What is it about?
KNbO3 is one of good lead-free ferroelectric–piezoelectric materials and environmentally friendly. We have proposed a new Landau thermodynamical potential model with 3 varying expansion coefficients, with which cubic-tetragonal-orthorhombic triple point near 15GPa and -73℃ in the experimental phase diagram of KNbO3 has been explained for the first time. We have calculated the temperature dependences of spontaneous strain, lattice constants, spontaneous polarization and dielectric constants, the electric field dependence of piezoelectric coefficients , and the behaviors of dielectric constant ( ) of KNbO3 crystal under various temperatures and pressures.
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Why is it important?
Phenomenological Landau theory is an efficient approach to study the phase transitions and properties of the condensed matter such as crystals. However, the phase transitions under several external parameters such as temperature and pressure can’t be explained exactly by Landau theory in which only the second order coefficient of the thermodynamical potential is assumed to be dependent on temperature. Therefore, based on the 8th-order potential model in which only the second order coefficient is assumed to be dependent on temperature and pressure, the cubic-tetragonal-orthorhombic triple point and the cubic-orthorhombic phase transition observed in KNbO3 crystal above 15.5GPa can’t be explained. The results of catastrophe theory show that structurally stable thermodynamical potential models with several expansion coefficients varying with external parameters such as temperature and pressure are quite necessary for exact description on qualitative features of phase transitions under variation of some external thermodynamical parameters. Our finding shows that one can explain reasonably the phase transitions in KNbO3 crystal including triple point with the 8th order thermodynamical potential model in which the second order coefficient and the two fourth order ones vary with temperature and hydrostatic pressure.
Perspectives
It is my pleasure that the possibility to explain qualitatively complex phenomena in condensed matter such as crystal on the basis of structural stable phenomenological models has been shown through this article. I am sure that the semi-empirical thermodynamical theory, Landau-Ginzburg-Devonshir theory, could be further developed on the basis of rigorous catastrophe theory.
Chol-Jin Kim
Kim Il Sung University
Read the Original
This page is a summary of: A new higher-order Landau–Ginzburg–Devonshire theory for KNbO3 crystal, Phase Transitions, October 2018, Taylor & Francis,
DOI: 10.1080/01411594.2018.1536269.
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