Optical and Dilatometric Properties of (CH₃)₂NH₂Al(SO₄)₂·6H₂O Crystals in Paraelectric and Ferroelectric Phases

Abstract

Experimental investigation of the thermal linear expansion 1(T) and temperature dependencies of the interference optical path difference D(T) were carried out for (CH 3 )2 NH2Al(SO 4 )2 • 6H2O crystal, in the range of 120-310 K for three crystal physical directions. Decrease in electrostriction coefficients g of crystal was observed on approaching the phase transition point Tc 150 K in ferroelectric phase. Relative temperature changes of the lengthening 51/1 and refractive index δn/(n -1) are of the same order of magnitude. Peculiarities of temperature dependence of dilatometric and optical parameters of (CH3)2NH2Al(SO4)2.6112O testify for qualitative reconstruction of chemical bonds in the crystal when temperature changes in the same phase.

Dilatometric and Optical Properties of (CH3)2NH2Ga(SO4)2 � 6H2O Crystals in Paraelectric and Ferroelectric Phases

“…The character of these temperature dependences, i.e. minimum at T c , is similar to those ones obtained for DMAAlS crystal [23]. Since the relative changes of the optical path difference D ij ¼ L i (n j --1) measured can be presented in the form [20] dD we have calculated the temperature dependences of dn j /(n j À 1) (Fig.…”

“…The spontaneous electrostriction coefficients Q Li of DMAGaS crystal remain almost unchanged in the wide temperature range of the ferroelectric phase 115-136 K (DQ L 0.06Q L ). This differs from the situation in DMAAlS crystals, where these coefficients decrease in the ferroelectric phase when approaching the corresponding PT point T c ¼ 150 K [23].…”

“…A negative coefficient a V of thermal volume expansion was detected in [3] for DMAGaS, too. Analogous relations between these different coefficients were observed in DMAAlS crystal [23]. Taking into account that the paraelectric phase of DMAGaS is center symmetrical, one can state that the spontaneous elongation dL si /L i is proportional to the square of spontaneous polarization P s 2 , similarly to the electrostriction effect.…”

“…4). The temperature dependences for DMAGaS and DMAAlS crystals clearly differ: negative temperature derivatives d/dT (ln (n j À 1)) are peculiar in the ferroelectric phase of DMAAlS for p and g directions of the optical indicatrix [23]. It is seen from Table 1 that the temperature sensitivity of characteristic parameters of the electron polarizability, (n i À 1) --1 dn i /dT, and dimensions of the crystal unit cell, a i (i ¼ m, g), are of the same order of magnitude.…”

“…Here dL si /L i is the difference between the relative elongation dL i /L i for T ¼ 121 K < T c and a linear approximation of this value from the temperature range T > T c [23]. Thus, dL si /L i is the relative elongation dL i /L i due to the spontaneous polarization P s .…”