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

Abstract

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 Tc1=417 K; into incommensurate phase at Tci=261 K, into ferroelectric phase (space group I2) at Tc3=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 Tc2=305 K. In temperature region Tc2>T>Tc1 ND4DSeO4 has momoclinic structure with the space group I2 (Z=3). The phase (x, T) diagram of N(H1-xDx)4 H1-xDxSeO4 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