The High-Temperature Polymorph of LiBF 4 .

The single-crystal-to-single-crystal phase transition is determined by using X-ray crystallography on LiBF 4 , resolving a longstanding ambiguity in the existence of a high-temperature polymorph of LiBF 4 . LiBF 4 possesses an endothermic phase change at 28.2 °C with Δ H = 1180 J mol -1 and Δ S = 3.92 J mol -1 K -1 based on DSC. Single-crystal X-ray diffraction shows that the low-temperature phase collected at 200 K is a twinned trigonal P system with a twin law indicating reflection through the 110 plane. The same crystal collected above the phase transition temperature at 313 K is a C-centered orthorhombic system, describable as the superposition of the two low-temperature twin geometries undergoing interconversion. The geometries of the high- and low-temperature phases are consistent with the calorimetry experiments and with previous NMR findings indicating BF 4 geometric reorientations above 300 K.