Can a 0K calculation explain the temperature dependence of the Electric Field Gradient at Cd impurities in rutile TiO2

L. A. ERRICO

Bonn, Alemania

Conferencia; XIII International Conference on Hyperfine Interactions & XVII International Symposium on Nuclear Quadrupole Interaction; 2004

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The electronic charge density r(r) in a solid and its temperature dependence can be studied by measuring the electric-field gradient tensor (EFG), which is very sensitive to small changes in r(r). In this sense, the Perturbed-Angular Correlation technique (PAC) is specially suited to study the temperature dependence of the EFG because its sensitivity is independent of temperature. In the case of metallic systems, the temperature dependence of the EFG is relatively well understood in the frame of a model that takes into account the influence of lattice vibrations. On the other hand, in the case of semiconducting and insulating systems, several behaviours have been observed, which have been explained (with more or less success) in terms of host properties or processes induced by the presence of the hyperfine probes (generally impurities in the systems under study). In the particular case of the wide band-gap semiconductor TiO2 (rutile structure), the strong dependence of the EFG tensor at Cd impurities with temperature [1] cannot be explained from simple considerations. In order to explain the thermal behaviour of the EFG tensor, the authors of that article  attributed this dependence to the nonisotropic thermal expansion of the rutile lattice. But they found no means to make a quantitative connection between the two effects [1]. In this communication we show how a 0K ab initio calculation can explain the temperature dependence of the EFG at Cd impurities in TiO2. Calculations were performed with the Full-Potential Linearized-Augmented Plane Waves (FP-LAPW) method using temperature-dependent lattice parameters. This method allows us to treat the electronic structure and the processes induced by the impurity in the host-lattice (atomic relaxations, impurity levels) in a fully self-consistent way, without the use of external parameters and arbitrary suppositions. Recently, we have shown that, at room temperature, the Cd impurities introduce strong lattice distortions in the TiO2 lattice [3]. Now, for a given temperature, we calculate (using the thermal expansion coefficients) the corresponding lattice parameters. These lattice parameters are used in the FP-LAPW calculations. For each set of lattice parameters (that correspond to a given temperature), we obtain the new structural distortions introduced by the Cd impurities and the resulting EFG tensor. We found that the structural distortions depends on the temperature considered and are at the origin of the strong temperature dependence of the EFG tensor.      This work was partially supported by CONICET, Fundación Antorchas, and ANPCyT (PICT98 03-03727), Argentina, and TWAS, Italy. L. A. E. is fellow of CONICET.      [1] J. C. Adams and G. L. Catchen, Phys. Rev. B 50 (1994) 1264. [2] P. Blaha, K. Schwarz, and J. Luitz, Wien97 (K. Schwarz, T.U. Wien, Austria, 1999), ISBN 3- 9501031-0-4. [3] L. A. Errico, G. Fabricius, M. Rentería, P. de la Presa, and M. Forker, Phys. Rev. Lett. 89 (2002) 55503.