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.