Phase Transitions in Layered Semiconductor - Ferroelectrics

IntroductionCuInP 2 S 6 crystals represent an unusual example of an anticollinear uncompensated twosublattice ferroelectric system (Maisonneuve et al., 1997).They exhibit a first-order phase transition of the order-disorder type from the paraelectric to the ferrielectric phase (T c = 315 K).The symmetry reduction at the phase transition (C2/c to Cc) occurs due to the ordering in the copper sublattice and the displacement of cations from the centrosymmetric positions in the indium sublattice.X-ray investigations have shown that Cu ion can occupy three types of positions (Maisonneuve et al., 1997).The ordering of the Cu ions (hopping between Cu 1 u and Cu 1 d positions) in the double minimum potential is the reason for the phase transition dynamics in CuInP 2 S 6 .In (Maisonneuve et al., 1997) it was suggested that a coupling between P 2 S 6 deformation modes and Cu + vibrations enable the copper ion hopping motions that lead to the onset of ionic conductivity in this material at higher temperatures.At low temperatures a dipolar glass phase appears in CuInP 2 S 6 weakly doped with antiferroelectric CuCrP 2 S 6 or ferroelectric CuInP 2 Se 6 (Macutkevic et al., 2008).The copper chromium thiophosphate CuCrP 2 S 6 crystallizes in a layered two-dimensional structure of the Cu I M III P 2 S 6 (M = Cr, In) type described above (Maisonneuve et al., 1995).It is formed by double sheets of sulfur atoms sandwiching the metal cations and P-P groups which occupy the octahedral voids defined by the sulfur atoms.At room temperature the crystal structure has a space group of C2/c (Colombet et al., 1982).At 64 K, the Cu positions are confined to those of an antiferroelectric order where the crystal structure has the space group of Pc (Maisonneuve et al., 1995).Thus, the mechanism of the dielectric transition is likely to involve hopping of the copper ions among two or more positions.Two phase transitions have been observed at 155 K and 190 K by dielectric measurement and differential scanning calorimetry (DSC).The crystal is antiferroelectric below 155 K and paraelectric above 190 K.For the intermediate phase between 155 and 190 K, a quasiwww.intechopen.com