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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 Xray 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 four-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.

Preparation and Crystal Structures of Ternary Rare Earth Silver and Gold Arsenides LnAgAs 2 and LnAuAs 2 with Ln = La–Nd, Sm, Gd, and Tb