Abstract
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 P2 1/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 G3 + 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.
Original language | English |
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Pages (from-to) | 29270-29282 |
Number of pages | 13 |
Journal | Physical Chemistry Chemical Physics |
Volume | 26 |
Issue number | 47 |
DOIs | |
State | Published - Nov 25 2024 |
Funding
We acknowledge the support of the Australian Research Council for this work that was facilitated by access to Sydney Analytical, a core research facility at the University of Sydney. BGM thanks the Australian Institute for Nuclear Science and Engineering for a PGRA scholarship and the Australian Nuclear Science and Technology Organisation for a United Uranium Scholarship. MSM gratefully acknowledges the financial support from the Comunidad de Madrid, Spain, through an Atracci\u00F3n de Talento Investigador fellowship (2020-T2/IND-20581). A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory [beamtime on NOMAD (IPTS #30368) and POWGEN (IPTS #26502) greatly acknowledged]. At UNLV, this material is based upon work performed under the auspices of the Consortium on Nuclear Security Technologies (CONNECT) supported by the Department of Energy/National Nuclear Security Administration under Award Number(s) DE-NA0003948. Supratik Mukherjee acknowledges DRDO, India, for the financial support provided through ACRHEM (DRDO/18/1801/2016/01038: ACRHEMPHASE-III). Ganapathy Vaitheeswaran acknowledges the CMSD University of Hyderabad for providing the computational facilities, and expresses gratitude to the Institute of Eminence, University of Hyderabad (UOH-IOE-RC3-21-046), for their financial assistance. We acknowledge the support of the Australian Research Council for this work that was facilitated by access to Sydney Analytical, a core research facility at the University of Sydney. BGM thanks the Australian Institute for Nuclear Science and Engineering for a PGRA scholarship and the Australian Nuclear Science and Technology Organisation for a United Uranium Scholarship. MSM gratefully acknowledges the financial support from the Comunidad de Madrid, Spain, through an \u201CAtracci\u00F3n de Talento Investigador\u201D fellowship (2020-T2/IND-20581). A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory [beamtime on NOMAD (IPTS #30368) and POWGEN (IPTS #26502) greatly acknowledged]. At UNLV, this material is based upon work performed under the auspices of the Consortium on Nuclear Security Technologies (CONNECT) supported by the Department of Energy/National Nuclear Security Administration under Award Number(s) DE-NA0003948. Supratik Mukherjee acknowledges DRDO, India, for the financial support provided through ACRHEM (DRDO/18/1801/2016/01038: ACRHEM-PHASE-III). Ganapathy Vaitheeswaran acknowledges the CMSD University of Hyderabad for providing the computational facilities, and expresses gratitude to the Institute of Eminence, University of Hyderabad (UOH-IOE-RC3-21-046), for their financial assistance.