Rb[Li(5)Si(2)O(7)] – 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[Li(5)Si(2)O(7)] 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 [LiO(4)]‐tetrahedra and [Si(2)O(7)]‐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, Eu(2+)‐doped Rb[Li(5)Si(2)O(7)] 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[Li(3)SiO(4)](2):Eu(2+) is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li(5)Si(2)O(7)] dominating the single‐crystal diffraction pattern.