Thermal Diffusivity and Critical Behaviour of Uniaxial Ferroelectric Pb₅Ge₃O₁₁

On the critical exponent of order parameter in Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals

“…The same value  has been obtained on the basis of ultrasonic velocity measurements [12]. According to our recent data concerned with the temperature evolution of thermal expansion coefficients [13], the  parameter determined for the ferroelectric phase is equal to 0.52 0.08  , thus agreeing well with the results [2,12]. This  value is peculiar for the temperature behaviours of both the heat capacity and the thermal expansion coefficients for the crystals that stay under conditions close to the tricritical point on their phase diagram and, moreover, it does not agree with the value 0.42   .…”

“…The careful recent studies for the thermal diffusivity [2] have shown that the critical exponent  derived from these data amounts to 0.5 for all the temperature range outside the region of critical fluctuations. Moreover, it has also been demonstrated that anomalous part of the thermal diffusivity in the ferroelectric phase is well described in the frame of the mean-field theory outside the critical fluctuation region [2]. The same value  has been obtained on the basis of ultrasonic velocity measurements [12].…”

“…The transition is accompanied by the point symmetry group change 2 / m F m and by appearance of spontaneous electric polarisation, which is almost parallel to the crystallographic axis a . Many authors have noticed that the phase transition in Sn 2 P 2 S 6 is close to a number of specific polycritical points on the (x, T)-phase diagram of Sn 2 P 2 (Se x S 1-x ) 6 solid solutions, namely a tricritical point (see, e.g., the recent work [2][3][4][5]), a Lifshitz point [1,3,6], or a tricritical Liftshitz point [7]. The results of our recent work [5] have testified that, at x = 0.28, the tricritical point should occur on the (x, T)-phase diagram of Sn 2 P 2 (Se x S 1-x ) 6 rather than the Lifshitz one, whereas the triple point characterised by the appearance of incommensurately modulated phase should occur at some higher concentrations of selenium (e.g., at 0.4 x  ).…”

Peculiarities of thermal expansion in ferroelectric lead germanate crystals with nonstoichiometry

“…Кристали германату свинцю Pb 5 Ge 3 O 11 (скорочено ГС) -це одновісні сегнетоелектрики, що зазнають фазового переходу (ФП) другого роду типу зміщення при температурі 450  C T К зі зміною точкової симетрії 3 6  [1]. Незважаючи на загалом добре розуміння механізму цього ФП, кристали ГС продовжують привертати значну увагу дослідників (див., наприклад, [2][3][4][5][6][7][8]). Крім суто фундаментального інтересу до цих модельних сегнетоелектриків, це пояснюється й перспективними можливостями їхніх практичних застосувань, зокрема як піроелектричних матеріалів, елементів сегнетоелектричної пам'яті та матеріалів для запису голограм.…”

Comparison of critical fluctuations in Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals

“…They also reveal characteristics very promising for practical applications [2]. Recently there has been an upsurge in the interest of researchers in those crystals, which has included studies by more refined experimental techniques and re-examination of some obscure points concerned with their phase diagram (see the works [3][4][5][6][7], among many examples).…”