CONICET | Buscador de Institutos y Recursos Humanos

Conferencia invitada.  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 not reduced by temperature effects as in other techniques. In the case of metallic systems, the temperature dependence of the EFG is relatively well understood in the framework 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 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 cannot be explained from simple considerations. In order to explain the thermal behaviour of the EFG tensor, the authors of this experiment attributed this dependence to the non-isotropic thermal expansion of the rutile lattice. But they found no means to make a quantitative connection between the two effects [1]. The semiconductors that crystallize in the cubic structure of the mineral bixbyite Mn2O3 have been subject of systematic studies with 111Inà111Cd tracers. In these compounds, the presence of dynamic hyperfine interactions, originated in the electron-capture decay after-effects, and a strong positive linear temperature dependence of the EFG appear selectively, depending on the fact that if the host cations present closed electronic shells (such as Sc, Y, and In) or if it has incomplete electronic shells (as is the case of the 4f-orbitals of the rare-earths). Lutetium is the only rare-earth that presents a closed-shell electronic structure in its 3+ oxidation state, converting this oxide into an interesting laboratory to check the models proposed to explain the phenomena mentioned above. The anomalous experimentally observed step-like EFG temperature dependence at 111Cd sites in Lu2O3 was explained in the framework of a two-state model that considers an extremely fast fluctuation between two static EFG configurations, which enabled the experimental determination of an acceptor energy level introduced by the Cd impurity in the band-gap of the semiconductor and the estimation of the oxygen vacancy density in the sample [2]. In this talk we show how a 0K ab initio calculation can explain the temperature dependence of the EFG at Cd impurities in TiO2 and Lu2O3. Calculations were performed with the Full-Potential Linearized-Augmented Plane Waves (FP-LAPW) method. In the first example (Cd-doped TiO2), some years ago we have shown that at 300 K 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 depend on the temperature considered and are at the origin of the strong temperature dependence of the EFG tensor. In the case of Cd-doped Lu2O3, the temperature dependence of the EFG can be understand from the ionization of an impurity single acceptor level introduced in the band-gap of the semiconductor by the Cd impurity [4]. Both examples show the capability of ab initio calculations to complement hyperfine experiments also at varying temperatures. [1] J. C. Adams and G. L. Catchen, Phys. Rev. B 50 (1994) 1264. [2] LA.Errico, M. Rentería, A.G.Bibiloni, and F.G.Requejo, Hyperfine Interact.120-21, 457 (1999). [3] L. A. Errico, G. Fabricius, M. Rentería, P. de la Presa, and M. Forker, Phys. Rev. Lett. 89 (2002) 55503. [4] L.A. Errico, M. Rentería, A. G. Bibiloni, and G. N. Darriba, Physica Stat. Solidi C 2, 3576 (2005).