Preparation and Crystal Structures of Ternary Rare Earth Silver and Gold Arsenides LnAgAs2 and LnAuAs2 with Ln = La–Nd, Sm, Gd, and Tb

Abstract

The 14 arsenides LnAgAs2 and LnAuAs2 (Ln = La-Nd, Sm, Gd, Tb) were prepared by reaction of stoichiometric mixtures of the elemental components at high temperatures and characterized by Xray diffractometry. The silver compounds LaAgAs2 and CeAgAs2 and the gold compounds LnAuAs2 (Ln = Ce-Nd, Sm, Gd, Tb) crystallize with HfCuSi2 type structure (P4/nmm, Z = 2). Of these, the structures of CeAgAs2 (a = 408.5(1), c = 1048.2(1) pm, conventional residual R = 0.017 for 261 structure factors and 12 variable parameters) and CeAuAs2 (a = 411.4(1), c = 1015.3(2) pm, R = 0.030 for 428 F values) were refined from four-circle diffractometer data. The silver compounds LnAgAs2 (Ln = Pr, Nd, Sm, Gd, Tb) are isotypic with the antimonide SrZnSb2 (Pnma, Z = 4) as demonstrated by a single-crystal structure refinement of PrAgAs2 (a = 2107.3(4), b = 401.7(1), c = 407.8(1) pm, R = 0.042 for 746 F values and 26 variables). The gold compound LaAuAs2 (I4/mmm, Z = 4, a = 416.9(1), c = 2059.5(3) pm, R = 0.038 for 303 F values and 13 variables) was found to be isotypic with the bismuthide SrZnBi2, again by a refinement from single-crystal diffractometer data. In the structures of CeAgAs2, LaAuAs2, and CeAuAs2 large displacement parameters perpendicular to the four-fold axes were found for one of the two arsenic positions. These structures could also be refined with split positions for these arsenic atoms, which allow for considerable As-As bonding, resulting in a formal charge of −1 for these atoms. Chemical bonding in these compounds can thus be rationalized by a simple model corresponding to the formula Ln+3T+1As−1As−3 (T = Ag, Au), where the superscripts indicate oxidation numbers.