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

Abstract

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 polarization and deformation plays an important role in the mechanisms of the displacive transitions from a paraelectric to ferroelectric or incommensurate phase in the vicinity of a Lifshitz point on the state diagram. Layered CuInP₂S₆ crystals undergo a first order and order-disorder type phase transition to a ferrielectric phase. For these crystals, different polarization mechanisms within a wide frequency range were clearly observed and found to be interrelated via dielectric and Raman spectroscopy measurements. These mechanisms include the internal vibrations of (P₂S₆)^4- anions; the external lattice modes in which In³⁺ and Cu⁺ ions participate; the relaxational critical slowingdown due to Cu⁺ ions which yield the order – disorder phase transition; and the low frequency relaxational dynamics which causes the “throw” of Cu⁺ ions into the interlayer space thus providing ionic conductivity.

Key Words:

• Phase transitions
• ferroelectrics
• ferrielectrics
• dielectric properties
• Raman scattering
• lattice dynamics