α-LiMB9O15 (M = Sr, Pb): flexible [B3O7] units leading to the low temperature phase of β-LiMB9O15 (M = Sr, Pb)

Abstract

Polymorphism is a very important phenomenon in borates and is helpful when studying their structure and properties. Low-temperature phase α-LiMB₉O₁₅ (M = Sr, Pb), crystallized in the chiral space groups P2₁2₁2₁ and P6₃, was synthesized via a high-temperature solid reaction, while high-temperature phase β-LiMB₉O₁₅ (M = Sr, Pb) crystallizes in a non-centrosymmetric space group R3c. All of these four phases have three-dimensional structures made up of fundamental building units [B₃O₇] with six-membered rings, yet the [B₃O₇] units construct B–O frameworks with different types of three-dimensional channels. The TG-DSC measurements and X-ray diffraction of powders at different temperatures show that α-LiSrB₉O₁₅ transforms into β-LiSrB₉O₁₅ at 840 °C, and the high-temperature β-phase compound is a congruent melting compound.