A new higher-order Landau–Ginzburg–Devonshire theory for KNbO₃ crystal

ABSTRACT

A Landau-Ginzburg-Devonshire(LGD) theory based on a structurally stable 8th-order potential with three varying expansion coefficients (a₁, a₁₁ and a₂) was proposed for phenomenological description for KNbO₃. The phase diagrams in the space of phenomenological parameters of the LGD potential model with co-dimension (Codim) of 2 and 3, invariant under the cubic point group m3m, were constructed. The values of the phenomenological coefficients of the potential model were determined by fitting experimental data. The theoretical pressure-temperature phase diagram which can explain the experimentally observed cubic-tetragonal-orthorhombic triple point and cubic-orthorhombic phase transition was constructed. Temperature dependences of spontaneous strain, lattice constants, dielectric constants and spontaneous polarization calculated with this potential model were in good agreement with the experimental data. Furthermore, the influence of the hydrostatic pressure on the dielectric constant (ε₃₃) in the vicinity of the triple point and the dependence of piezoelectric coefficients on the applied electric field were predicted.

KEYWORDS:

• KNbO₃ crystal
• LGD theory
• structural stability
• phase transitions
• ferroelectric properties

ORCID

Kye-Ryong Sin http://orcid.org/0000-0003-4084-8703