Successive phase transitions have been found in crystalline state of the title compound by X-ray diffractometry with synchrotron radiation and by adiabatic calorimetry. The transitions are at 173.5 and 174.2 K with a total entropy change of 7.26 J K-1 mol(-1). The phase transitions are associated with a change of the periodicity of lateral molecular stacking in the monoctinic lattices with the common b axis, Z = 4, 16, and 12, for high, intermediate and low temperature phases, respectively.