Studying the temperature dependence of the electric-field gradient at impurity sites in oxides using ab initio calculations. Two examples.

L.A.ERRICO (CONFERENCISTA INVITADO), M. RENTERÍA, G.N. DARRIBA, AND E.L. MUÑOZ

Århus

Workshop; International Workshop “Quantum Theory of Solids (QTS-5)”; 2009

University of Århus, Dinamarca

**Invited Plenary Talk**The electronic charge density rho(r) in a solid and its temperature dependence can be studied bymeasuring the electric-field gradient tensor (EFG), which is very sensitive to small changes in ρ(r). Inthis sense, the Perturbed-Angular Correlation technique (PAC) is specially suited to study thetemperature 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 simpleconsiderations. In order to explain the thermal behaviour of the EFG tensor, the authors of thisexperiment attributed this dependence to the non-isotropic thermal expansion of the rutile lattice. Butthey 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 havebeen subject of systematic studies with 111In
111Cd tracers. In these compounds, the presence ofdynamic hyperfine interactions, originated in the electron-capture decay after-effects, and a strongpositive linear temperature dependence of the EFG appear selectively, depending on the fact that if thehost cations present closed electronic shells (such as Sc, Y, and In) or if it has incomplete electronicshells (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 ofthe EFG at Cd impurities in TiO2 and Lu2O3. Calculations were performed with the Full-PotentialLinearized-Augmented Plane Waves (FP-LAPW) method.In the first example (Cd-doped TiO2), some years ago we have shown that at 300 K the Cdimpurities introduce strong lattice distortions in the TiO2 lattice [3]. Now, for a given temperature, wecalculate (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 fromthe ionization of an impurity single acceptor level introduced in the band-gap of the semiconductor bythe Cd impurity [4]. Both examples show the capability of ab initio calculations to complement hyperfine experiments also at varying temperatures.REFERENCIAS[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).