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

The 14 arsenides LnAgAs 2 and LnAuAs 2 (Ln = La-Nd, Sm, Gd, Tb) were prepared by reaction of stoichiometric mixtures of the elemental components at high temperatures and characterized by X-ray diffractometry. The silver compounds LaAgAs 2 and CeAgAs 2 and the gold compounds LnAuAs 2 (Ln = Ce-Nd, Sm, Gd, Tb) crystallize with HfCuSi 2 type structure (P4/nmm, Z = 2). Of these, the structures of CeAgAs 2 (a = 408.5(1), c = 1048.2(1) pm, conventional residual R = 0.017 for 261 structure factors and 12 variable parameters) and CeAuAs 2 (a = 411.4(1), c = 1015.3(2) pm, R = 0.030 for 428 F values) were refined from foul-circle diffractometer data. The silver compounds LnAgAs 2 (Ln = Pr. Nd, Sm, Gd, Tb) are isotypic with the antimonide SrZnSb 2 (Pnma, Z = 4) as demonstrated by a single-crystal structure refinement of PrAgAs 2 (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 LaAuAs 2 (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 SrZnBi 2 , again by a refinement from single-crystal diffractometer data. In the structures of CeAgAs 2 , LaAuAs 2 , and CeAuAs 2 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 + 3 T + 1 As - 1 As - 3 (T = Ag, Au), where the superscripts indicate oxidation numbers.