Phase Transition in Tl₂TeO₃: Influence and Origin of the Thallium Lone Pair Distortion

A new modification of thallium tellurite, β-Tl2TeO3, has been synthesized by methanolothermal reaction, and its phase transition has been studied by single-crystal X-ray diffraction. At a temperature of 440(10) °C an irreversible phase transition from a monoclinic structure (β-Tl2TeO3: P21/c (No. 14), Z = 4, a = 8.9752(18) Å, b = 4.8534(6) Å, c = 11.884(2) Å, β = 109.67(2)°, V = 487.47(15) Å3 at 25 °C) to an orthorhombic structure (α-Tl2TeO3: Pban (No. 50), Z = 8, a = 16.646(2) Å, b = 11.094(2) Å, c = 5.2417(8) Å, V = 968.0(3) Å3 at 25 °C) is observed. Both structures are characterized by ψ-tetrahedral TeO32- anions. In the orthorhombic structure ψ-trigonal bipyramidal [TlO4] units are found together with ψ-tetrahedral [TlO3] units whereas in the monoclinic structure the coordination polyhedron around Tl(I) can be best described as a ψ-square pyramide, ψ-[TlO4]. The electronic structure of Tl2TeO3 in both modifications has been studied to explain the influence of the lone pairs. It can be conclusively shown that the minimization of antibonding ns-metal/2p-oxygen interactions is the driving force for “lone pair” distortions which determines the structures of Tl2TeO3.