Temperature dependences of the optical path difference, variable part of the refractive index and thickness of triglycine sulphate crystal for three crystallophysic directions are studied in the temperature range of 39–70 °C, including the ferroelectric phase transition at Tc=49 °C, using the Jamen type optical interferometer. Temperature dependences of the spontaneous changes of the characteristics studied in the range of 39–49 °C are fitted by the power-like low Y ∼ τ²β with doubled averaged effective critical indices 2β=0.87–0.95. The 2β values being different from the unity is explained by the essential temperature dependence of the coefficients of electrooptic, inverse piezoelectric and electrostriction effects in the range close to the phase transition point.
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