Intrinsic luminescence of rare-earth oxyorthosilicates

The spectra and decay kinetics of luminescence and the excitation and reflection spectra of the luminescence of orthosilicates A ₂SiO₅ (A = Y, Lu, Sc, Gd), both nominally pure and doped with cerium, are measured using time-resolved VUV spectroscopy in the ranges of energies ?ν = 1.5–16 eV and temperatures T = 8–3000 K. The band structure of the crystals is calculated in the local electron density approximation (LDA). The origin of the intrinsic luminescence in the crystals studied is established, and the assumption regarding the existence of self-trapped excitons and their structure is made.     

Potential and limitations of rare-earth luminescence in inverse opals

Inverse opals exhibiting strong photonic effects can cooperate with luminescent materials imbedded in their voids. While this has been demonstrated for dye molecules in the past, inspection of the literature reveals only few experiments on the potential of rare earths in this context. The efforts described focus on the spectral and spatial modification of emissive properties, whereas we here pursue the study of corresponding absorption properties. However, arbitrary incorporation of the luminescent species disturbs the photonic effects, therefore, principal preparative efforts need to be conducted to preserve both, the photonic properties of the opaline hosts and the luminescence efficiency of the guests. In this paper, we describe issues regarding the morphology and efficiency of inverse opal-incorporated rare-earth fluorides and oxifluorides and present an interior coating method for efficiency improvement. In addition, we were in this context, able to demonstrate the feasibility of energy transfer from Tb³⁺ modified host walls to Eu³⁺ guest species.     

Understanding the influence of nanoenvironment on luminescence of rare-earth ions

This paper presents an overview of the recent results on upconversion and photoluminescence of rare-earth ions in nanoenvironments. The role of the rare-earth ion concentration, crystal size and crystal phase on the up and downconversion emission of rare-earth ions in oxide nanocrystals and their underlying mechanisms are discussed. It is also found that the luminescence lifetime of the excited state rare-earth ions is sensitive to the particle crystalline phase and size. The analysis suggests that the modifications of radiative and nonradiative relaxation mechanisms are due to local symmetry structure of the host lattice and crystal size respectively     

Morphology dependent luminescence properties of rare-earth doped lanthanum fluoride hierarchical microstructures

Here, we demonstrate an ionic liquid-assisted hydrothermal method for preparing Tb³⁺ and Eu³⁺ doped LaF₃ hierarchical microstructures and the morphology is modified by hydrothermal reaction time, temperature of heating and ionic liquid concentration. The mechanism related to morphology control is proposed and discussed. It is also found that PL intensity, decay time and quantum efficiency are sensitive to the morphology. The average decay times are 2.9 ms and 4.8 ms for Eu³⁺ doped LaF₃ microstructures prepared at 10 min and 3 h reaction time, respectively. The average decay time is increased from 4.8 ms to 5.8 ms after heating the sample at 500 °C. The quantum efficiency varies from 34% to 67% with changing morphology. Analysis suggests that morphology plays an important role on efficiency of rare-earth doped materials.     

Investigation of luminescence and photoelectrochemical properties of rare-earth metal-doped anode oxide films on aluminum

A study is made of the luminescence of rare-earth-metal (REM)-doped anode oxide films (AOF) grown on aluminu. It is shown that REMs introduced into an AOF in the course of its formation exert a considerable influence on the position of the maximum half-width and the intensity of the luminescence spectrum of the matrix, i.e., Al₂O₃. Photoelectrochemical properties of AOF (Eu) are investigated. A dependence of the AOF photopotential on the conditions of its formation is shown. Belarusian State University of Information Science and Radioelectronics, 6, P. Brovka St., Minsk, 220600, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 593–596, July–August, 1998.     

27Al and 23Na double-rotation NMR of sodalites.

The 27Al NMR spectra of calcium tungstate aluminate sodalite (CAW), Ca8[Al12O24](WO4)2, and the 23Na NMR spectra of sodium aluminosilicate sodalites of general composition Na9[Si6Al6O24]A2 with A = B(OH)4- (SBS), SCN- (SRS) and A2 = SO4(2-) (SSS), MoO4(2-) (SMS) have been measured using magic-angle spinning (MAS) and double-rotation (DOR) techniques. Rotor synchronized pulse excitation is applied in the DOR experiments. Dramatic line narrowing is observed in the DOR spectra of all samples. The 27Al DOR NMR spectra of CAW measured at 9.4 and 11.7 T and spinning rates of 800-1150 Hz of the outer and 5 kHz of the inner rotor show seven sharp central lines accompanied by a manifold of spinning sidebands. These lines correspond to the seven crystallographically inequivalent Al sites of the CAW framework derived from X-ray structure analysis. From the difference of the line positions in the 9.4 and 11.7 T spectra the quadrupole coupling constant, QCC, quadrupole induced shift, sigma qs, and isotropic chemical shift, delta cs, of each Al site have been calculated. QCC values in the range of 5 to 9 MHz are obtained which reflect the strong tetragonal distortion of the AlO4 tetrahedra in CAW. delta cs shows only small changes in the range between 74.4 and 77.2 ppm. A tentative assignment of all lines to the distinct Al sites is derived from the correlation between QCC and a "shear strain parameter" describing quantitatively the distortion of the AlO4 tetrahedra.(ABSTRACT TRUNCATED AT 250 WORDS)     

The mechanism of color center production in iron doped photochromic sodalites

Bromide sodalites doped with iron (0.75 mg/cm³) have been studied by the Mossbauer technique, before and after heat treatment in hydrogen atmosphere and after coloration by u.v. light and by electron bombardment. The measurements show that u.v. or electron irradiation causes Fe²⁺ ions to convert to Fe³⁺. The number of converted ions (? 10¹⁷/cm³ for u.v. irradiation, and 5 × 10¹⁷/cm³ for electron irradiation) agrees with the expected number of F centers estimated from optical measurements. The thermal recovery time at room temperature of the Fe³⁺ ions back to their Fe²⁺ state was found to be several months.