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 degrees C with Delta H = 1180 J mol(-1) and Delta 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 BF4 geometric reorientations above 300 K.