OPTICAL INVESTIGATIONS OF PHASE TRANSITIONS IN (NH4) 2 SbF5 CRYSTAL

Results of optical thickness and refractive indices measurements for (NH4)2SbF5 crystal in the temperature range 100-310 K are presented. The measurements were done with interference method using He-Ne laser (λ = 632.8 nm). At To = 292 K and Ta = 168 K two continuous phase transitions with critical indices /3 1 = 0.38 and β2 ≤ 0.57 were observed. Additional anomaly of temperature changes of interference order with maximum at Ta 132 K were found. This anomaly resembles a diffused transition. Temperature-averaged values of derivatives of refractive indices dn g /dT = -3.3 x 10 -5 K and dn m /dT = -2.9 x 10 -5 K -1 evidenced a decrease in optical anisotropy (birefringence) along the b-axis on heating run

Critical indices of the ferroelectric phase transition in TGS crystals

Temperature dependencies of retardation, electron susceptibility and linear thermal expansion for three crystal-physic directions are obtained by means of optical investigations of the ferroelectric phase transition in TGS crystal using the James-type interferometer. Temperature dependencies of the spontaneous changes of the characteristics studied in the 39–49◦ C range are fitted by the power low Y ~ τ²β with double critical indices 2β=0.87–0.95. Difference of 2β values from the unity is explained by the essential temperature dependence in the range close to the phase transition point for the coefficients of electrooptic, reversed piezoelectric and electrostriction effects.Шляхом оптичних вимірювань сегнетоелектричного фазового переходу в кристалі тригліцинсульфату за допомогою інтерферометра Жамена одержано температурні залежності оптичної рiзниці ходу, електронної сприйнятливості та лінійного розширення для трьох кристалофізичних напрямків. Температурні залежності спонтанних змін досліджуваних характеристик в області 39–49◦ C апроксимовані степеневими залежностями Y ~ τ²β з подвійними критичними індексами 2β=0.87–0.95. Відмінність 2β від одиниці пояснюється суттєвою температурною залежністю поблизу точки фазового переходу коефіцієнтів електрооптичного, оберненого п’єзоелектричного ефектів та електрострикції

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.).Обгрунтовується новий кристалооптичний метод вимірювання температури на базі інверсії знаку двозаломлення ∆n та температурної залежності ∆n(T). Наведено відповідні характеристики деяких кристалів, які підтверджують високі метрологічні та експлуатаційні характеристики методу. Відзначається, що метод розв’язує деякі проблеми, що виникають перед термометрією на базі термоелектрики і термоопору, а також відкриває більш високі можливості вимірювань температури у важких умовах (в області високих електричних та магнітних полів, деталей, що обертаються і ін.)

Structural and morphological properties of CdSe1-xSx thin films obtained by the method of high-frequency magnetron sputtering

CdSe1-xSx (x= 0.3, 0.4 and 0.6) thin films were deposited on quartz and silicon substrates by the method of high-frequency magnetron sputtering. The chemical composition analysis and crystal structure refinement was examined with using X-ray fluorescence spectroscopy and X-ray diffraction data. CdSe1-xSx thin films crystallizes in hexagonal structure (structure type – ZnO, space group P63mc (No. 186)). The lattice parameters (a, c and V), crystallite size (D), strain (ε), dislocation density (δ) and the texture coefficient TC(hkl) was estimated from X-ray diffraction analysis. Units-cell parameters decrease with increasing S content in CdSe1-xSx thin film

Optical refractive properties and phonon spectra of Na2SO4 single crystal

Na2SO4 crystal was grown by slow evaporation method from an aqueous solution. Using the PXRD method, the crystal structure of Na2SO4 was investigated at room temperature, and it was found that the crystal belongs to the orthorhombic symmetry of the space group Fddd no. 70 with the following unit cell parameters a 5.8568 4 , b 12.2957 7 , c 9.809 1 , and V 706.52 8 3. The dispersion of the refractive indices of Na2SO4 crystal in the visible and ultraviolet ranges was measured at room temperature and the principle refractive indices of optical indicatrix were determined to be nX 1.4703, nY 1.4847, and nZ 1.47669 for the wavelength amp; 955; 500 nm. The first principle calculations of the phonon properties of the crystal were performed using the lattice dynamics method and DFPT approach. The polarized normal incidence reflection spectra of Na2SO4 crystal were measured in the infrared range 50 1300 cm amp; 8722;1 by using FTIR spectrometer and synchrotron radiation. The calculated reflection spectra of Na2SO4 crystal are in very good agreement with the experimental ones. In Na2SO4 crystal, the Reststrahlen band in the spectral range of 1120 1200 cm amp; 8722;1 is confirmed experimentally and theoreticall

Investigations of free electrons in doped silicon crystals derived from Fourier transformed infrared measurements and ab initio calculations

The reflection spectra of n and p type silicon crystals doped with phosphorus and boron were measured for the free carrier concentrations of 1.1 1015 cm amp; 8722;3 amp; 8722; 1.2 1020 cm amp; 8722;3 in the far and mid infrared range between 20 3000 cm amp; 8722;1 using synchrotron radiation and Fourier transformed infrared technique. Transmittance spectra could be measured for lower sample carrier concentrations from the range studied. The measured reflection spectra were fitted by using the Drude relation and the parameters of free electron conductivity electron effective mass m and momentum scattering time amp; 61556; were obtained for the n and p type doped silicon. Additionally, the calculations of the band electronic structure and the electric conductivity amp; 61555; of the crystals were performed in the framework of the density functional theory for different carrier concentrations and temperatures. The study main findings are 1 the substantial decrease of the momentum scattering time amp; 61556; and 2 the clear increase of the electron effective mass m with an increase of the carrier concentrations Nc for both n and p type doped silicon crystal

Optical absorption spectra in the far infrared range and phonons of CdSe1 xTex thin films

Optical reflection spectra of CdSe1 xTex thin films deposited on quartz substrates are measured using the synchrotron radiation in the spectral range of 20 500 cm amp; 8722;1. The absorption bands of CdSe, CdTe, CdSe0.75Te0.25, CdSe0.5Te0.5, and CdSe0.25Te0.75 films are localized in the range of 20 220 cm amp; 8722;1. The imaginary parts of the dielectric function amp; 949;2 amp; 955; amp; 8722;1 of CdTe1 xSex crystals calculated in the framework of the density functional theory are in good agreement with the experimental reflection spectra of CdTe1 xSex films. The eigenvectors of the dynamical matrix CdTe1 xSex crystals are analyzed for several phonon modes to understand the difference between the frequency dependences of the calculated vibration density of states and the imaginary part of dielectric function amp; 949;2 amp; 955; amp; 8722;1 . Small features of the experimental reflective spectra of CdSe1 xTex films in the ranges 50 70 cm amp; 8722;1 and 80 120 cm amp; 8722;1 are explained using the analysis of results of the molecular dynamics. During comparative molecular dynamics calculations, it was found that the vibration density of states of a thin CdTe slab with a surface to volume number of atoms relation of NS NV 0.2, experienced a redshift of approximately 30 cm amp; 8722;1. This shift was observed in comparison with the CdTe single crysta

Band electronic structure and dielectric functions of (C3N2H5)2SbF5 crystals

Structural and electronic properties of the ferroelastic crystal (C3N2H5)2SbF5 of the molecular type were studied by ab initio methods in the framework of the density functional theory. Band electronic structure, density of electronic states and dielectric functions in the range of valence electrons excitations of the crystal in the monoclinic phase (space group no. 11) have been obtained using the plane waves, ultrasoft pseudopotentials and van-der-Waals corrections. The electronic values obtained are discussed from the viewpoint of the layer-type crystal structure of (C3N2H5)2SbF5.Strukturalne i elektronowe właściwości ferroelektrycznego kryształu (C3N2H5)2SbF5 typu molekularnego zostały obliczone w ramach teorii funkcjonału gęstości (DFT) z wykorzystaniem odpowiedniej metody z pierwszych zasad (ab initio). Pasmowa struktura elektronowa, gęstość stanów elektronowych i funkcje dielektryczne w zakresie wzbudzenia elektronów walencyjnych kryształu zostały obliczone dla strukturalnej fazy jednoskośnej (grupa przestrzenna no. 11) z wykorzystaniem płaskich fal, super pseudopotencjałów miękkich i uwzględnienia poprawek na oddziaływania międzyatomowe typu van-der-Waalsa. Otrzymane wielkości elektronowe zostały omówione pod kątem warstwowej struktury krystalicznej (C3N2H5)2SbF5