INFLUENCE ON DONOR ELECTRON ENERGIES OF THE CHEMICAL-COMPOSITION OF K, NA AND CA ALUMINOSILICATES

The chemical composition of KxNa1-xAlSi3O8 and NayCa1-yAl2-ySi2+yO8 cr ystals (alkali and plagioclase feldspars) is shown to determine the op tical transition energies of electrons trapped at donor centres within them. Optical resonances in the infrared region 1.2-1.5 eV can be int erpreted as arising from the 1s-2p transition of an ideal Bohr hydroge n donor. Shifts in the energy positions with sample composition are we ll accounted for by variations in the dielectric constant. Within the context of the Bohr model, the effective electron mass and donor radii can be determined. The mass is found to be 0.76m(e) in the K-Na serie s but slightly higher, at 0.79m(e) in CaAl2Si2O8. The ground-state don or radii are determined as 1.62 Angstrom, 1.63 Angstrom and 1.68 Angst rom in the K, Na and Ca end members, respectively; these values closel y match the mean (Si, Al-Si)-O bond lengths in the material for tetrah edra with mean Al content.