Point defects inα−Al2O3:Mgstudied by electrical conductivity, optical absorption

Abstract

Total and partial ionic and electronic c-axis dc conductivities were measured for singlecrystal A1203.8 ppm Mg as a function of oxygen pressure and temperature. The sample is unsaturated at T & 1360'C but is saturated in equilibrium with a spinel precipitate phase at lower temperatures.The results are analyzed to give ionic mobility, thermodynamic parameters of oxidation reduction of A1~03, and the solubility of MgO as a function of temperature. Oxidized samples are grayish purple due to the presence of a broad absorption band with maximum at 480 nm. Discoloration can occur by reduction (at high temperature) or by precipitation (at T & 1000 C); both proceed from the surface, involving migration of oxygen out of or of Al into the crystal. An explanation for the rapid diffusion regulating precipitation belo~1000'C is proposed. Recoloration at low temperature proceeds without a front and possibly involves oxygen diffusing into the sample along dislocations or subgrain boundaries. Hole-release studies at low temperature indicate that the hole binding energy in the defect MgAb E, &0.68 eV. The combined results of the present work suggest that free holes move as large, rather than small, polarons and that E, =0.68 eV.

Beryllium Diffusion of Ruby and Sapphire

“…In April 2002, we initiated a set of experiments to determine the origin of the color induced by Be diffusion. We believed that we were observing trapped-hole-center absorption, similar to that observed much earlier in both natural and synthetic corundum (Mohapatra and Kröger, 1977;Wang et al, 1983;Emmett and Douthit, 1993;Häger, 1993). Because holes only form in corundum under highly oxidizing conditions, the concentrations of Mg-or Be-hole pairs are very sensitive to the oxygen partial pressure at which they are equilibrated in the furnace.…”

“…Fe 2+ also can be charge compensated by tetravalent donor ions (e.g., Si 4+ or Ti 4+ ) forming donor-acceptor pairs; by hydrogen, which is also a donor; or, in the absence of either of these options, by an oxygen vacancy or an aluminum interstitial ion. Mg 2+ in corundum is also an acceptor (Mohapatra and Kröger, 1977;Wang et al, 1983). It can be charge compensated by tetravalent donor impurities (e.g., Si 4+ or Ti 4+ ) or, in their absence, by an oxygen vacancy or a hole.…”

“…One of the unusual properties of the Mg 2+ -trapped-hole coloration in corundum is its sensitivity to oxygen partial pressure (oxygen concentration in the furnace atmosphere; Mohapatra and Kröger, 1977;Wang et al, 1983;Emmett and Douthit, 1993). If corundum containing Mg 2+ is heat treated in pure oxygen, the color saturation is maximized.…”

Impurity Centers in Oxides Investigated by γ-γ Perturbed Angular Correlation Spectroscopy and <i>Ab Initio</i> Calculations

“…[2] This model cannot be distinguished from the physically different case where there are two fractions of probes, f d undergoing a fluctuating field characterized by the perturbation factor [3] and f s subjected only to the static field. Indeed, Eq.…”

“…Sometimes, however, an additional heat treatment in an oxygen atmosphere is necessary for removing competing intrinsic positive defects, such as oxygen vacancies, in order to achieve this situation. This is known to take place, e.g., for Al 2 O 3 doped with Mg [3], and the hole-concentration increases strongly with rising oxygen partial pressure. In crystalline quartz two O --type defects are known, O -next to Al 3+ [4] or to Ge 4+ [5], which show absorption bands typical for bound hole polarons.…”