THE IMPROPER FERROELASTIC PHASE-TRANSITION OF LETOVICITE, (NH4)(3)H(SO4)(2) - AN OPTICAL BIREFRINGENCE, X-RAY-DIFFRACTION AND RAMAN-SPECTROSCOPIC STUDY

The improper ferroelastic phase transition of synthetic letovicite has been studied using optical birefringence, hard mode Raman spectroscop y and X-ray diffraction measurements. The critical point is the L-poin t on the surface of the Brillouin zone of the rhombohedral high-temper ature phase. The domain pattern of the ferroelastic phase is related t o the arms of the L-star leading to orientational domains of the speci es <(3)over bar mF2/m>. The integrated intensity of the A(g)-Raman-mod e near 483 cm(-1), the thermal evolution of the spontaneous strain and coupled quantities like the morphic optical birefringence show a rapi d saturation on approaching the transition point near 413 K. The satur ation has been fitted using Landau-type free energy expansions with hi gh power terms of the order parameter. The non-classical thermal varia tion of the order parameter is compatible with a Slater instability of infinite order.