The Dipole Ordering and the Ionic Conductivity in the NASICON-Like Structures of the Na3Sc2(PO4)3 Type

Abstract

Problems of the dipole ordering and ionic conductivity of Na₃Sc₂(PO₄)₃ polycrystal that has three polymorphic phases—α, β, and γ—are studied. The features of the α-Na₃Sc₂(PO₄)₃ crystal structure and the dipole ordering and relaxation polarization in the α and β phases are specified. The origin of the dipole ordering in the α phase and the partial dipole disordering in the β-Na₃Sc₂(PO₄)₃, as well as the high ionic conductivity in the β and γ phases of the Na₃Sc₂(PO₄)₃ polycrystal are associated with phase transformations α → β, β → γ that result in structural changes of the rhombohedral {[Sc₂(PO₄)₃]^3–}_3∞ crystalline framework. A model explaining the emergence of the dipole ordering and the ionic conductivity phenomena in Na₃Sc₂(PO₄) is proposed.