Ternary Arsenides A2Zn2As3 (A = Sr, Eu) and Their Stuffed Derivatives A2Ag2ZnAs3

The ternary arsenides A2Zn2As3 and the quaternary derivatives A2Ag2ZnAs3 (A = Sr, Eu) have been prepared by stoichiometric reaction of the elements at 800 °C. Compounds A2Zn2As3 crystallize with the monoclinic Ba2Cd2Sb3-type structure (Pearson symbol mC28, space group C2/m, Z = 4; a = 16.212(5) Å, b = 4.275(1) Å, c = 11.955(3) Å, β = 126.271(3)° for Sr2Zn2As3; a = 16.032(4) Å, b = 4.255(1) Å, c = 11.871(3) Å, β = 126.525(3)° for Eu2Zn2As3) in which CaAl2Si2-type fragments, built up of edge-sharing Zn-centered tetrahedra, are interconnected by homoatomic As–As bonds to form anionic slabs [Zn2As3]4– separated by A2+ cations. Compounds A2Ag2ZnAs3 crystallize with the monoclinic Yb2Zn3Ge3-type structure (Pearson symbol mC32, space group C2/m; a = 16.759(2) Å, b = 4.4689(5) Å, c = 12.202(1) Å, β = 127.058(1)° for Sr2Ag2ZnAs3; a = 16.427(1) Å, b = 4.4721(3) Å, c = 11.9613(7) Å, β = 126.205(1)° for Eu2Ag2ZnAs3), which can be regarded as a stuffed derivative of the Ba2Cd2Sb3-type structure with additional transition-metal atoms in tetrahedral coordination inserted to link the anionic slabs together. The Ag and Zn atoms undergo disorder but with preferential occupancy over four sites centered in either tetrahedral or trigonal planar geometry. The site distribution of these metal atoms depends on a complex interplay of size and electronic factors. All compounds are Zintl phases. Band structure calculations predict that Sr2Zn2As3 is a narrow band gap semiconductor and Sr2Ag2ZnAs3 is a semimetal. Electrical resistivity measurements revealed band gaps of 0.04 eV for Sr2Zn2As3 and 0.02 eV for Eu2Zn2As3, the latter undergoing an apparent metal-to-semiconductor transition at 25 K.