Detection and characterization of a reversible solid-solid phase transition at 147 K in crystalline 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione

Single-crystal X-ray diffraction in the temperature range 19–296 K has revealed that solid 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione undergoes a reversible monoclinic (high-temperature) to triclinic (low-temperature) phase transition at Tc = 147 ± 1 K. Unit-cell parameters, accurately determined at eight temperatures below and seven above Tc, show no discontinuities, but the derived expansivities are discontinuous at 147 K. A linear dependence on temperature is shown by the square of the spontaneous strain and the deviation from the monoclinic value (90°) for the unit-cell angle γ. The phase transition qualifies as a second-order pure ferrodistortive transformation.