A hole center causing rose coloration in AlPO4 and GaPO4

In synthetic AlPO4 crystals and Sn-doped GaPO4 hole centers localized on an oxygen showing an almost isotropic hyperfine splitting from a 31P nucleus are observed after X-ray irradiation and analyzed by EPR. They are evidently caused by substitution of Al and Ga, respectively, by a divalent cation, most likely Mg2+. From the variation of the EPR linewidths with temperature an activation energy of 19 kJ/mole for thermal hopping of the hole is found in AlPO4. These centers cause a rose coloration and the first absorption band in AlPO4 has an oscillator strength near 0.3, in the same range as for the smoky quartz centers. It is interpreted as arising from optical transfer of the hole within the PO4 unit. Thermoluminescence emission is observed with destruction of the centers in AlPO4. From glow curves an activation energy of 136 kJ/mole is obtained which in this case represents the distance of the hole from the valence band. The properties of the center in AlPO4 are compared to those of a similar center in certain natural rose-colored quartz crystals. (author)