The structural origin and boundaries of thermal transitions in stillwellite-type LnBSiO5

Abstract

Stillwellite-type borosilicates, LnBSiO5 represent a family of promising non-linear optical materials. The incorporation of different rare earths into the Ln sites results in complicate patterns of phase transitions. We herein analyze the high-temperature (HT) structural transformations of LnBSiO5 (Ln = Nd, Ce, La) using a suite of in situ HT methods: single-crystal structural study, powder X-ray diffraction and Raman spectroscopy, accompanied by thermal analysis. It is shown that phase transitions between the low- and high-temperature polymorphs of LnBSiO5 originate from the order-disorder in borate chains and are greatly dependent on the nature of the rare earth in the Ln sites. We showed that the high-temperature polymorphs of LnBSiO5 (Ln = Nd, Ce, La) are always metastable at ambient conditions, regardless of lanthanide cation type.