The electronic potential of SbSI crystal in the region of phase transition

Gaigalas²,

Žigas³

et al. 2005

Open Physics

This paper presents the theoretical investigation of energy levels of valence bands (VB) and core levels (CL) of the ferroelectric SbSI single crystals in antiferroelectric and ferroelectric phases. Since the best approximation for the deep VB levels is a calculation by the Hartree-Fock method, the molecular model of a SbSI crystal was used for calculations. This model of the crystal was also used for calculations of the total density of states. It was found that the VB and CL of this ferroelectric semiconductor are sensitive to the small lattice distortion at the phase transition, and that an average of the total density of states, when all atoms participate in oscillations of all normal modes, are more similar to the experimental X-ray photoelectron spectra (XPS). The experimental splitting of CL obtained by XPS was compared with the theoretically calculated one by two different methods. The cluster model calculations showed that the splitting of the CL in SbSI might be caused by photoelectron emission from the atoms, which have different valence state, at the surface.

Section: Introductionmentioning

confidence: 99%

Theoretical investigation of the electronic structure of a ferroelectric SbSI cluster at a phase transition

Gaigalas²,

Žigas³

et al. 2005

Open Physics

“…Strong exponential absorption edge anomalies of SbSI and SbSBr crystals are observed in the temperature range from T c − T to T c + T when E|| c and E⊥ c (here T c = 295 K and T = 10 K for SbSI; T c = 22.8 K and T = 10 K for SbSBr) [12,15,16]. Band structure dependence on λ mode phonon amplitude at the point k in the Brillouin zone is described by the second-order perturbation theory [17]:…”

Section: Resultsmentioning

confidence: 99%

Origin of the Optical Anomalies Near the Ferroelectric Phase Transition in SbSI and SbSBr Crystals

Ferroelectrics Letters Section

Žigas²,

Žaltauskas³

et al. 2008

Self Cite

Experimental investigation of SbSI exponential absorption tail of slope σ/kT (T ) and isoabsorption energy E K (T ) at K = const reveal that in ferroelectric phase transition region at T c the slope σ/kT (T ) has anomalous behavior. Explanation of this phenomena presented using electronic potential V (z) dependence upon high frequency mode B 1u symmetry normal coordinates in c(z) direction taking into account thermal fluctuations of the atoms in x − y plane. The splitting of B 1u normal modes in double-well V (z) explains anomalous behavior σ/kT (T ) at the ferroelectric phase transition region in SbSI and SbSBr crystals.

“…On passage into the AFEP (T C2 > T > T C1 = 295 K) and FEP (T < T C1 ), the position parameters normal to the c-axis are substantially unchanged. In AFEP (T = 300 K) the displacements of equilibrium position occur for all Sb3 atoms along the c-axis z 0 = 0, 02 Å and for all Sb4 atoms (z 0 = −0, 02 Å) [37]. In FEP (T < T C1 ) all Sb and S atoms move along the c-axis toward these I sites, which leads to the removal of the mirror plane symmetry.…”

Section: Investigation Of Sbsi Crystal Valence Bandmentioning

confidence: 99%

“…Lukaszewicz et al [17] determined the crystal structure of SbSI in the temperature region at 170-465 K. They found that there are three phases in the phase diagram of SbSI: ferroelectric phase (FEP) III (space group Pna2 1 ) below T C1 = 295 K, antiferroelectric phase (AFEP) II (P 2 1 2 1 2 1 ) in the temperature region (T C2 = 295-410 K), and high temperature paraelectric phase (PEP) I above 410 K. AFEP is characterized by the double polar chains [(SbSI) 1 ] 2 , which are ordered antiferroelectrically: these cause a disorder in the crystal lattice. Experimental investigation of X-ray absorption, fluorescence, and electron emission [37,38] adjusts the energy band structure of SbSxSe1−xI (x = 0.25; 0.5; 1), SbSBr. Its explanation necessitates for a more precise calculation of the contribution of separate atoms to the total density of states.…”

Section: Introductionmentioning

confidence: 99%

Electronic Structure and Piezoelectric Properties of SbSI Crystals

Piezoelectric Materials

Žigas²,

Sereika³

et al. 2016

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 B 1u symmetry in the microwave range and semisoft modes in the IR range. The A u and B g 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 A u symmetry normal mode interacting with the degenerate A u symmetry electronic states in the valence band top. The pseudo-Jahn-Teller effect is induced due to the same mode interacting with A u symmetry electronic states in the valence band and B g 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 crystallographicx(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 p (z) of Sb atoms in soft mode.