Temperature dependence of elastic behaviour of Rb2ZnCl4by Brillouin spectroscopy

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Abstract
Temperature dependence of elastic stiffness is measured in the range -100 to +70 degrees C. At the lock-in transition near -81 degrees C, only slight slope changes appear for some stiffnesses. Weak anomalies are observed near the incommensurate-commensurate transition at 29 degrees C for c22, c33, c44, c55 and c66. No broadening of Brillouin lines is detected. The results are discussed in terms of coupling between strain and soft modes which are allowed by symmetry. The value of 0.6+or-0.1 for the exponent 2 beta of the order parameter squared is determined from the temperature dependence of c44. The evolution of c33 in the incommensurate phase below the 29 degrees C transition agrees with a relaxational behaviour of the susceptibility associated with the ampliton mode normal coordinate, characterised by a critical slowing down of the relaxation time. This result is compatible with the overdamped ampliton mode observed by Raman scattering.