Electro-optic measurement of the order parameter of NH4Cl

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Abstract
We present here measurements of the temperature variation of the electro-optic effect in the low temperature phase of NH4Cl and we show how they are related to the order parameter of the order-disorder transition. These measurements are made difficult by the presence of parasitic strain birefringence, and by the existence of domains which produce a change of sign of the electro-optic effect. The results are compared with previous measurements of the order parameter by the piezoelectric effect and second harmonic generation, and their interpretation, following the Landau theory or power law with critical exponents, is discussed in relation to the existence of a multicritical point under high pressure. An exponential tail of the order parameter, extending over several degrees in the high temperature phase is systematically found in all these experiments. The origin of this residual order is probably related to the stresses produces by crystalline defects