Search

Single crystals of LiCaAlF6 undoped and Eu, Na co-doped were studied by electron paramagnetic resonance, radioluminescence and thermally stimulated luminescence techniques applied in a correlated manner. The undoped samples exposed to X-ray irradiation exhibited two hole-like charge trapping centers creation, the molecular ions of the form: ClF- and F2^- - F2^- dimer. Their trap depths and frequency factors were determined as follows: Et1=1.7 eV and Et2=1.1. eV for trap depths and f ~ 10^13 s-1 for frequency factor, respectively. It was found that the europium preferable charge state is 2+ in the LiCaAlF6:Eu,Na samples, however, some amount of the Eu3+ is also present. Moreover, there were two Eu2+ centers: the dominating Eu2+(Ca) and the low-content Eu2+(Li). The amount of the latter is easily governed by the sodium admixture while the former is insensitive to the Na co-doping. Eu and Na co-doping affected the defects distribution and incorporation in the LiCaAlF6 host.

The present work reports results of the electron paramagnetic resonance (EPR), optical absorption, radio- and photoluminescence (RL and PL) complex study of the Lu2Si2O7:Pr and Lu2Si2O7:Ce pyrosilicate crystals. In both crystals, the EPR spectra demonstrate the presence of characteristic signals originating from the Yb3+, Er3+, Nd3+, Dy3+, Gd3+ and V3+ ions existing in the material as uncontrolled impurities. The corresponding spectra (except for Gd3+) have been analysed in detail and g- and hyperfine tensors are determined for all these ions in the lutetium pyrosilicate for the first time. Optical absorption, RL and PL measurements in the Lu2Si2O7:Pr crystal demonstrated presence of only characteristic Pr3+ transitions. In addition, the Pr3+- Pr3+ energy transfer was observed and confirmed experimentally.

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.

The processes of hole localization in the Y3Al5O12 and Lu3Al5O12 single crystals were investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It was found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming O- hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both YAG and LuAG crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as Mg2+ and Al_{Y} or Al_{Lu} antisite ion that leads to increase of the thermal stability of the trapped hole to app. 150 K. TSL measurements show two composite glow peaks in the temperature range of 77 - 280 K, the temperature positions of which well correlate with the thermal stability of the O- centers. The hole thermal ionization energy was determined from a numerical fit of the TSL peaks within the model of second order kinetics. It is in the range of 0.25 - 0.26 eV for the O- STH center, and increases to 0.41 - 0.45 eV for O- center stabilized by the acceptor. Revealed O- centers can be attributed to O- small polarons formed mainly due to the hole stabilization by short-range interaction with the surrounding lattice.

We study deep trapping states in Ce3+-doped garnet crystals with the composition (Lu,Y,Gd)3(Ga,Al)5O12, recently shown as having remarkably high light yield. We use thermally stimulated luminescence (TSL) technique above room temperature and determine the composition Gd3Ga3Al2O12 as the host showing the lowest concentration of traps. This host consistently manifest very low afterglow comparable to that of the standard BGO crystal. We also perform TSL glow peak analysis based on the initial rise technique to evaluate trap depth and other characteristics associated with TSL peaks.

Luminescence characteristics of single crystals (SC), single crystalline films (SCF), powders and ceramics of YAlO_3 and YAlO_3:Ce have been studied at 4.2-300 K under photoexcitation in the 4-20 eV energy range and X-ray excitation. The origin and structure of defects responsible for various exciton-related emission and excitation bands have been identified. The ~5.6 eV emission of YAlO_3 SCF is ascribed to the self-trapped excitons. In YAlO_3 SC, the dominating 5.63 eV and 4.12 eV emissions are ascribed to the excitons localized at the isolated antisite defect Y^{3+}_Al and at the Y^{3+}_{Al} defect associated with the nearest-neighbouring oxygen vacancy, respectively. Thermally stimulated release of electrons, trapped at these defects, takes place at 200 K and 280 K, respectively. The formation energies of various Y^{3+}_{Al}-related defects are calculated. The presence of Y_{Al} antisite-related defects is confirmed by NMR measurements. The influence of various intrinsic and impurity defects on the luminescence characteristics of Ce^{3+} centers is clarified., Comment: 21 pages, 11 figures

Systems in which particles can self-assemble into mono- or bilayers can form variety of stable and metastable structures on a nanometer length scale. For this reason confinement has a particularly strong effect on such systems. We discuss in some detail effects of confinement on lamellar and cubic phases with double-diamond structure. Structural deformations in slit geometry are described for large and small unit cells of the structure (in units of the thickness of the monolayer) and for various strengths of interactions with the confining surfaces. We show how the structural changes of the confined fluid are reflected in the measurable solvation force between the confining walls., Comment: 38 pages, 15 figures (revtex)

The system of bose liquid + impurity is considered. The energy spectrum as well as effective mass of impurity is calculated. For the case of the $ ^3He$ atom in superfluid $^4He$ numerical calculations are performed., Comment: 10 pages, LaTeX