A new representation of the thermodynamic potential of lithium niobate

Within the framework of the phenomenological theory new thermodynamic models of lithium niobate with Landau potentials of the fourth and sixth degrees have been constructed. The coefficients of these potentials have been calculated using known experimental values of the material constants of the linear equations of the piezoelectric effect, as well as electro-optical and acousto-optical constants obtained at room temperature. When constructing the potentials, the temperature behavior of the permittivity, spontaneous polarization and deformation has been taken into account. Both potentials allow calculating the full set of piezoelectric, electro-optical and acousto-optical constants of lithium niobate in a wide range of temperature changes. Computational experiments have been carried out to calculate the deformation and spontaneous polarization in the range of 300–1400 K. The results of calculations obtained using the sixth-degree potential have shown good agreement with the results of experimental studies.