The Investigation of the Ferroelectric Phase Transition in Cluster Systems of Order-Disorder Type III. Free Energy

1991 ◽  
Vol 163 (2) ◽  
pp. 355-367 ◽  
Author(s):  
I. R. Yukhnovskii ◽  
N. A. Korynevskii
Keyword(s):  
Phase Transition ◽  
Free Energy ◽  
Ferroelectric Phase Transition ◽  
Ferroelectric Phase ◽  
Cluster Systems ◽  
Type Iii ◽  
Disorder Type

Origin of ferroelectric phase transition in SbSClxI1-x mixed crystals

2014 ◽  
Vol 28 (30) ◽  
pp. 1450209 ◽  
Author(s):  
A. Audzijonis ◽  
L. Žigas ◽  
R. Sereika ◽  
R. Žaltauskas
Keyword(s):  
Phase Transition ◽  
Free Energy ◽  
Temperature Dependence ◽  
Ferroelectric Phase Transition ◽  
Ferroelectric Phase ◽  
Harmonic Approximation ◽  
Temperature Dependent ◽  
Normal Coordinate ◽  
Unit Cell Parameters ◽  
Cell Parameters

The measurements of SbSCl x I 1-x(x = 0.2) temperature dependent capacitance were carried out. The temperature of ferroelectric phase transition TC ≈340 K was measured experimentally. TC of SbSCl x I 1-x was calculated theoretically in anharmonic and harmonic approximations. TC was calculated in anharmonic approximation using temperature dependence of mean potential energy of Sb atoms as a function of the soft B1u symmetry normal coordinate along c(z)-axis. Moreover, TC was calculated in harmonic approximation using temperature dependence of vibrational thermodynamic functions (Helmholc free energy). TC dependence from unit cell parameters a, b and from mixture composition x was carried out.

Origin of ferroelectric phase transition in SbOxS1−xI mixed crystals

2015 ◽  
Vol 29 (23) ◽  
pp. 1550167
Author(s):  
A. Audzijonis ◽  
L. Žigas ◽  
R. Žaltauskas ◽  
R. Sereika
Keyword(s):  
Phase Transition ◽  
Transition Temperature ◽  
Potential Energy ◽  
Ferroelectric Phase Transition ◽  
Ferroelectric Phase ◽  
Normal Coordinates ◽  
Mixture Formula ◽  
Composition Formula ◽  
Energy Formula ◽  
Disorder Type

The dependence of the potential energy [Formula: see text] of Sb atoms on [Formula: see text] symmetry normal coordinates for [Formula: see text] crystal in ferroelectric phase with a varying mixture [Formula: see text] is investigated in the current paper. The potential energy of Sb atoms is characterized by a high anharmonicity within the temperature range 295–377 K. The large anharmonicity of the potential energy [Formula: see text] implies that [Formula: see text] crystals exhibit intermediate behavior between displacement and order–disorder type phase transitions. The phase transition temperature [Formula: see text] depends on the mixture composition [Formula: see text].

scholarly journals The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe

2021 ◽  
Vol 7 (1) ◽  
Author(s):  
Đorđe Dangić ◽  
Olle Hellman ◽  
Stephen Fahy ◽  
Ivana Savić
Keyword(s):  
Phase Transition ◽  
Thermal Conductivity ◽  
Phase Transitions ◽  
Thermoelectric Materials ◽  
Ferroelectric Phase Transition ◽  
Lattice Thermal Conductivity ◽  
Ferroelectric Phase ◽  
Structural Phase ◽  
High Symmetry ◽  
Structural Phase Transitions

AbstractThe proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity κ. However, the κ of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in κ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating κ based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates κ near the phase transition. Our findings elucidate the influence of structural phase transitions on κ and provide guidance for design of better thermoelectric materials.

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