Electron-deficient copper pnictides: A2Mg3Cu9Pn7 (A = Sr, Eu; Pn = P, As) and Eu5Mg2.39Cu16.61As12

Abstract

A series of new magnesium copper pnictide intermetallics, A₂Mg₃Cu₉Pn₇ (A = Sr, Eu; Pn = P, As) and Eu₅Mg_2.39Cu_16.61As₁₂, were synthesized through high temperature flux reactions. By the single-crystal X-ray diffraction technique, their structures were accurately determined. Sr₂Mg₃Cu₉P₇, Eu₂Mg₃Cu₉P₇, Sr₂Mg₃Cu₉As₇ and Eu₂Mg₃Cu₉As₇ crystallize in the Zr₂Fe₁₂P₇ structure type (S.G. P, no. 174) with cell parameters: a = 9.7736(19)/9.7417(7)/10.0550(10) Å/10.0410(8) Å, c = 3.9280(16)/3.9008(5)/4.0544(9)/4.0364(6) Å, respectively, while Eu₅Mg_2.39Cu_16.61As₁₂ adopts the Zr₅Co₁₉P₁₂ structure type (S.G. P2m, no. 189) with cell parameters of a = 13.2521(6) Å and c = 4.0910(3) Å. These compounds can be viewed as an extensive branch of the ternary RE−M−Pn system (RE = rare-earth elements; M = transition metals; Pn = pnictogen) with the metal-to-nonmetal ratio being close or equal to 2 : 1. Interestingly, with the divalent alkaline-earth cations being introduced, these metal-rich phases exhibit electron deficiency according to the DFT calculations. Eu₂Mg₃Cu₉As₇ is metallic based on the resistivity measurement, consistent with theoretical prediction, and the magnetic susceptibility data of Eu₂Mg₃Cu₉As₇ confirmed the divalent oxidation state and weak ferromagnetic coupling for the europium cations.