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 crystals (alkali and plagioclase feldspars) is shown to determine the optical transition energies of electrons trapped at donor centres within them. Optical resonances in the infrared region 1.2-1.5 eV can be interpreted as arising from the 1s-2p transition of an ideal Bohr hydrogen donor. Shifts in the energy positions with sample composition are well 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 series but slightly higher, at 0.79m(e) in CaAl2Si2O8. The ground-state donor radii are determined as 1.62 Angstrom, 1.63 Angstrom and 1.68 Angstrom in the K, Na and Ca end members, respectively; these values closely match the mean (Si, Al-Si)-O bond lengths in the material for tetrahedra with mean Al content.