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Abstract
(N(CH3)4)2ZnBr4 undergoes a ferroelastic phase transition from an orthorhombic mmm structure to a monoclinic 2/m phase. Birefringence and optical indicatrix rotation about the monoclinic axis have been measured in the ferroelastic phase and their temperature dependences have been related to the order parameter of the transition. The spontaneous strain has also been determined by means of X-ray diffraction. It is shown that in the temperature range covered, this quantity is proportional to the optical polarization coefficient B6, which becomes spontaneous at the transition. This fact has allowed us to obtain a numerical estimate of the p66 elastooptic coefficient at the transition point.