The model of phase transitions in the crystals of the NH₄HSeO₄ type

Abstract

NH₄HSeO₄ crystals and their deuterated analogs undergo phase transitions and posess a number of interesting properties. Experimental investigations of the crystals by NMR, X-ray diffraction, optical and other methods were realized in ^1-4: All crystals have a high temperature phase of still not defined symmetry. In this phase an increase of proton conductivity is observed ². With temperature decrease the NH₄HSeO₄ crystals undergo the successive phase transitions: into paraelectric phase (space group I1) at T_c1=417 K; into incommensurate phase at T_ci=261 K, into ferroelectric phase (space group I2) at T_c3=98 K. There are three molecules per unit cell in monoclinic and triclinci phases ¹. Monoclinic phase at temperatures below T=271 K, incommensurate and apprently ferroelectric phases are unstable since after being for several hours in this temperature region the crystal undergoes phase transition into orthorhombic phase (P2₁2₁2₁) and turns to have four molecules per unit cell ^3,4. The orthorhombic phase is observed for deuterated crystals with deuterium concentration higher thant 45% below T_c2=305 K. In temperature region T_c2>T>T_c1 ND₄DSeO₄ has momoclinic structure with the space group I2 (Z=3). The phase (x, T) diagram of N(H_1-xD_x)₄ H_1-xD_xSeO₄ is shown in Fig.1. Dependently on the conditions the crystal can have one of two following successions of phase transitions: superionic phase → monocolinic phase → orthorhombic phase; superionic phase → monoclinic phase → incommensurate phase → ferroelectric phase → low-temperature phase