THE ELECTRIC-FIELD GRADIENT AT CD-111 IN VANADIUM-OXIDES

The electric field gradient (efg) of Cd-111 in polycrystalline V2O5 wa s studied using perturbed angular correlation (PAC) spectroscopy, with the In-111 activity ion-implanted at 400 keV. Between the individual steps of an isochronal annealing program, a distinct efg (nu(Q1) = 88. 1(3) MHz, eta1 = 0.62(2)) was recorded the contribution of which incre ased with annealing temperature up to 74% at 870 K. Corresponding X-ra y analysis of inactive V2O5 samples, which underwent the same annealin g treatment, proved that the sample always stayed as V2O5. Since V2O5 has only one equivalent cation site, it is concluded that this efg bel ongs to Cd-111 at this site. Oxidation of a vanadium foil at T = 675 a nd 800 K at p(O2) = 200 mbar also yielded this efg. From PAC measureme nts in VO2, two well-defined efg's were found above and below the meta l-semiconductor transition at 340 K, which are tentatively attributed to the monoclinic and the tetragonal phase.