Rb[Li₅ Si₂ O₇ ] - A Latecomer in the Family of Alkali Lithosilicates Hiding a Green-Emitting Lithosilicate

In order to expand the field of alkali lithosilicates, a new representative of the substance class with a previously unknown structure type was found based on solid-state synthesis. The novel compound with the sum formula Rb[Li₅ Si₂ O₇ ] crystallizes in the orthorhombic space group Pbcm (no. 57) with a=7.6269(3), b=9.5415(4), and c=9.4095(3) Å by means of single-crystal X-ray diffraction. The structure consists of a highly condensed lithosilicate framework, built up of corner- and edge-linked [LiO₄ ]-tetrahedra and [Si₂ O₇ ]-units, and the rubidium ions aligned in channels. Suitable crystals of the material were obtained using sealed tantalum ampoules as reaction tube at a temperature of 750 °C. The new compound was further characterized via powder diffraction, Rietveld analysis, and EDX measurements. At first glance, Eu²⁺ -doped Rb[Li₅ Si₂ O₇ ] reveals an intense green luminescence. In-depth crystal analysis shows that a core-shell formation is present even for apparently high quality single-crystals. As a minority phase, the known green phosphor RbLi[Li₃ SiO₄ ]₂ :Eu²⁺ is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li₅ Si₂ O₇ ] dominating the single-crystal diffraction pattern.