systematic phase analytical work we have now obtained the first rare earth based palladium pnictide oxides Nd 10 Pd 3 As 8 O 10 and Sm 10 Pd 3 As 8 O 10 .…”