Synthesis and Luminescence of Eu3+-Doped Narsarsukite Prepared by the Sol-Gel Process

The sol-gel synthesis and structural characterisation of narsarsukite powders are reported. Samples doped with different Eu³⁺ amounts (Ti/Eu = 20 and 200), calcined at 800°C, have been characterised by powder XRD, ²³Na, ²⁹Si MAS NMR, Raman and luminescence spectroscopies. Eu³⁺-doped narsarsukite displays a high and stable room-temperature luminescence. The presence in narsarsukite of two different Eu³⁺ local environments is inferred based both on the distinct ⁵D₀ ⁷F₀ lines observed and on the local field splitting of the ⁷F_1,2 levels. For low lanthanide contents, the Eu³⁺ ions are essentially localised in a centrosymmetric environment characterized by a low-energy ⁵D₀ ⁷F₀ line and a relatively long ⁵D₀ lifetime (3.56–3.96 ms). In contrast, at high lanthanide contents the Eu³⁺ ions are also present in a second local site with a less covalent first coordination shell. This corresponds to a high-energy ⁵D₀ ⁷F₀ line and a short ⁵D₀ lifetime (0.84–0.99 ms). Therefore, it is likely that Eu³⁺ ions substitute for both Ti⁴⁺ and Na⁺, although the former ions are preferentially replaced at low Eu³⁺ content.