Thermal decay of the V-band in MgO quenched from high temperature

Experimental studies of the effects of quenching and annealing in the range 650 to 1250 K on the so called V-band absorption in MgO single crystals have been carried out. Optical absorption in the visible and infrared spectral range of irradiated and unirradiated crystals, together with density changes attendant upon heat treatment, were used to gain insight into the nature of the centers and the kinetic processes involved. It is shown that the enhancement by prior quenching of the radiation induced 2.3 eV V-band absorption is reversible with quench temperature; at any annealing temperature it approaches a limiting value which differs from specimen to specimen. This behavior is similar to that observed earlier for the 3296 cm−1 V−OH band which suggests a connection between the V-band enhancement and the concentration of V−OH complexes present, namely, that the enhancement is due to an increase in VOH concentration in the quenched and irradiated crystals. Isothermal annealing of the V-band enhancement in the range 650 to 850 K reveals that the decay process is bimolecular with an activation energy of 1.5 eV. The similarity of this behavior to the decay of V−OH along with the results of bulk density measurements during annealing lend strong support to the model suggested by Henderson and Sibley that the V-band enhancement results from the increased solubility of Mg(OH)2 in the form of cation vacancy OH− complexes with increasing temperature.