posted on 2024-01-17, 15:35authored byLaura
A. Sonnenberg, Shujit Chandra Paul, Stephanie L. Wunder, Michael J. Zdilla
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.