L.A. Kappers

Growth and properties of Bi2TeO5 single crystals

The authors report the successful growth of Bi2TeO5 single crystals by the Czochralski method. This crystal is a new prospective piezoelectric material. Boules as large as 25 mm diameter and 50 mm length were grown, providing pale yellow, optically transparent specimens suitable for indepth physical investigations.

Electron traps and transfer efficiency of cerium-doped aluminate scintillators

Abstract Comparative measurements of thermoluminescence and scintillation light outputs of gamma-ray irradiated Ce;LuAlO 3 (LuAP) and Ce:YAlO 3 (YAP) reveal that electron trapping significantly depresses transfer efficiency in these scintillator materials, but fails to explain fully either their performance differential or their departures from ideal efficiency. In the limit of short radiation times, the ratio of integrated thermoluminescence light output to integrated scintillation light output is 0.14 in LuAP and 0.02 in YAP.

Pressure and temperature dependence of chromium photoluminescence spectra in fluoride elpasolites

Abstract Photoluminescence spectra of K 2 NaGaF 6 Cr 3+ and K 2 NaScF 6 :Cr 3+ , measured as functions of hydrostatic pressure, reveal a blue shift of the emission band culminating in a pressure-induced transition from a broad-band fluorescence spectrum to a highly structured phosphorescence spectrum. The blue shift is interpreted in terms of a pressure dependent local compressibility. The high-pressure, low-temperature emission spectrum of K 2 NaScF 6 :Cr 3+ reveals a splitting of the zero-phonon line indicative of a transition to a lower symmetry phase.

Radiation-induced oxygen interstitials in MgO☆

Abstract Neutron irradiation of single crystals of MgO near 65° C produces a series of three distinct yet similar ESR spectra arising from O-2 molecules. The O-2 molecules are formed when oxygen interstitials produced by the radiation combine with indigenous oxygen ions. We propose that magnesium vacancies act as the stabilizing site for the O-2 molecules.

Pressure dependence and thermal quenching of chromium photoluminescence in Cs2NaYCl6:Cr3+

Abstract Photoluminescence spectra of the chromium-doped elpasolite Cs 2 NaYCl 6 :Cr 3+ were measured at ambient and near liquid nitrogen temperatures as a function of pressure in a diamond anvil cell. A pressure-induced blue shift of the broad 4 T 2g → 4 A 2g fluorescence band was observed, culminating in a crossing of the 4 T 2g and 2 E g excited states as evidenced by the appearance of a structured 2 E g → 4 A 2g phosphorescence spectrum. Resolved vibronic structure of the latter permitted determination of vibration frequencies at high pressure. Evidence of a phase transition was observed near the highest pressures attained, ≈ 11 GPa. The pressure dependence of the peak energy of the broad band was interpreted in terms of a pressure-dependent local compressibility. Measurements of photoluminescence lifetimes as functions of pressure and temperature established that the activation energy for thermal fluorescence quenching increases linearly at the rate of 1668 ± 52 cm −1 GPa . A semi-empirical, two-accepting mode, linear- and quadratic-coupling model, calibrated at ambient pressure, predicts a rate of 1758 cm −1 GPa , in substantial agreement with experiment.

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.

Afterglow Suppression and Non-Radiative Charge-Transfer in CsI:Tl,Sm

Suppression of afterglow in co-doped CsI:Tl is found to be an order of magnitude more effective in CsI:Tl, Sm than in CsI:Tl, Eu. Rate equations predict that deep electron traps introduced by co-doping with samarium effectively scavenge electrons from shallow traps associated with thallium, thus suppressing afterglow in the time domain of tens of milliseconds. In addition, combined radioluminescence and thermoluminescence experiments suggest that electrons released by samarium recombine non-radiatively with trapped holes, thus providing a mechanism for suppression of hysteresis. Ab initio quantum chemistry calculations support the conclusion that non-radiative charge-transfer transitions in CsI:Tl, Sm are enabled by the presence of low-energy excited states of within the ground configuration.

Thermal quenching and electron traps in LSO

It is demonstrated by comparison of thermoluminescence and scintillation light outputs of LSO as functions of radiation time that a previously suggested thermal quenching correction is inappropriate. Approximate solutions of rate equations are employed to infer absolute trap concentrations and to explore the effects of thermal quenching on the shapes of thermoluminescence glow curves.

Photochromic effects in Bi2TeO5:Cr crystals

Bismuth tellurite is a new non-linear optical material with interesting photorefractive properties. Chromium builds into the crystal in 6+ valence state (chromate anion). The exposure of the Cr-doped crystals with white light creates new absorption bands in the visible and near infrared range. This photochromic effect is related to a Cr6+→Cr5+ charge transfer. An appropriate thermal annealing eliminates the color restoring the Cr6+ state. The temperature and wavelength dependence of the photochromic effect has been investigated.