INSTITUTO DE FISICA LA PLATA
Unidad Ejecutora - UE
congresos y reuniones científicas
APW+lo and TDPAC study of the Electric-Field Gradient at the cation sites of the (44Ti(EC)->)44Sc-doped Sc2O3Semiconductor
D. RICHARD; E.L. MUÑOZ; T. BUTZ; L.A. ERRICO; M. RENTERÍA
Conferencia; 3rd Joint International Conference on Hyperfine Interactions & International Symposium on Nuclear Quadrupole Interaction; 2010
We report a combined theoretical and experimental study of the Electric-Field-Gradient tensor (EFG) at both inequivalent Sc sites in the semiconductor Sc2O3. This semiconductor crystallizes in the bixbyite structure with two six-fold coordinated inequivalent cation sites called C and D. The first is highly asymmetric whereas the second is axially symmetric. 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), which allows us to treat the electronic structure and the atomic structural position refinements in a fully self-consistent way. Our results are compared with experimental data determined by Time-Differential Perturbed-Angular Correlation (TDPAC) spectroscopy using the Leipzig 6-detector TDPAC spectrometer with LaBr3(Ce) scintillators and using the first excited I=1 state of the 44Ti(EC)->44Sc isotope as radioactive tracer. There is excellent agreement between the experimental EFGs and those predicted by ab initio calculations. It is clear from the comparison of the experimental EFGs and the Point-Charge Model (PCM) that the PCM can not describe even approximately the measured electric-field gradients at cation sites in pure scandium sesquioxide. In this simple situation, where the 44Sc probe atom is not an impurity in the material under study, the tracer does not introduce structural distortions that are usually not taken into account in the PCM when an impurity is concerned, and it that does not introduce impurity levels in the band gap of the semiconductor, which are usually critical for the origin of the electric-field gradient. Nevertheless, in this simple case, the PCM seems to fail. This can be only due to a poor 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. We also found an experimental site preference for the 44Ti impurity that can be understood by performing ab initio calculations for the Ti-doped impurity system using super-cells.