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 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.

Intriguing magnetic and electrical transport behavior in novel CeTAs2 (T=Cu, Ag, Au) compounds

“…The patterns were indexed in the tetragonal HfCuSi 2 -type structure with the lattice parameters close to * Corresponding author. those reported in the literature [2,4,5]. However, in the case of CeAgAs 2 a few reflections showed some small splitting indicative of orthorhombic distortion, as already reported for this compound in Ref.…”

Quaternary rare-earth arsenides REAg1−Zn As2 (RE=La–Nd, Sm, Gd–Dy) with tetragonal SrZnBi2- and HfCuSi2-type structures

“…RECu 1+x As 2 [19], REAuAs 2 [15], and REZn 0.67 As 2 [18]. Interestingly, such a crossover does not take place in REAgAs 2 , which basically adopts only HfCuSi 2 -type structures, subject to symmetry-lowering distortions of the As square net [15,25].…”

“…Indeed, the most prevalent ternary rare-earth transition-metal pnictides are HfCuSi 2 -type phases REM 1-x Pn 2 , where the M site is generally defective if it is occupied by a d-block element other than a monovalent one (i.e., from the Cu triad) [9][10][11]. Among arsenides, the only known representatives thus far are REMAs 2 (M = Cu [12,13], Ag [14][15][16], Au [15]), which are stoichiometric, and REZn 1-x As 2 , which are substoichiometric (REZn 0.67 As 2 ) [17,18]. However, a small Cu excess can be accommodated in lightly stuffed HfCuSi 2 -type phases RECu 1.1 As 2 [19,20].…”

Role of magnetism in superconductivity ofBaFe2As2: Study of5dAu-doped crystals

“…: d 8 ) and expansion of the crystal structure due to its extended orbitals, which is noted by transition metal-arsenide bond lengths of ~ 2.40 Å in 122, 35 and ~ 2.74 Å in LaAuAs 2 , which has similar tetrahedral coordination around the transition metal. 36 In the periodic table, Au sits to the right of 5d Ir and Pt, and is just below Cu (3d) and Ag (4d). Although there are no studies on Ag-122 (presumably because it can be +1, may not form a coordination with As, and is too large to sit in interstitial sites), there was a doping study of Cu into 122.…”