Transparent and luminescent thin films of partially substituted La10 (Si O4) 6 O3: Eu3+ apatite-type silicates

Partially substituted oxyapatite silicates, La8 M2 (Si O4) 6 O3: Eu3+ (M=Na, K, Mg, Ca, Sr, Y, and Gd), were prepared as powders and thin films by the conventional solid-state reaction and the sol-gel method, respectively. X-ray diffractometry and Fourier transform infrared spectroscopy were used to evaluate crystallinity and structure of the silicates. Transparent thin-film samples fabricated on quartz glass substrates exhibited intense red emissions due to Eu3+ upon irradiation with UV light through the charge-transfer excitation from O2- to Eu3+. The excitation wavelength, which was 268 nm for the nonsubstituted silicate, was shifted by the substitution in the host silicates. This shift was explained by the change in the lattice constant c and accordingly, the Eu-O distance. Our results then demonstrate that fine tuning of the excitation was possible in the chemically stable and transparent oxyapatite thin films.