Toxic, radioactive, and disordered: a total scattering study of TlTcO₄

A detailed variable temperature neutron total scattering study of the potential nuclear waste matrix TlTcO₄ was conducted. The long-range average structure of TlTcO₄ undergoes an orthorhombic Pnma to tetragonal I4₁/amd phase transition below 600 K, consistent with previous synchrotron X-ray diffraction studies. However, several anomalies were observed in the Rietveld refinements to the neutron powder diffraction data, such as large atomic displacement parameters at low temperature and a shortening of the Tc-O bond distance upon heating. Modelling the short-range local structure of both the low- and high-temperature data required a lowering of symmetry to the monoclinic P2₁/c model due to the stereochemical activity of the Tl⁺ 6s² lone pairs. Density functional theory calculations also verified this model to have a lower ground state energy than the corresponding long-range average structure. It is concluded that at low temperatures, the Tl⁺ 6s² lone pairs are 'frozen' into the structure. Upon heating, the rigid TcO₄ tetrahedra begin to rotate, as governed by the Γ₃⁺ and M₄⁺ modes. However, there is a disconnect between the two length scales, with the 6s² lone pair electrons remaining stereochemically active on the local scale, as observed in the neutron pair distribution function fits. The orthorhombic Pnma to tetragonal I4₁/amd phase transition is seemingly the result of a change in the correlation length of the Tl⁺ 6s² lone pairs, leading to a larger unit cell volume due to their uncorrelated displacements.