Electronic Structure and Piezoelectric Properties of SbSI Crystals

The SbSI crystals are investigated in the paraelectric and ferroelectric phases. The calculations have been performed to find the symmetry and normal coordinates of vibrational modes. We have observed that potential energy with double well create the soft mode of B1u symmetry in the microwave range and semisoft modes in the IR range. The Au and Bg symmetry, top electronic levels of the highest valence band, are degenerate in the paraelectric phase. It is shown that the Jahn-Teller effect is caused by Au symmetry normal mode interacting with the degenerate Au symmetry electronic states in the valence band top. The pseudo-Jahn-Teller effect is induced due to the same mode interacting with Au symmetry electronic states in the valence band and Bg symmetry states in the conduction band bottom. Concerning these two effects, the normal mode force constant K decreases and vibrational constants undergo changes during the phase transition. The theoretical deformation along the crystallographic—x(a), y(b), and z(c)—axes were studied for Sb atoms. The major change of piezoelectric modulus takes place in the ferroelectric phase near the phase transition temperature. At lower temperatures piezoelectric modulus changes become slow. The value as well as anomalous temperature dependence of piezoelectric modulus and Δz(T) is influenced by the change of mean potential energy V¯pz of Sb atoms in soft mode

SbSI kristalų juostinės sandaros ir optinių savybių tyrimas pseudopotencialo metodu [Rankraštis] : daktaro disertacija : fiziniai mokslai, fizika, kondensuotosios medžiagos (P 260)

Bibliogr.: p. 102-106, 108Vytauto Didžiojo universitetasŠvietimo akademij

Electronic band structure and optical properties of SbSI crystals studied by pseudopotential

Darbas atliktas 1992-2002 m. Vilniaus pedagoginiame universitete, Kietojo kūno optikos laboratorijoje, Bendrosios fizikos katedrojeAut. darbų sąrašas: p. 32Vytauto Didžiojo universitetasŠvietimo akademij

Birefringence and refractive indices of ferroelectric SbSI

Electronic structure and refractive indices of SbSI crystal in paraelectric and ferroelectric phases were investigated by full-potential linearized augmented plane wave method with density functional theory. The temperature dependence of refractive indices along a-, b-, and c-axes and birefringence Dn¼nc na as a function of photon energy were calculated near the phase transitions. The theoretical results were compared with experimental measurements of birefringence. Comparison between calculated increment of the birefringence (Dn0¼nc na) and experimental spontaneous polarization P2s indicates the existence of the second-order phase transition at Tc2 240K and confirms its relation to the PsVytauto Didžiojo universitetasŠvietimo akademij

Antiferroelectric phase transition in SbSI and SbSeI crystals

This work represents existence of the high temperature antiferroelectric phase transitions in SbSI, SbSeI and BiSeI crystals. The antiferrolectric phase transition is found by a measured capacitance change at a frequency of 1 kHz for the SbSI and SbSeI crystals grown by the Bridgman–Stockbarger technique in the temperature range of 270–440 K. Therefore, SbSI has three phases: ferroelectric (), antiferroelectric () and paraelectric (). SbSeI has two phases: antiferroelectric () and paraelectric (). The antiferroelectric phase transition is of an intermediate type between displacive and order–disorder typesVytauto Didžiojo universitetasŠvietimo akademij

Experimental activities in physics teaching: joint work with teachers

Vytauto Didžiojo universitetasŠvietimo akademij

Origin of ferroelectric phase transition in SbOxS1−xI mixed crystals

art. no. 1550167The dependence of the potential energy V(z) of Sb atoms on B1u symmetry normal coordinates for SbOxS1−xI crystal in ferroelectric phase with a varying mixture x 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 V(z) implies that SbOxS1−xI crystals exhibit intermediate behavior between displacement and order–disorder type phase transitions. The phase transition temperature TC depends on the mixture composition xVytauto Didžiojo universitetasŠvietimo akademij

Optical properties of the BiSI crystal

Vytauto Didžiojo universitetasŠvietimo akademij

Reflection and vibrational spectra of SbSCl(0.1)I(0.9) crystals in the ferroelectric phase-transition region

SbSCl(0.1)I(0.9) crystals were grown from the vapor phase and reflection spectra were recorded using a Fourier-transform IR spectrometer. The optical parameters and optical functions along the c axis were calculated using an improved Kramers–Kronig technique with two confining spectral limits. The reflection spectra were analyzed using the oscillator parameter fitting technique for comparison of results. The vibrational frequencies ωL and ωT were evaluated. The ferroelectric phase-transition temperature was estimated as 330 K for SbSCl0.1I0.9 from experimental reflection measurements and theoretical investigation of the potential energy of Sb atoms for B1u soft normal modes of SbSI and SbSCl(0.1)I(0.9) crystalsVytauto Didžiojo universitetasŠvietimo akademij

Origin of ferroelectric phase transition in SbSClxI1−x mixed crystals

art. no. 1450209The measurements of SbSClxI1−x (x = 0.2) temperature dependent capacitance were carried out. The temperature of ferroelectric phase transition TC ≈ 340 K was measured experimentally. TC of SbSClxI1−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 outVytauto Didžiojo universitetasŠvietimo akademij