Phase Transitions in $[N(CH_3)_4]_2ZnCl_4$ and $[N(CH_3)_4]_2CuCl_4$ Crystals Related with the Incommensurate Modulation Existence

The continuous phase transitions are observed in the crystals with incommensurate phase. They are transitions of parent-incommensurate phases $(T_i)$; transitions between metastable states; incommensurate-commensurate phases $(T_c)$. It was set that phase transition between parent and incommensurate phase is a continuous second-order phase transition with a critical index β = 0.5. The transition between metastable states is a continuous phase transition through the intermediate temperature region - incommensurate phase. The wave vector changes with the temperature here and wave vectorwe q* = q₁ - q₂ appears, where q₁, q₂ denote commensurate values of incommensurability wave vector of neighboring metastable states. It was shown that the phase transition between incommensurate and commensurate phases is a continuous phase transition.