Ternary Arsenides A2Zn5As4 (A = K, Rb): Zintl Phases Built from Stellae Quadrangulae

Stoichiometric reaction of the elements at high temperature yields the ternary arsenides K2Zn5As4 (650 °C) and Rb2Zn5As4 (600 °C). They adopt a new structure type (Pearson symbol oC44, space group Cmcm, Z = 4; a = 11.5758(5) Å, b = 7.0476(3) Å, c = 11.6352(5) Å for K2Zn5As4; a = 11.6649(5) Å, b = 7.0953(3) Å, c = 11.7585(5) Å for Rb2Zn5As4) with a complex three-dimensional framework of linked ZnAs4 tetrahedra generating large channels that are occupied by the alkali-metal cations. An alternative and useful way of describing the structure is through the use of stellae quadrangulae each consisting of four ZnAs4 tetrahedra capping an empty central tetrahedron. These compounds are Zintl phases; band structure calculations on K2Zn5As4 and Rb2Zn5As4 indicate semiconducting behavior with a direct band gap of 0.4 eV.