Calorimetric investigation of the ferroelectric overline43m-mm2 phase transition in boracite crystals

The isobaric molar heat capacity of CrCl, FeI, CuCl, NiBr and ZnBr boracite at the ferroelectric overline43m-mm2 phase transition is reported. The magnitude of the rise in C_p at the transition, and the large upper bound values of ∆H and ∆S prove that the phase transition is first order. The values of ∆H follow the trend ZnBr ≫ NiBr > CrCl ≫ CuCl > FeI reflecting possible structural dissimilarities amongst the boracites. Thermal annealing of single crystal boracite samples of NiBr and CrCl is found to remove multiple peaking of the heat capacity at the transition resulting in single peak heat capacity curves. The multiple peaking is thought to arise from internal stresses within the crystal.