Search

This work reports the results of EPR and NMR study of the Ce3+ incorporation in LaAlO3 single crystals grown by the micro-pulling-down method in the range of the Ce concentrations from x=0.1 at.% up to 100 at.%. From EPR measurements, Ce3+ g tensor parameters were obtained as function of Ce concentration. The g tensor has orthorhombic symmetry even in the trigonal phase (x < 10 at.%) suggesting that the incorporation of Ce at La site lowers lattice symmetry near this ion. The local properties of the La1-xCexAlO3 crystals were further studied by 27Al and 139La high-resolution NMR measurements. It was found that 139La chemical shift has the Fermi contact interaction origin. It linearly increases with Ce concentration up to 165 ppm at x = 0.5. Due to this strong Fermi contact interaction, separated peaks corresponding to different Ce-O-La spin transfer passways are resolved in the 139La NMR spectra. On the other hand, no Fermi contact interaction is visible in 27Al NMR spectra. However, these spectra contain satellite peak which intensity linearly increases with increase of Ce concentration leaving position of this peak unchanged. This was interpreted as manifestation of crystal structure modification in vicinity of Ce ions in agreement with EPR data. Thus, optical properties of Ce3+ in LaAlO3 will be namely determined by the local crystal structure near this ion.

Distribution of aluminum and gallium atoms over the tetrahedral and octahedral sites in the garnet structure was studied in the mixed Lu3Al5-xGaxO12 crystals using the 27Al and 71Ga MAS NMR together with the single crystal 71Ga NMR. The experimental study was accompanied by theoretical calculations based on the density functional theory in order to predict the tendency in substitutions of Al by Ga in the mixed garnets. Both experimental and theoretic results show a non-uniform distribution of Al and Ga over the tetrahedral and octahedral sites in the garnet structure, with strong preferences for Ga, having larger ionic radius than Al, to occupy the tetrahedral site with smaller volume in the garnet structure. The quadrupole coupling constants and chemical shift parameters for Al and Ga nuclei have been determined for all the studied compounds as well as electric field gradients at Al and Ga nuclei were calculated in the framework of the density functional theory.

Photoluminescence, thermally stimulated luminescence (TSL) and EPR characteristics of the Ce3+ doped single crystals of multicomponent Y1Lu2GaxAl5-xO12 and Lu3GaxAl5-xO12 garnets with different Ga contents (x = 0, 1, 2, 3, 4, 5) excited in the Ce3+ - related absorption bands are investigated in the 9 - 500 K temperature range. The distribution of Ga3+ and Al3+ ions in the crystal lattice is determined by the NMR method. The relative number of Ga3+ ions in the tetrahedral crystal lattice sites, the maxima positions of the TSL glow curve peaks and the corresponding trap depths are found to decrease linearly with the increasing Ga content. At the same time, the reduction of the activation energy Ea of the TSL glow curve peaks creation under irradiation in the 4f - 4d1 absorption band of Ce3+ is strongly nonlinear. To explain this effect, the suggestion is made that Ea is the energy distance between the excited 5d1 level of Ce3+ and a defect level located between the 5d1 level and the bottom of the conduction band and arising from the Ga3+ ion perturbed by the nearest neighboring Ce3+ ion. The electrons thermally released from the excited Ce3+ ions are suggested to be trapped at the perturbed Ga3+ ions resulting in the appearance of electron Ga2+ centers. In spite of the fact that the paramagnetic Ga2+ ions were not detected by EPR, the described above process was found for Fe3+ impurity ions, namely the electron transfer from the 5d1 excited levels of Ce3+ to Fe3+ was directly detected by EPR.

We report the observation of cluster (local) superconductivity in the magnetoelectric Pb(Fe1/2Sb1/2)O3 ceramics prepared at a hydrostatic pressure of 6 GPa and temperatures 1200-1800 K to stabilize the perovskite phase. The superconductivity is manifested by an abrupt drop of the magnetic susceptibility at the critical temperature TC 7 K. Both the magnitude of this drop and TC decrease with magnetic field increase. Similarly, the low-field paramagnetic absorption measured by EPR spectrometer drops significantly below TC as well. The observed effects and their critical magnetic field dependence are interpreted as manifestation of the superconductivity and Meissner effect in metallic Pb nanoclusters existing in the ceramics. Their volume fraction and average size were estimated as 0.1-0.2% and 140-150 nm, respectively. The superconductivity related effects disappear after oxidizing annealing of the ceramics., Comment: 9 pages, 5 figures