B. Andriyevsky

Electronic and Optical Properties of Strontium Barium Niobate Single Crystals

Electronic structure of Sr x Ba1- x Nb2O6 crystals in the energy range involving the valence and conduction bands has been studied theoretically and experimentally using the DFT-based ab initio VASP code and the spectroscopic ellipsometry method with synchrotron radiation. The Sr0.61Ba0.39Nb2O6 sample of the congruently melting composition is characterized by the greatest difference of the experimental dielectric function ϵ(E) compared to the non congruently melting compositions of Sr x Ba1- x Nb2O6 (x = 0.40, 0.55, 0.65, 0.75). Electronic band structures of Sr0.60Ba0.40Nb2O6 and LiNbO3 crystals in the energy range of 6–8 eV and the corresponding dielectric functions ϵ(E) in the photon energy range of 3–10 eV have been found to be similar, what is caused by similar octahedral NbO6 complexes in the crystals.

Manifestation of phase transformations in optical spectra of Na0.5Bi0.5TiO3 crystals between 25°C and 350°C

The pseudo dielectric function ⟨ε⟩ (E) = ⟨ε 1⟩ (E) + i⟨ε 2⟩ (E) of sodium bismuth titanate Na0.5Bi0.5TiO3, has been obtained in the spectral range of electronic excitations (2–9.5 eV) by spectroscopic ellipsometry using the synchrotron radiation source BESSY II. The spectrum contains a broad absorption band at 4.2–4.5 eV. The temperature dependence in the range of 25–350°C is presented by the related susceptibilities ⟨χ 1⟩ (T) and ⟨χ 2⟩ (T) at 5 eV. A clear peculiarity in the temperature behavior of ⟨χ 1⟩ (T) and ⟨χ 2⟩ (T) between 180 and 320°C has been revealed and discussed. In this range an extremum-like temperature dependence of intensity of the light reflected from the sample surface, has been revealed and can be explained by the diffraction of light on the grid of the elastic domain structure of the sample.

Band structure and optical spectra of ferroelectric triglycine sulphate

First-principal calculations of the band structure, density of electronic states, and dielectric functions of the triglycine sulphate crystal (TGS) together with the corresponding experimental dielectric functions in the range of 4–10 eV have been presented. Calculations have been performed in the frame of density functional theory using the CASTEP code. The experimental dielectric functions ε′(ω) and ε″(ω) have been obtained for different temperatures by ellipsometry using the synchrotron radiation source BESSY II. The theoretical dielectric functions ε′(ω) and ε″(ω) of TGS crystal agree satisfactorily with corresponding experimental dependencies. The character and magnitude of temperature changes of dielectric functions in the vicinity of phase transition point depend on the crystal orientation and photon energy. The greatest maximum-like temperature changes of dielectric functions observed at the phase transition point are detected for the photon energy ω ≈ 9.5 eV.

Effect of hydrostatic pressure on structural and electronic properties of TGS crystals (first-principle calculations)

First principle calculations of the effect of hydrostatic pressure on the structural and electronic parameters of TGS crystals have been carried out within the framework of density functional theory using the CASTEP code. The volume dependence of total electronic energy E(V ) of the crystal unit cell satisfies the third-order Birch-Murnaghan isothermal equation of state. For the pressure range of 5...5 GPa, the bulk modulus was found to be equal to K = 45±5 GPa. The relative pressure changes of the unit cell parameters were found to be linear in the range of 5...5 GPa. Crossing of the pressure dependencies of enthalpy corresponding to the ferroelectric and non-ferroelectric phases at P = 7.7 GPa testifies to the probable pressure induced phase transition in TGS crystal.

Crystal optical method for temperature measuring

A new crystal optical method for temperature measuring based on the sign inversion of birefringence ∆n and temperature dependence of ∆n(T ) is substantiated. The respective characteristics of some crystals are presented, which confirm good metrological and exploitation properties of the method. It is noted that the method solves some problems which arise in thermometry based on thermoelectricity and thermoresistance. The proposed method also gives better possibilities for measuring the temperature in hard conditions (in the region of high electric and magnetic fields, rotating workpieces etc.).

Spectral Ellipsometry Study of SBN Single Crystals in Visible and Ultraviolet Region

The pseudo-dielectric function (E) = (E) + i(E) of strontium-barium niobate single crystals, Sr x Ba1-x Nb2O6 (SBN), was studied at room temperature for five nominal compounds indices x (x = 0.40, 0.55, 0.61, 0.65, 0.75) by spectroscopic ellipsometry using the synchrotron light ellipsometer in the photon energy range of 1.5– 10 eV. The spectra of (E) obtained appear to be similar to the analogous spectrum of LiNbO3 crystal, where the NbO6 octahedral groups are responsible for the features of (E). Differences in the dielectric function of SBN with various x have been found and analyzed. The nonlinear dependences of optical values on the compound index x have been found in SBN.

Electronic Properties of KDP and DKDP Crystals: Ab-Initio Calculations and Spectral Ellipsometry Experiment

Electronic band structure, density of states and complex dielectric function ϵ(E) of KDP (KH2PO4) crystal at Fdd2 and space groups of symmetry corresponding to the ferroelectric and paraelectric phases, have been calculated within the density functional theory using the VASP code. The experimental dielectric function ϵ(E) of KDP and DKDP (KD2PO4) crystals have been studied by the spectroscopic ellipsometry method in the photon energy range of 5–18 eV using synchrotron radiation. Temperature dependences of the dielectric function ϵ(T) and the intensity of reflected light IR (T) of KDP and DKDP crystals have been measured and discussed.

Dielectric properties of (NH4)2SO4 crystals in the range of electronic excitations

Spectra of the real and imaginary parts of the pseudo-dielectric permittivity, (1)(E) and (2)(E), of ferroelectric ammonium sulfate crystals, (NH(4))(2)SO(4), have been measured in the range of electronic excitations 4.0 to 9.5 eV by ellipsometry using synchrotron radiation. Temperature dependences of the corresponding susceptibilities, chi(1)(T) and chi(2)(T), obtained for the photon energy E = 8.5 eV, related to excitations of oxygen p-electrons, reveal sharp peak-like temperature changes near the Curie point T(C) = 223 K. The large temperature-dependent increase of the imaginary part of the susceptibility chi(2)(T), together with a simultaneous decrease of the real part of the susceptibility chi(1)(T), take place at the phase transition. These anomalies have been ascribed mainly to the SO(4) group of the crystal structure.

Influence of uniaxial stresses on electronic and optical properties of β-K2SO4 crystal

Abstract In view of possible practical applications of transparent crystals for mechanical stress sensors, theoretical investigation of piezo(elasto)optic effect in crystals might be useful for searching for proper materials, possessing large coefficients of elasto-optic effect, and for determination of uniaxial stress directions of maximum piezo-optical sensitivity. The influence of the uniaxial stresses on the electronic band structure, density of states, and optical properties of potassium sulphate crystal β-K2SO4 have been studied in the framework of the density functional theory using the ab initio CASTEP code. Increase in the band gap, Eg of the crystal takes place for three crystallographic directions and for moderate uniaxial stresses, σ < 1.5 GPa. Dependencies of principal refractive indices and polarizabilities upon principal uniaxial stresses are discussed together with other features of electronic structure of the crystal.

Ultraviolet vacuum ultraviolet optical functions for SrTiO3 and NdGaO3 crystals determined by spectroscopic ellipsometry

Complex dielectric functions e(E) = e1(E) + e2(E) were experimentally evaluated within the spectral range E = 2–25 eV and E = 2–20 eV for SrTiO3 and NdGaO3 single crystals, respectively, using synchrotron-based spectroscopic ellipsometry measurements. The ellipsometric spectra were evaluated within a framework of optical layer model taking into account sample surface roughness and anisotropy of NdGaO3. The parameters of Herzinger-Johs oscillator model were fitted to reproduce sufficiently all features of the optical spectra within the spectral range 2–10 eV. Only slight differences were revealed for spectra polarized along b and c crystallographic axes of the NdGaO3, which can confirm weak optical anisotropy.