Thermodynamics of Ferroelectric and Optical Properties in KNbO3

TL;DR

This paper develops a thermodynamic model for KNbO3 that unifies ferroelectric and optical properties, accurately predicting temperature-dependent behaviors and optical dispersion without additional fitting.

Contribution

It introduces a comprehensive thermodynamic framework that separates lattice and electronic polarization contributions, enabling precise predictions of ferroelectric and optical responses in KNbO3.

Findings

Model accurately predicts temperature dependence of electro-optic coefficients.

Quantitative agreement with experimental dielectric and refractive index data.

Captures optical dispersion in near infrared to visible spectrum.

Abstract

Potassium niobate (KNbO3) is a prototypical perovskite ferroelectric with large electro-optic and nonlinear optical responses, high optical damage thresholds and a rich sequence of temperature-driven phase transformations, making it a promising platform for tunable photonic devices. In this work, we develop a thermodynamic model for the coupled ferroelectric and optical properties of KNbO3. By separating the total polarization into lattice and electronic contributions, the model provides a unified description of both the anisotropic ferroelectric and optical properties. The thermodynamic coefficients are determined by fitting to experimental measurements of the spontaneous polarization, dielectric susceptibilities, lattice parameters, and refractive indices. Without any further fitting, the model quantitatively predicts the temperature dependence of the electro-optic and piezoelectric coefficients in close agreement with experimental measurements. By utilizing the electronic polarization equation of motion, the model further captures the optical dispersion in the near infrared to visible spectrum. This work provides the thermodynamic foundation for future studies of coupled ferroelectric and optical phenomena in KNbO3.