Crystal structure, electronic structure and physical properties of the new quaternary chalcogenides Tl2NdAg3Se4 and Tl2NdAg3Te4

Abstract

The chalcogenides Tl₂NdAg₃Q₄ (Q = Se, Te) were prepared by heating the elements in stoichiometric ratios under exclusion of air between 973 K and 1073 K. Both compounds adopt new structure types, with Tl₂NdAg₃Se₄ crystallizing in the space group Pnma with a = 15.9937(14) Å, b = 4.3420(4) Å, c = 14.3676(12) Å, V = 997.75(15) ų, Z = 4, while Tl₂NdAg_3−xTe₄ adopts a different structure with related motifs, crystallizing in the noncentrosymmetric space group Cmc2₁ with a = 4.5968(2) Å, b = 16.8620(6) Å, c = 44.7131(15) Å, V = 3465.8(2) ų, Z = 12, when x = 0.05. In both cases, linear chains of [AgQ₄] tetrahedra are interconnected via common corners and edges to build a three-dimensional network, which encompasses the Tl and Nd atoms in linear channels. The differences lie in their three-dimensional connection, resulting in a larger, noncentrosymmetric unit cell of the telluride. Electronic structure calculations indicate semiconducting properties, in accord with the transport property measurements in both cases. A special feature of both materials is the extraordinarily low thermal conductivity.