Dipole ordering and critical behavior of the static and dynamic propertiesin three-dimensional and layered MM ' P2X6 crystals (M, M '-Sn, Cu, In; X-S, Se)

The thermodynamic properties and lattice dynamics of crystals MM'P2X6 With cations in different valence states have been investigated. For Sn2P2S(Se)( 6) crystals with a three-dimensional lattice, the interaction between polar ization and deformation plays an important role in the mechanisms of the di splacive transitions from a paraelectric to ferroelectric or incommensurate phase in the vicinity of a Lifshitz point on the state diagram. Layered Cu InP2S6 crystals undergo a first order and order-disorder type phase transit ion to a ferrielectric phase. For these crystals, different polarization me chanisms within a wide frequency range were clearly observed and found to b e interrelated lia dielectric and Raman spectroscopy measurements. These me chanisms include the internal vibrations of (P2S6)(4-) anions; the external lattice modes in which In3+ ' and Cu+ ions participate; the relaxational c ritical slowing-down due to Cu+ ions which yield the order-disorder phase t ransition; and the low frequency relaxational dynamics which causes the "th row" of Cu+ ions into the interlayer space thus providing ionic conductivit y.