| Autor: |
Mullens BG; School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia. brendan.kennedy@sydney.edu.au., Marlton FP; Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia. frederick.marlton@uts.edu.au., Saura-Múzquiz M; Departamento de Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040, Madrid, Spain., Everett M; Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA., Li C; Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA., Manjon-Sanz AM; Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA., Tucker MG; Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA., Poineau F; Department of Chemistry, University of Nevada Las Vegas, Las Vegas, Nevada, 89154, USA., Louis-Jean J; Department of Chemistry, University of Nevada Las Vegas, Las Vegas, Nevada, 89154, USA., Mukherjee S; Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, 500046 Hyderabad, Telangana, India., Mondal S; Advanced Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, 500046 Hyderabad, Telangana, India., Vaitheeswaran G; School of Physics, University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, 500046 Hyderabad, Telangana, India. vaithee@uohyd.ac.in., Kennedy BJ; School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia. brendan.kennedy@sydney.edu.au. |
| Abstrakt: |
A detailed variable temperature neutron total scattering study of the potential nuclear waste matrix TlTcO 4 was conducted. The long-range average structure of TlTcO 4 undergoes an orthorhombic Pnma to tetragonal I 4 1 / 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 P 2 1 / c model due to the stereochemical activity of the Tl + 6s 2 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 + 6s 2 lone pairs are 'frozen' into the structure. Upon heating, the rigid TcO 4 tetrahedra begin to rotate, as governed by the Γ 3 + and M 4 + modes. However, there is a disconnect between the two length scales, with the 6s 2 lone pair electrons remaining stereochemically active on the local scale, as observed in the neutron pair distribution function fits. The orthorhombic Pnma to tetragonal I 4 1 / amd phase transition is seemingly the result of a change in the correlation length of the Tl + 6s 2 lone pairs, leading to a larger unit cell volume due to their uncorrelated displacements. |
