A. Kvedaravičius

The nature of anharmonicity and phase transition in SbSBr crystal

The ferroelectric crystal structure of SbSBr is composed of [Sb(S, Br)]∞ chains parallel to the [001] axis. In paraelectric phase, the crystal structure is disordered. The theoretical investigation of dependence of potential energy VP (z) of Sb atoms on amplitudes of B1u vibration symmetry coordinates along c (z)-axis have showed that each atom is shifted from the xy mirror plane by up or down along [001]. The theoretical investigation of the average potential energy of all Sb atoms in unit cell revealed that in paraelectric phase in region of phase transition has double-well shape. This strong anharmonicity of is created by interaction between phonons.

Origin of Ferroelectric Phase Transitions of BixSb1-xSI Mixed Crystals

The nature of normal modes’ anharmonism of SbSI—type crystals is investigated in this work. In order to explain the nature of anharmonism we apply the normal vibration modes average potential energys dependence on amplitudes of normal coordinates along c (z)—axis. It is proved that the variation of potential barrier between two wells of double-well potential energy Vp(z) is caused by temperature T, quasihidrostatic pressure p, and mixture x. The temperature dependence of ferroelectric phase transition on quasihidrostatic pressure p and mixture x is determined for SbSI and BixSb1-xSI crystals. Applying the equality is theoretically proved that there is no ferroelectric phase transition in BixSb1-xSI crystals where x > 0.7. The ferroelectric phase transition in SbSI—type crystals and BixSb1-xSI crystals is caused by the phonon interactions dependence on temperature, pressure and mixture.

The Nature of the Ferroelectric Phase Transition in the Modified SbSI Ceramics

The modified SbSI (ceramic) has been grown by Bridgman method. The dielectric permitivity of modified SbSI along c(z)-axis has been measured using LCR meter as a function of temperature at fixed frequency 1 KHz. The nature of ferroelectric phase transition in the modified SbSI has been discussed using ab initio calculations. Theoretical investigations reveal how ferroelectricity and phase transition in the modified SbSI are closely related to deformation of the unit cell and the phonon interaction. Dielectric measurements reveal that substitution of the individual (I−) with (Cl−) ions creates the deformation of the unit cell and increase the temperature of the ferroelectric phase transition TC. Deformations of the unit cell change the interaction between phonons and the anharmonicity of soft B1u normal mode.

Lattice Dynamics of Ferroelectric SbSBr Crystal

The temperature dependence of phonon dispersion, total and partial phonon DOS of SbSBr crystal have been determined in paraelectric phase at T = 30 K and in ferroelectric phase at T = 11 K using FP—LAPW method in the framework of DFT along with the GGA approximation. Temperature dependence of total and partial phonon DOS for Sb, S and Br atoms where used for calculation of the vibration free energy of the unit cell Fvib and contribution from Sb, S and Br atoms to the free energy Fi,μ along c(z) —axis in quasiharmonic approximation in paraelectric and ferroelectric phases in temperature range from 11 K to 294 K. For investigation of softening effect nature of phonons along c(z) —axis the mean-squared displacements of Sb, S and Br atoms in x -, y -, z - direction have been calculated in quasiharmonic approximation in ferroelectric and paraelectric phases.

The Nature of Piezoelectricity of Ferroelectric SbSI Crystal

The contribution of Sb atoms soft mode to the temperature dependences of the piezoelectric modulus e(T) and atomic equilibrium positions difference Δz(T) has been studied theoretically, when SbSI crystal is deformed along crystallographic x(a), y(b) and z(c)—axes. The largest change of e(T) occurs in the ferrophase near the phase transition (when T ≈ TC ). At T < TC , the e(T) changes become slow. It has been found that value and anomalous temperature dependence of e(T) and Δz(T) is influenced by the change of mean potential energy of Sb atoms in soft mode due to its interaction with phonons.

Investigation of the Vibration Spectra of a SbSBrxI1-x Crystal in Harmonic and Quasiharmonic Approximation

The vibration spectra of SbSBrxI1-x (x = 0 ÷ 0.75) crystal have been determined in quasiharmonic approximation by diagonalization of dynamical matrix. Applying the model atom A with its own parameters (mass and force constant) we were able to get exact normal modes’ vibration frequencies. The obtained normal modes’ frequencies were superposed with experimental data. It is proved that in Brillouin zones point k = 0 the lower frequency IR (infrared) mode of B1u symmetry and lower frequency R (Raman) mode of B1g symmetry are soft-alike modes whereas in Brillouin zones point k = 0.5 an acoustic and Raman mode become active, but lower frequency IR mode becomes inactive. The interaction between phonons creates the anharmonicity of soft-alike IR, R and A vibration modes.