Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroelectric and displacive phase transitions by describing what happens in the low-temperature phase in terms of phonon softening and instability. A new derivation of the generalized Lyddane-Sachs-Teller (LST) relation for materials with strong anharmonic phonon damping is also presented which leads to the expression $arepsilon_{0}/arepsilon_{infty}=|omega_{LO}|^{2}/|omega_{TO}|^{2}$. The theory provides a microscopic expression for $T_c$ as a function of physical parameters, including the mode specific Gr"uneisen parameter. The theory also shows that $omega_{TO} sim (T_{c}-T)^{1/2}$, and again specifies the prefactors in terms of Gr"uneisen parameter and fundamental physical constants. Using the generalized LST relation, the softening of the TO mode leads to the divergence of $epsilon_0$ and to a polarization catastrophe at $T_c$. A quantitative microscopic form of the Curie-Weiss law is derived with prefactors that depend on microscopic physical parameters.

Soft mode theory of ferroelectric phase transitions in the low-temperature phase / L. Casella, A. Zaccone. - In: JOURNAL OF PHYSICS. CONDENSED MATTER. - ISSN 0953-8984. - 33:16(2021 Jan 13). [10.1088/1361-648X/abdb68]

Soft mode theory of ferroelectric phase transitions in the low-temperature phase

A. Zaccone
Ultimo
2021

Abstract

Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroelectric and displacive phase transitions by describing what happens in the low-temperature phase in terms of phonon softening and instability. A new derivation of the generalized Lyddane-Sachs-Teller (LST) relation for materials with strong anharmonic phonon damping is also presented which leads to the expression $arepsilon_{0}/arepsilon_{infty}=|omega_{LO}|^{2}/|omega_{TO}|^{2}$. The theory provides a microscopic expression for $T_c$ as a function of physical parameters, including the mode specific Gr"uneisen parameter. The theory also shows that $omega_{TO} sim (T_{c}-T)^{1/2}$, and again specifies the prefactors in terms of Gr"uneisen parameter and fundamental physical constants. Using the generalized LST relation, the softening of the TO mode leads to the divergence of $epsilon_0$ and to a polarization catastrophe at $T_c$. A quantitative microscopic form of the Curie-Weiss law is derived with prefactors that depend on microscopic physical parameters.
Lyddane-Sachs-Teller relations; dielectrics; ferroelectrics; lattice dynamics; structural phase transitions
Settore FIS/03 - Fisica della Materia
Settore FIS/02 - Fisica Teorica, Modelli e Metodi Matematici
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/825262
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