Peculiarities of the Piezooptic Effect in Incommensurate Crystals

Abstract

The influence of uniaxial mechanical stress on the modulated structure which consists of a superstructure sequence is investigated in [N(CH3),],MeCl, (Me = Cu, Fe, Zn) crystals. The peculiarities of the effective piezooptic coefficients n : within the incommensurate phase of these crystals are studied by Senarmont's method. The influence of uniaxial mechanical stress disturbs the conditions of modulated structure interaction with defects what leads to the appearance of the widening of metastable states and to anomalies in the physical characteristics. It causes the change of the region of the localization of the incommensurability wave vector k on the commensurate values of higher order. IntroductionThe authors of [l to 31 have drawn the conclusion that anomalies of the effective piezooptic coefficients zi"j within the incommensurate (IC) phase are connected with peculiarities of the order parameter and the behaviour of the elastic constants Cij. Anomalies observed in the vicinity of phase transitions were explained by the influence of the term a@/&, proportional to the value aT,laaj, i.e. to the shift of the transition temperature under the influence of uniaxial stress. According to [4] the displacement of the anomaly corresponding to the thermooptical memory effect under the influence of mechanical stresses cj cannot be explained only by the temperature shift of the phase transition at T,. It is necessary to take into account soliton density changes under the action of mechanical stresses o? Besides, the deformation of the soliton structure (the change of the regular distribution along the crystal sample) may proceed in this case, as has been shown theoretically [5]. The experimental confirmation of this fact has been reported in [6]. Besides, the displacement of the ') Kyrylo and Methodiy str. 8, UA-290005 Lviv, Ukraine. , ) Lesna str. 16, PL-25509 Kielce, Poland. 15*

Specific Sequence of Commensurate Long-Periodic Regions Inside the Incommensurate Phases

“…However, these modes are not in accordance with the experimental results and theoretical considerations [ 17, 181 stating the parity of appearing and annihilating solitons. On the other hand, up to now, we do not know what does take place inside the transition regions (the regions separating the LPC phases) [14].…”

“…The temperature dependences of 6(An,) and 6(An,) are shown in Fig. 7a Such a behaviour could be understood if a deformation of the optical indicatrix is supposed to occur during the transition from the ferroelectric phase with P, to the ferroelectric phase with P, [14]. The anomalous behaviour of &(Anc) = f(T) results from the appearance of spontaneous strain.…”