Hydrothermal Synthesis, Structural Investigation, Photoluminescence Features, and Emission Quantum Yield of Eu and Eu−Gd Silicates with Apatite-Type Structure

For the first time, Eu8.18Na1.08(SiO4)6(OH)1.62-2yOy (Eu apatite) and Eu3.96Gd3.96Na1.2(SiO4)6(OH)1.86-2yOy (Eu−Gd apatite) were synthesized at mild hydrothermal conditions. Powder X-ray analysis suggested a preferential substitution of Ln3+ by Na+ ions in 4f position for both materials. The substitution of Eu3+ by Gd3+ ions alters the distribution of the Ln3+ ions between 4f and 6h sites, which impacts their local symmetry, optical properties, and quantum efficiency. These materials are efficient room-temperature emitters with a maximum external quantum yield of ca. 21.4%. The abnormal increase in the relative intensity of the Eu3+ intra-4f6 5D0 → 7F4 transition is attributed to the presence of interstitial oxygen or incommensurate ordering of the oxygen around the 6h position, which results in a distortion of the Eu3+ local symmetry. Evidence of concentration quenching effects will be discussed based on the 5D0 lifetime and the emission quantum yield dependence on the Eu3+ concentration.