Relaxation of electronic defects in pure and dopedLa2O3observed

Abstract

Slow recovery processes of the electronic environment following the electron-capture decay of 111 In can reduce the amplitude of the perturbed ␥␥-angular correlation of a nuclear decay. This effect was used to quantitatively extract recovery rates of an electrically stable environment at the probe ion 111 Cd in La 2 O 3 . The recovery rates depend on the availability of electrons at the probe site, which in turn is governed by the concentration of electron sources and the transport mechanisms. Both properties are experimentally analyzed by variations of the temperature and oxygen partial pressure and by doping with two ͑Ba, Mg͒ and four-valent ions ͑Ce, Zr͒. Tunneling processes between defect levels in the band gap are proposed to account for the temperature dependence of the recovery rates. Unexpectedly, an enhanced electron availability is observed at temperatures below 200 K. The electric field gradients of substitutional 111 Cd and those generated by intrinsic defects and dopants are analyzed. A comparison to the probe ion 111m Cd, not affected by electron capture, is presented.

Perturbed Angular Correlation Studies in SrTiO3 Single Crystals

“…In the present work, we extend our previous PAC experiments on 11 'in/ 11 'Cd probes in BaTi0 3 BaHf0 3 and SrHf0 3 perovskites [7,8] to SrTi0 3 single crystals, using implanted ''' In + ions. On the basis of the cubic structure of the compound, one expects a vanishing EFG on substitutional, defect-free cation sites.…”

“…Among the point defects most probably oxygen vacancies are the predominant defect type as proposed by Wenzel et al [ 13] in CoO and NiO. For 111 In in the binary oxides Cr0 2 , Ga 2 0 3 , Zr0 2 , Hf0 2 and La 2 0 3 , electronic defects have been made responsible for the appearance of additional EFG's [9,11,14,15]. Electronic defects always led to variations of the hyperfine interaction fractions/), which are reversible with temperature.…”

“…On the basis of the cubic structure of SrTi0 3 one would expect a vanishing EFG for 11 'in/ 111 Cd probe nuclei on unperturbed lattice sites. In all perovskite oxides studied so far [2,7,8], large fractions of such substitutional implantations have been identified.…”

“…The sample surface was oriented along the intersecting line between the centers of two adjacent detectors so that the 7-rays were detected at 45° to the surface normal. Data were taken in a high-vacuum chamber at 26 -300 K or in air or high vacuum between 300 K and 700 K. Further details of the experimental set-ups used and data analysis can be found in [8,10,11],…”

“…Investigations of the hyperfine interactions in ternary oxides using Perturbed Angular Correlation (PAC) spectroscopy with radioactive 11 'in or 18 'Hf nuclei have been carried out since several years [1 -8], Many of these investigations served to study the possible scaling of electric field gradients (EFG) with the cation-oxygen bond lengths [3,5], the structural, magnetic or electric phase transitions [4,6,7], and the nature of electronic or ionic defects and their influence on the electric conductivity [9 -11]. While in many compounds the probe atoms behave as ideal "observers" on substitutional cation sites, the "'in tracers were found to have a strong influence on the phase transitions in the case of the antiferromagnetic ordering in CuFe0 2 [4] and the spin-Peierls transition of CuGe0 3 [6], Previous PAC studies in a number of perovskite oxides revealed, besides substitutional probe sites, defect complexes which, however, are not understood up to now [2, 8 -11].…”