Rb[Li5Si2O7] - 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[Li5Si2O7] crystallizes in the orthorhombic space group Pbcm (no. 57) with a=7.6269(3), b=9.5415(4), and c=9.4095(3) angstrom 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 [Si2O7]-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 degrees C. The new compound was further characterized via powder diffraction, Rietveld analysis, and EDX measurements. At first glance, Eu2+-doped Rb[Li5Si2O7] 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[Li3SiO4](2):Eu2+ is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li5Si2O7] dominating the single-crystal diffraction pattern.

Automated summary

A new alkali lithosilicate compound Rb[Li5Si2O7] was discovered with a highly condensed lithosilicate framework. The intense green luminescence of Eu2+-doped Rb[Li5Si2O7] was revealed, with in-depth crystal analysis showing a core-shell formation even in high quality single crystals. The known green phosphor RbLi[Li3SiO4](2):Eu2+ was identified as the source of luminescence within the tiny core surrounded by a larger matrix of Rb[Li5Si2O7].