Phase Transitions in [N(CH3)4]2ZnCl4 and [N(CH3)4]2CuCl4 Crystals Related with the Incommensurate Modulation Existence

S. Sveleba a, O. Semotyuk a, I. Katerynchuk a, I. Kunyo a, I. Karpa a and Yu. Pankivskyi b a Department of Nonlinear Optics, Lviv National Ivan Franko University, Tarnavskogo str. 107, Lviv, UA-79017, Ukraine b Department of Ecology, Ukrainian National University of Forestry, O. Kobylyanskoyi str. 1, Lviv, UA-79005, Ukraine

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Received: 14 11 2007; Revised version: 27 04 2008; In final form: 8 05 2008;

The continuous phase transitions are observed in the crystals with incommensurate phase. They are transitions of parent-incommensurate phases (Ti);transitions between metastable states;incommensurate-commensurate phases (Tc). 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 vector 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.

DOI: 10.12693/APhysPolA.114.791 PACS numbers: 61.44.Fw, 64.70.Rh, 78.20.Fm, 65.40.Ba