O.R. Gilliam

Basic spectroscopic properties of bismuth tellurium oxide, Bi2TeO5

Optical absorption measurements in the visible and infrared and reflectivity measurements in the visible and ultraviolet are reported for single crystals of bismuth tellurium oxide (Bi2TeO5). Strong dependence of these measurements on the light polarization is found for this recently grown non-linear material. Deviation from the Urbach rule (see Appendix) is believed to result from non-stoichiometry; approximate values of the Urbach parameters are given for the [0 1 0] polarization.

ESR OF PLATINUM IMPURITY IONS IN ZNWO4 SINGLE CRYSTALS

Abstract In zinc tungstate single crystals grown in platinum crucibles, a new platinum-impurity center was observed by ESR showing C 1 symmetry below 26 K and C 2 symmetry above 50 K. The Pt impurity resides as Pt 3+ in a Zn 2+ lattice site where reduction to C 1 symmetry is attributed to a “frozen-in” oscillation of the Pt between two sites determined by the double-well lattice potential for the Pt ion.

Colouration, impurities and non-local charge-compensation in ZnWO4

Abstract The characteristic pink colour of undoped ZnWO 4 single crystals having minimal Fe 2+ concentration is attributed primarily to Cr 3+ impurity ions. Other suggested sources of this colour, namely OH - ions, W 5+ type colour centres or oxygen vacancies are ruled out. The Me 5+ type dopants which enhance the crystals transparency in the visible region are shown to be charge-compensated non-locally by Fe 3+ ions.

ESR of W5+-H centers in γ- or UV-irradiated ZnWO4 single crystals doped by Li

Abstract Gamma of UV (254 nm) irradiation of an “as-received” ZnWO 4 :Li crystal, or, UV (366 nm) bleaching of a reduced crystal, produces a W 5+ paramagnetic defect for irradiation at 77 K. The ESR spectra indicate C 1 symmetry for this center. It is characterized by a strong 183 W hyperfine interaction as well as weaker 183 W, 1 H and 67 Zn superhyperfine interactions, permitting the development of a defect model. The defect is designated as the W 5+ H center. It is related to a Mo impurity center reported in reduced ZnWO 4 crystals, although both its production and stability are different.

An electron spin resonance study of vanadium-doped alpha -TeO2 single crystals

An ESR study of vanadium-doped paratellurite indicates that vanadium enters the lattice as V4+, equally populating the four inequivalent cation sites. The 3d1 electron exhibits hyperfine interaction with the I=7/2 51V nucleus and superhyperfine interaction with two neighbouring I= 1/2 nuclei identified as protons by ENDOR experiments. Spin Hamiltonian parameters computed from ESR measurements show that each V4+ ion has a single C2 symmetry axis along one of the (110) directions. The neighbouring hydrogens appear to enhance the symmetry and stability of the defect structure. Group theoretical arguments show that there are two possible linear combinations of d wavefunctions that could represent the ground state of V4+ in alpha -TeO2. However, a determination of the lower-lying state was not feasible using only the ESR data.

Paramagnetic defects in ZnWO4 single crystals induced by electron irradiation at room temperature

Abstract Intrinsic paramagnetic defects in undoped zinc tungstate single crystals have been observed by ESR after electron irradiation at room temperature. The only observed hole-type defect is attributed to a hole trapped at an O2− ion near a zinc vacancy. Two electron-excess centers are each due to a single electron trapped at an oxygen vacancy, where the defects differ in the oxygen vacancy positions. All of the defects exhibit superhyperfine interactions with neighboring 183W nuclei. Spin-Hamiltonian parameters are reported and discussed.

Paramagnetic resonance of V4+ in tetragonal GeO2☆

Abstract Anisotropic g and A tensors are found for V 4+ in single crystal tetragonal GeO 2 . The orbital mixing parameter λ , normalized radial average 〈1/ r 3 〉/ N and core polarization x / N are calculated, and λ is not anomalously low as reported from polycrystalline measurements.

Luminescence of rutile-like GeO2 crystals

Abstract Thermoluminescence glow peaks and emission spectra data are reported for undoped and Al doped GeO2 tetragonal crystals over the temperature range from 45 to 700 K. For the pure crystal the emission peaks near 475 nm below 150 K but at higher temperatures it is characterised by red emission near 720 nm. For samples irradiated at room temperature the Al:GeO2 shows 575 nm emission below 75°C but primarily red signals at higher temperature. Separation and activation energies of component glow peaks are recorded. The thermoluminescence spectra alter after repeated irradiation and measurement. Further, comparisons are made between the thermoluminescence spectra and those obtained during X-ray or electron beam irradiation. The cathodoluminescence spectra differ at 300 K with a dominant 575 nm peak for the Al:GeO2 and bands near 300, 400, 540, 575 and 720 nm for the pure sample. Below room temperature there are significant differences although both show extra bands near 75 to 100 K peaked at 310 and 370 ...

The role of OH- Ions in the charge compensation of impurities in ZnWO4 single crystals

Abstract Antimony and vanadium are observed to reduce the colour of ZnWO 4 single crystals. It is suggested that charge compensation for these dopants may occur via the substitution of OH - ions for O 2- . The structure and the angular dependence of the IR absorption bands, as well as OH-OD exchange experiments, are in accordance with this charge-compensation mechanism.

and centres in single crystals characterized by ESR and ENDOR spectroscopy

ESR and ENDOR methods are used to characterize a new centre with symmetry in reduced single crystals. Present ENDOR results also confirm that for the recently reported centre in UV- or -irradiated crystal an observed superhyperfine interaction results from a nearby hydrogen nucleus. Comparison of spin-Hamiltonian parameters shows the similarity of and centres in crystals.