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

For the first time, Eu 8.18 Na 1.08 (SiO 4 ) 6 (OH) 1.62-2 y O y (Eu apatite) and Eu 3.96 Gd 3.96 Na 1.2 (SiO 4 ) 6 (OH) 1.86-2 y O y (Eu−Gd apatite) were synthesized at mild hydrothermal conditions. Powder X-ray analysis suggested a preferential substitution of Ln 3+ by Na + ions in 4f position for both materials. The substitution of Eu 3+ by Gd 3+ ions alters the distribution of the Ln 3+ 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 Eu 3+ intra-4f 6 5 D 0 → 7 F 4 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 Eu 3+ local symmetry. Evidence of concentration quenching effects will be discussed based on the 5 D 0 lifetime and the emission quantum yield dependence on the Eu 3+ concentration.