Luminescence excitation characteristics of Ca, Na and K-aluminosilicates (feldspars) in the stimulation range 5–40 eV: determination of the band-gap energies

The first comprehensive survey of band-edge features in the ternary group of naturally occurring aluminosilicates (the feldspars) is presented. Synchrotron-based luminescence excitation of KAlSi 3 O 8 , NaAlSi 3 O 8 and CaAl 2 Si 2 O 8 allows the measurement of the evolution of the band-gap across the system, which, at 8 K, is found to vary from 7.86 eV in NaAlSi 3 O 8 to 7.7 eV and 7.62 eV in KAlSi 3 O 8 and CaAl 2 Si 2 O 8 , respectively: the band-gap energies are typically 0.1 eV smaller at 300 K. In comparison with measurements made on natural and synthetic hydrothermal α-quartz, where both the direct and indirect band-gap structures are distinctly observable, no significant post-edge band structure is discernable in the feldspars. In Ca-rich material, the luminescence is attenuated by more than two orders of magnitude for excitation energies up to 3 eV above the band-gap, partly supporting the proposition (derived from previous cyclotron resonance experiments) that the bands are simply parabolic and that the materials may be direct band-gap insulators. The luminescence excitation experiments also allow an initial survey to be made of sub-band-gap features in the materials, including low mobility, temperature sensitive band-tail states and mid-gap defects.