Specific Heat and Linear Birefringence in Incommensurate Phase of NH₄HSeO₄ Crystal

Abstract

Precise measurements of the specific heat and linear birefringence changes were performed for the NH₄HSeO₄ crystal. Phase transitions at about 100 K, 252 K and 260 K were confirmed. The continuous second order phase transition at 260 K to the incommensurate phase has been described by the 3D XY model. Thermal parameters (as the excess enthalpy ΔH, and the excess entropy ΔS) of the N-IC transition were estimated. The critical index ≈ 0.832 was determined below T_i from the linear birefringence increment. Hysteretic behaviour of the linear birefringence in the incommensurate phase was observed at various temperature rates.

Acknowledgments

This work was supported by the University of Wroclaw (grant 2016/W/IFD/2009).