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

The thermodynamic functions of ferroelectric and paraelectric SbSI

A first-principle method is used to calculate phonon density of states, Helmholtz free energy, internal energy, and entropy for ferroelectric and paraelectric SbSI. Theoretical phase transition temperature was obtained using the difference of the Helmholtz free energy, internal energy, and entropy term between ferroelectric and paraelectric phases on temperature. The obtained value is in reasonable agreement with the experimental second-order phase transition temperature Tc2 = 233 KVytauto Didžiojo universitetasŠvietimo akademij

On the heat capacities of SbSI and SbSBr

We investigated the heat capacities of SbSI and SbSBr using ab-initio calculations based on the density-functional theory. The full-potential linearized augmented plane wave method was used with the generalized gradient approximation. To benchmark our calculations, we compared the lattice heat capacity at constant volume with the available experimental data. Our investigations indicate that theoretical results from first-principles are consistent with experimental measurements. The observed discrepancies are created due to anharmonic effects at the phase transition region and may be influenced by crystal growing techniques or by crystal deviation from stoichiometryVytauto Didžiojo universitetasŠvietimo akademij

The thermodynamic functions of SbSBr crystal

Using density functional theory methods, the phonon density of states, Helmholtz free energy, internal energy, and entropy of ferroelectric and paraelectric phases are investigated. The temperature dependence of the free energy indicates that vibrational entropy contributes to the destabilization of the ferroelectric phase. The vibrational entropy of Sb, S, and Br atoms is attributed to the stabilization of ferroelectric SbSBr at the temperature Tc. Calculations indicated that SbSBr in ferroelectric phase become more stable than in paraelectric phase at temperatures lower than 22.8 K. The calculated temperature of ferroelectric phase transition is in good agreement with the experimental dataVytauto Didžiojo universitetasŠvietimo akademij

Electronic structure and electron charge density in the interatomic bonds of BiSBr and BiSeBr crystals

Electronic structure and electronic charge density in the interatomic bonds are investigated with ab initio calculations based on the density-functional theory. The full potential linearized augmented plane-wave method was used with the generalized gradient approximation. Considering the partial density of states the electron charge density distribution in the Bi, S, Se and Br atomic bonds is caused by Bi-6p, S-3p, Se-4p, Br-4p orbital hybridization. Electronic charge distribution of one BiSBr and BiSeBr molecule range suggest that the Bi–S, Bi–Se and Bi–Br bonds are covalentionic type. Bi–S and Bi–Se bonds are strong covalent with a not great ionicity factor (fP i = 0.060, Bi–S; fP i = 0.039, Bi–Se). Bi–Br bonds are covalent type with a larger ionicity factor (fP i = 0.147, Bi–Br)Vytauto Didžiojo universitetasŠvietimo akademij

Electron-phonon interaction in BiSeI and BiSI crystals

Vytauto 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

Puslaidininkinių-feroelektrinių kristalų virpesių spektrų ir elektroninės struktūros tyrimas [rankraštis]

Vytauto 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

Electronic conduction model in ferroelectrics based on phonon-assisted tunneling emission

Vytauto Didžiojo universitetasŠvietimo akademij