APW+lo and TDPAC study of the electric field gradient at the cation sites of the (44Ti(EC)-->)44Sc-doped Sc2O3 semiconductor

D. RICHARD; E. L. MUÑOZ; T. BUTZ; L. A. ERRICO; M. RENTERÍA

CERN 2010, Ginebra, Suiza.

Conferencia; 3rd Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions; 2010

Comité Organizador de la reunión

We report on an ab initio study of the Electric-Field-Gradient tensor (EFG) at both nonequivalent Sc sites in the semiconductor Sc2O3. This semiconductor crystallizes at the bixbyite structure, presenting two six-fold coordinated cation sites called C and D. The first is highly asymmetric and the second presents axial symmetry.  The calculations were performed applying  the Full-Potential Augmented-Plane Waves plus local orbitals (FP-APW+lo) method, in the framework of the Density Functional Theory (DFT),  that allows us to treat the electronic structure and the atomic structural position refinements in a fully self-consistent way. Our results are compared with recent  dedicated experimental data determined by Time-Differential g-g Perturbed-Angular Correlation (TDPAC) spectroscopy, at the performant TDPAC spectrometer of the  IEP II at Leipzig , using the first excited I=1- state of the (44Ti(EC)à)44Sc isotope as radioactive tracer. It is clear from the comparison of the experimental electric-field gradients and our ab initio results that simple models such as the Point-Charge Model (PCM) can not describe even approximately the measured electric-field gradients at cation sites in pure scandium sesquioxide. In this simple situation, were the 44Sc probe atom is not an impurity in the material under study,  the tracer does not introduce structural distortions that are not usually taken into account in the point-charge model when an impurity is concerned and that does not introduce impurity levels  in the band gap of the semiconductor, which are usually critical in the origin of the  electric-field gradient. Nevertheless, in this simple case, the Point-Charge Model seems to fail.  This can be only due to a bad description of the electronic distribution around the probe atom, which is not taken into account with the Sternheimer antishielding factor that is proposed in the PCM to describe the polarization of the core electrons of the probe atom.