Birefringence of mechanically stressed Rb 2 ZnC l4 crystals
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HYSICAL PROPERTIES OF CRYSTALS
Birefringence of Mechanically Stressed Rb2ZnCl4 Crystals V. Yu. Kurlyaka, V. Yo. Stadnyka, V. B. Stakhuraa, and Z. O. Kohutb a Ivan b
Franko National University of Lviv, Lviv, 79005 Ukraine Lviv Polytechnic National University, Lviv, 79013 Ukraine e-mail: [email protected] Received October 22, 2014
Abstract—The spectral and temperature dependences of the birefringence of mechanically free and uniaxially stressed Rb2ZnCl4 crystals have been investigated. It is shown that annealing a crystal leads to step changes in the birefringence at the phase transition from the incommensurate to commensurate phase, whereas the birefringence of as-prepared crystals changes continuously. It is established that the birefringence of Rb2ZnCl4 crystal is sensitive to uniaxial pressure; however, uniaxial stress does not change the character of its dispersion and temperature dependences. A new “pseudoisotropic” state of this crystal and a baric shift of the points of paraelectric phase–incommensurate phase–commensurate phase transitions to different temperature regions are found. DOI: 10.1134/S1063774515060164
INTRODUCTION Rubidium tetrachlorozincate Rb2ZnCl4 (RTCZ) crystals are typical representatives of one-dimensional modulated incommensurate structures of the А2ВХ4 type. They undergo a sequence of phase transitions (PTs) standard for these crystals: paraelectric phase (PP) (Ті = 302 K) → incommensurate phase (IP) (Тс = 192 K) → commensurate ferroelectric phase [1–3]. The high-temperature phase І of RTCZ crystal is paraelectric; it is characterized by space group Pnam (mmm) and number of formula units Z = 4, like for crystals of the K2SеO4 group. The intermediate phase ІІ (Тс < Т < Ті) is incommensurately modulated in the a direction with wave vector q = (1 – δ)a/3 (δ is the incommensurability parameter) [2, 3]. The low-temperature phase ІІІ (Pna21 (mm2), q = a/3 [1–3]) is an improper ferroelectric phase with spontaneous polarization along the c axis and a triple unit-cell parameter along the pseudohexagonal a axis. Further cooling of an RTCZ crystal leads to its phase transformation at 74 K into a monoclinic structure with a peculiar space group C1c1, which is doubled with respect to the Pna21 structure along the b and c axes and has 48 formula units [3, 4]. The IP of RTCZ crystal was investigated by electron diffraction. At temperatures below Тi the diffraction pattern contains additional diffuse spots, corresponding to the occurrence of additional modulation in the a direction in the IP [5, 6]. The temperature dependences of birefringence Δni of RTCZ crystal, measured previously by the Senarmont method at one wavelength, showed that devia-
tions from the linear dependence δ(Δni) are observed in the initial phase at temperatures much lower than Ті. The IP-commensurate phase (CP) transition is accompanied by small step variations in δ(Δni). The temperature hysteresis was found to be ~1 K [7–9]. A study of the electro-optical effect showed the absence of linear effect in the IP, because it macrosc
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