Isothermal annealing of a 620 nm optical absorption band in Brazilian topaz crystals

Abstract

Isothermal decay behaviors, observed at 515, 523, 562, and 693K, for an optical absorption band at 620nm in gamma-irradiated Brazilian blue topaz were analyzed using a kinetic model consisting of O− bound small polarons adjacent to recombination centers (electron traps). The kinetic equations obtained on the basis of this model were solved using the method of Runge–Kutta and the fit parameters describing these defects were determined with a grid optimization method. Two activation energies of 0.52±0.08 and 0.88±0.13eV, corresponding to two different structural configurations of the O− polarons, explained well the isothermal decay curves using first-order kinetics expected from the kinetic model. On the other hand, thermoluminescence (TL) emission spectra measured at various temperatures showed a single band at 400nm in the temperature range of 373–553K in which the 620nm optical absorption band decreased in intensity. Monochromatic TL glow curve data at 400nm extracted from the TL emission spectra observed were found to be explained reasonably by using the knowledge obtained from the isothermal decay analysis. This suggests that two different structural configurations of O− polarons are responsible for the 620nm optical absorption band and that the thermal annealing of the polarons causes the 400nm TL emission band.