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Using birefringence measurements by Senarmont's method (λ = 633 nm) the interaction of incommensurate structure with defects for (N(CH3)4)2FeCl4 crystals is investigated. It is found that in the incommensurate phase both the amplitude and phase of order parameter give their contributions in the spontaneous increment of birefringence with temperature.

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Long‐period anomalies caused by gradients of X‐ray defect concentration and temperature as well as electric fields and mechanical stresses are found in the temperature dependences of birefringence and dielectric characteristics for [N(CH3)4]2CuCl4 and [N(CH3)4]2ZnCl4 crystals in the region of IC phase existence. The obtained anomalies are explained by the influence of pinning force and external action gradients on the incommensurate structure.