Polymorphism of LiMO2 Compounds and High-Pressure Single-Crystal Synthesis of LiBO2

Abstract

Single crystals of γ‐LiBO₂, which is the phase stable at high (P, T), have been grown at 15 kbar and at 950°C by using a flux technique. The crystals were retained metastably by quenching to room pressure and temperature. γ‐LiBO₂ is tetragonal with four molecules per unit cell (a=4.1961±0.0004, c=6.5112±0.0007 Å), Space Group $\text{I4̄2d}$ . The arrangement is that of a zinc‐blende‐like structure, with every atom tetrahedrally coordinated. The structure has been refined from single‐crystal x‐ray‐diffraction data with a precision corresponding to an R factor of 0.028. The interatomic distances are B–O=1.483 Å and Li–O=1.957 Å. The polymorphism of the LiMO₂ compounds (M=B, Al, Ga, In, Tl) is discussed. It is shown that there is a common trend among these compounds. They tend to assume a NaCl‐like structure. LiAlO₂ and LiGaO₂ have such an arrangement at high (P, T), while LiInO₂ and LiTlO₂ have a NaCl‐like structure at room pressure and temperature. The only exception is LiBO₂.