Diffuse reflectance and photoluminescence spectroscopic investigations of Sr₂RE₈(SiO₄)₆O₂ (RE = rare earth) apatites

Apatite-type compounds have attracted attention due to their flexible crystalline structure besides high chemical stability, which increases the interest in understanding and modifying their properties to achieve the requirements for applications in catalysis and energy devices. In order to attend these demands, this paper investigates the optical properties of apatite-type Sr-2 RE (8)(SiO4)(6)O-2 ceramics (RE = rare earth) by using diffuse reflectance and photoluminescence spectroscopies. Intraconfigurational f-f transitions, charge transfer bands, and energy gaps were identified and detailed discussed in terms of their relationships with the structural features of the rare earth apatites. Although all materials have the same hexagonal space group (P6 (3) /m, # 176), the band gaps varied between 3.51 and 3.95 eV, for Eu- and Yb-containing apatites, respectively, showing the influence of their ionic radii and their electronic configurations on this property. In this sense, diffuse reflectance spectroscopy emerges as a powerful tool to study the optical properties of rare earth-containing materials as a complementary technique to photoluminescence spectroscopy. In this work, these techniques were discussed in association, which allowed us to provide a complete set of optical information that improves the understanding of apatite-type materials, as required in advanced technological demands.