Anomalously large formula unit volume and its effect on the thermal behavior of LiBF4.

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 P3(1)21 with a=4.892(5) A, c=11.002(12) A, V=228.0(4) A3, and Z=3 at 200 K. The twin is generated by a 2-fold rotation about the [10] 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) A are observed. The formula unit volume (FUV=V/Z) of 77.9 A3 for LiBF4 at 298 K is considerably larger than 72.7 A3 for NaBF4 and 72.5 A3 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.