Synthesis and structure of Sr14Zn1+xAs11 and Eu14Zn1+xAs11 (x ≤ 0.5). New members of the family of pnictides isotypic with Ca14 AlSb11, exhibiting a new type of structural disorder

Systematic investigation of the ternary systems A-Zn-As (A = Sr, Eu) led to the discovery of two new ternary phases, Sr14Zn1+xAs11 and Eu14Zn1+xAs11 (x <= 0.5). Both title compounds were structurally characterized by single-crystal X-ray diffraction. The basic structure of these new arsenides is akin to that of the numerous other pnictides adopting the tetragonal Ca14AlSb11 structure type (space group I4(1)/acd (No. 142); Z = 8). The main difference is the presence of an interstitial Zn atom, located in a nearly trigonal-planar coordination environment of three As atoms. Four such fragments share common edges with the regular ZnAs4 tetrahedron, to form an unusual new structural unit with the formula [Zn1+xSb8]. The small uptake of additional zinc is needed to augment the overall valence electron count, thereby driving the system to an electronically charge-balanced state. Upon partial substitution of the nominally divalent Sr with a trivalent La, the equivalent of electron doping, the structure rids itself of the interstitial zinc, as demonstrated on the example of Sr13.30(2)La0.70ZnAs11, which was also structurally characterized by single-crystal X-ray diffraction as part of this study. Electronic structure calculations for the idealized composition Sr(14)ZnAs(11 )confirm the speculated electron-deficient assignment, and suggest intrinsic semiconducting behavior for the one-electron richer Sr14Zn1+xAs11 (x= 0.5) and Sr13LaZnAs11, respectively.