Ternary arsenides a(2)zn(5)as(4) (a = k, rb): zintl phases built from stellae quadrangulae

Stoichiometric reaction of the elements at high temperature yields the ternary arsenides K(2)Zn(5)As(4) (650 °C) and Rb(2)Zn(5)As(4) (600 °C). They adopt a new structure type (Pearson symbol oC44, space group Cmcm, Z = 4; a = 11.5758(5) ?, b = 7.0476(3) ?, c = 11.6352(5) ? for K(2)Zn(5)As(4); a = 11.6649(5) ?, b = 7.0953(3) ?, c = 11.7585(5) ? for Rb(2)Zn(5)As(4)) with a complex three-dimensional framework of linked ZnAs(4) 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 ZnAs(4) tetrahedra capping an empty central tetrahedron. These compounds are Zintl phases; band structure calculations on K(2)Zn(5)As(4) and Rb(2)Zn(5)As(4) indicate semiconducting behavior with a direct band gap of 0.4 eV.