Thermal, optical and spectroscopic investigation in ferroelastic incommensurate (N(CH3)4)2CuCl4crystal

The three phase transitions (commensurate to incommensurate (I to II), incommensurate to ferroelastic (II to III), and ferroelastic to ferroelastic (III to IV)) in (N(CH 3 ) 4 ) 2 CuCl 4 are investigated by adiabatic calorimetry, thermal expansion, optical measurements and Raman spectroscopy. A possible order-disorder contribution associated with organic groups movements is discussed according to the calorimetric and Raman measurements. The experimental results seem to show that the disorder prevails in the spectral characteristics in the phase transition sequence. The birefringence measurements for the II to III phase transition display information about the primary and secondary order parameter behaviour which are compared with those predicted by a phenomenological model which takes into account the symmetries involved in the phase transition. Finally, a study about the critical behaviour of the I to II transition is also carried out.