Electronic and structural properties of the α-Fe2O3:Ta semiconductor. Experimental EFG determination and ab initio calculations.

G. N. DARRIBA; E. L. MUÑOZ; P. D. EVERSHEIM; M. RENTERÍA

Ginebra

Conferencia; 3rd Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions. HFI/NQI 2010; 2010

ISOLDE/CERN

In this work we present results from Time-Differential Perturbed-Angular Correlations (PAC) experiments in alpha-Fe2O3 singlecrystals (in their corundum structure) implanted with 181Hf(181Ta) ions at the ion accelerator facility of the H-ISKP at the Bonn University. The magnitude, asymmetry and orientation of the EFG were determined measuring the spin-rotation curves as a function of the singlecrystal orientation (for three different configurations of the sample) with respect to the laboratory system. The PAC experiments were carried out at 973 K in order to have only the electric-quadrupole interaction in the spectra, since above the Neel temperature (TN=955 K) the system has a paramagnetic behaviour. The experimental results are compared with ab initio calculations performed in the framework of the Density Functional Theory (DFT) and with predictions of the Point Charge Model. The ab initio calculations were carried out with the FP-APW+lo method (embodied in the WIEN2K code) with an impurity dilution of 1:12 (with respect to the cations). This theoretical and experimental study enables to determine the structural relaxations introduced by the impurity in the host lattice, and the charge state of the impurity in the band gap of the system under study. The Ta impurity introduces a double donor level in α-Fe2O3, which seems to be partially ionized at 973 K, supported by the good agreement between the experimental EFG and the ab initio prediction for this charge state of the impurity. We discuss here the validity to compare the ab initio calculations carried out at 0 K (where the system has an antiferromagnetic behaviour) with the experimental results taken at 973 K (where the system has a paramagnetic behaviour).