Anomalously Large Formula Unit Volume and Its Effect on the Thermal Behavior of LiBF₄

Abstract

The crystal structure of LiBF₄ has been determined by single-crystal X-ray diffraction measurements. LiBF₄ crystallizes as a merohedral twin in the trigonal space group P3₁21 with a = 4.892(5) Å, c = 11.002(12) Å, V = 228.0(4) Å3, and Z = 3 at 200 K. The twin is generated by a 2-fold rotation about the [11̄0] direction. The lithium cation is coordinated by four fluorine atoms in a distorted tetrahedral manner, wherein two Li−F distances of 1.862(5) and 1.846(5) Å are observed. The formula unit volume (FUV = V/Z) of 77.9 ų for LiBF₄ at 298 K is considerably larger than 72.7 ų for NaBF₄ and 72.5 ų for AgBF₄, despite the smaller size of Li+, indicating loose ionic packing of LiBF₄. The thermodynamic evaluation of the decomposition temperature for LiBF₄ was performed using the empirical relationship between the standard entropy and the FUV obtained. The results indicate that the large FUV of LiBF₄ contributes to its higher decomposition temperature compared to that of LiPF₆.