The High-Temperature Polymorph of LiBF₄

The single-crystal-to-single-crystal phase transition is determined by using X-ray crystallography on LiBF₄, resolving a longstanding ambiguity in the existence of a high-temperature polymorph of LiBF₄. LiBF₄ possesses an endothermic phase change at 28.2 °C with ΔH = 1180 J mol^-1 and ΔS = 3.92 J mol^-1K^-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 BF₄ geometric reorientations above 300 K.