Phase transitions in the ferroelectrics (Sn1−x In(2/3)x )2P2S6 at high pressures

New ferroelectric solid solutions (Sn_1−x In_(2/3)x )₂P₂S₆ were investigated at high hydrostatic pressures. The range in which the incommensurate structure exists was determined. A dynamic shift of the incommensurate-ferroelectric phase transition temperature with increasing rate of change of temperature and the appearance of “reverse hysteresis” were observed. The characteristic features in the appearance of the latter effect in these crystals are investigated. Fiz. Tverd. Telsa (St. Petersburg) 41, 1276–1278 (July 1999)     

About the existence of a Lifshitz point on the phase diagram of Sn2P2(Se x S1– x )6 solid solutions: acoustic and optical studies

On the basis of combined studies of the acoustic and optical properties of Sn₂P₂(Se _x S_{1– x} )₆ solid solutions with x?=?0 and 0.28 it is shown that two special points exist on the x,T-phase diagram of these ferroelectric crystals, a tricritical point and a triple one. With increasing Se concentration, the phase transition changes its character at x?≈?0.28 from second-order to first-order. The phase transition becomes split at x?≈?0.4, with the appearance of an intermediate incommensurate phase.     

Dynamic shift of the phase-transition temperature in (Sn1−x In(2/3)x)2P2S6 crystals with incommensurate phase

The dielectric properties of ferroelectric (Sn_1?x In_(2/3)x )₂P₂S₆ crystals were studied in the region of incommensurate phase transitions occurring upon fast cooling and heating (0.2–11 K/min). The dynamic phase-transition shift observed in these materials was examined.     

Influence of Covalence on the Metal-Insulator Temperature in the CuIr 2 (S 1− x Se x ) 4 Spinels

Influence of the covalence on the metal-insulator (M-I) temperature in the CuIr 2 (S 1 m x Se x ) 4 spinel series is considered. For this purpose a vacancy model and the effective number of valence electrons per molecule are used. Calculations of the vacancy model parameters, i.e., the ion packing coefficients, the filling coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing selenium concentration x , leading to a decrease of the metal-insulator temperature. It was observed that the metal-insulator temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the metal-insulator transition. These effects are explained within the framework of the mixed valence of the iridium ions including structural defects.     

Photovoltaic structures using AgSb(S x Se1−x )2 thin films as absorber

Photovoltaic structures were prepared using AgSb(S _x Se_1?x )₂ as absorber and CdS as window layer at various conditions via a hybrid technique of chemical bath deposition and thermal evaporation followed by heat treatments. Silver antimony sulfo selenide thin films [AgSb(S _x Se_1?x )₂] were prepared by heating multilayers of sequentially deposited Sb₂S₃/Ag dipped in Na₂SeSO₃ solution, glass/Sb₂S₃/Ag/Se. For this, Sb₂S₃ thin films were deposited from a chemical bath containing SbCl₃ and Na₂S₂O₃. Then, Ag thin films were thermally evaporated on glass/Sb₂S₃, followed by selenization by dipping in an acidic solution of Na₂SeSO₃. The duration of dipping was varied as 3, 4 and 5 h. Two different heat treatments, one at 350 °C for 20 min in vacuum followed by a post-heat treatment at 325 °C for 2 h in Ar, and the other at 350 °C for 1 h in Ar, were applied to the multilayers of different configurations. X-ray diffraction results showed the formation of AgSb(S _x Se_1?x )₂ thin films as the primary phase and AgSb(S,Se)₂ and Sb₂S₃ as secondary phases. Morphology and elemental detection were done by scanning electron microscopy and energy dispersive X-ray analysis. X-ray photoelectron spectroscopic studies showed the depthwise composition of the films. Optical properties were determined by UV–vis–IR transmittance and reflection spectral analysis. AgSb(S _x Se_1?x )₂ formed at different conditions was incorporated in PV structures glass/FTO/CdS/AgSb(S _x Se_1?x )₂/C/Ag. Chemically deposited post-annealed CdS thin films of various thicknesses were used as window layer. J–V characteristics of the cells were measured under dark and AM1.5 illumination. Analysis of the J–V characteristics resulted in the best solar cell parameters of V _oc = 520 mV, J _sc = 9.70 mA cm^?2, FF = 0.50 and η = 2.7 %.     

Elastic properties of (Pb y Sn1−y )2P2S6 solid solutions

Elastic properties of (Pb _y Sn_1?y )₂P₂S₆ solid solutions were studied using Brillouin scattering technique. Different scattering geometries were used for sound velocities determination that make it possible to find all components of the stiffness tensor. The concentration dependencies of volume compressibility, the Grüneisen parameter and Debye temperature were investigated. The results obtained were used to analyze chemical bonding with substitution of tin by lead at room temperature in the crystals under consideration.     

Thermal diffusivity and 3D-XY critical behavior of ferroelectric semiconductors (PbxSn1−x)2P2Se6

An ac photopyroelectric calorimeter has been used to study the thermal diffusivity of the ferroelectric semiconductors family (Pb_xSn_1−x)₂P₂Se₆ (x=0–1) from 30 K to room temperature. Phase transitions have been found for x=0, 0.05, 0.2, 0.47 but not for x=1 in the full temperature range. A continuous phase transition has been found for x=0, 0.05, 0.2 and 0.47 which corresponds to the paraelectric commensurate to incommensurate phase. It has been possible to study the critical behavior of this transition for x=0, 0.05 and the critical parameters obtained have been α=−0.019, A⁺/A⁻=1.00 and α=−0.026, A⁺/A⁻=1.03, respectively, having fitted at the same time both the low and high temperature branches of the transition as rigorous critical theory indicates; these results agree with the theoretical prediction from renormalization group theory that this kind of transition complies with the 3D-XY universality class (α_theor=−0.014, A⁺/A⁻=1.06), which has been experimentally confirmed only in a few materials. A first order incommensurate to ferroelectric phase transition has been characterized in x=0, 0.05 at lower temperature.     

Depth profile of the lattice constant of the Cu-poor surface layer in (Cu2Se)1−x(In2Se3)x evidenced by grazing incidence X-ray diffraction

The surface of CuInSe₂ or related compounds is an essential part of heterojunctions for photovoltaic applications. It is generally accepted that the reconstruction of the surface of slightly Cu-deficient film results in a Cu-depleted defect layer. Since this surface layer generally exhibits a CuIn₃Se₅ stoichiometry the existence of a defect chalcopyrite phase CuIn₃Se₅ has also been proposed. Although efforts have been made to identify this phase structurally by means of grazing incidence X-ray-diffraction (GIXRD), it has not been detected so far. However, the existence of a Cu-depleted surface layer fits well in the existing theory, which models the functioning of photovoltaic devices. In this contribution we present for the first time evidence for a Cu-depleted surface layer measured by X-ray diffraction. A careful analysis of GIXRD-data at incident angles from 0.3 to 10° by comparing the measured shape and width with simulated spectra confirms structural changes in the surface layer. The thickness of this layer varies from 5 to 60 nm with the integral Cu-content of the film. Hence GIXRD provides a unique means for non-destructive depth profiling.     

Electrical and thermal properties of a-(Se70Te30)100−x(Se98Bi2)x (0⩽x⩽20) alloys

In electrical properties, the dc conductivity and photoconductivity measurements have been made in vacuum evaporated thin films of a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system, in the temperature range (308–355 K). It has been observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation has also been observed which follow the power law (I_ph∝F^γ). Transient photocurrent exhibits the non-exponential decay time. All these parameters show that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system.     

Structural deformations of the cubic lattice of the Zn1 ? x Fe x Se (x = 0.001) crystal

The structural state of a Zn_{1 ? x} Fe _x Se (x = 0.001) crystal has been studied using thermal neutron diffraction. The diffraction patterns of the cubic crystal have been found to contain diffuse scattering regions concentrated in the vicinity of the strong Bragg reflections. It has been shown that the diffuse scattering effects are due to local transverse displacements of the crystal lattice atoms, and these displacements are induced by iron ions that demonstrate the static Jahn-Teller effect of the tetragonal type in the ZnSe compound.     

Structural features of the system of cubic crystals Zn0.999Fe0.001S1 ? x Se x (x = 0, 0.2)

The structural state of cubic single crystals Zn_0.999Fe_0.001S_{1 ? x} Se _x (x = 0, 0.2) obtained by the chemical transport method has been investigated using thermal neutron diffraction for the first time. It has been found that the diffraction patterns of these crystals contain the previously unknown effects of diffuse scattering caused by local statistic atomic displacements in the metastable fcc lattice.     

Critical behavior of single crystal CuCr2Se4−x Br x (x=0.25)

The doping of bromine has prominent effect on the transport properties of CuCr₂Se₄, where the conductivity of CuCr₂Se_4?x Br _x changes from metallic behavior with x=0 to semiconductor with x=1. In this work, the critical behavior of single crystal CuCr₂Se_4?x Br _x with x=0.25, which lies at the boundary between the metallic and insulating state, is investigated through the magnetization under high magnetic field. The critical exponents β=0.39±0.03 and γ=1.25±0.05 are obtained by the Kouvel–Fisher method, and δ=4.79±0.01 is generated by the critical iso-thermal analysis. The obtained exponent β is close to the prediction of the three-dimensional (3D) Heisenberg model, while γ approaches that of the 3D-Ising model. Although the bromine doping has prominent effect on the transport behavior, the critical exponents are hardly changed. The obtained results indicate that the ferromagnetism is established through one-dimensional Cr–Se–Cr bonds. However, the nearest-neighbor magnetic interaction belonging to the 3D-Heisenberg model also contributes to the ferromagnetic exchange.     

Features of the plasma sputtering of polycrystalline Pb1 − x Sn x S films

The processes of the sputtering and modification of surfaces of polycrystalline films of the ternary solid solution Pb_{1 ? x} Sn _x S (x = 0.9–1.0) in a high-density Ar plasma of high-frequency low-pressure inductive discharge are studied. Films with thicknesses of 1–4 μm are grown on glass substrates using the “hot-wall” method and consist of plate-like crystallites. It is established that the sputtering rate for lead-tin sulfide films does not exceed 2.0 nm/s, which is determined by the presence of oxygen-containing compounds on the surfaces. In the case of plate-like crystallites with nanodimensional thicknesses, the effect of smoothing of the developed surfaces of the polycrystalline Pb_{1 ? x} Sn _x S layers during plasma treatment is observed; this is important for fabricating multilayer device structures.     

237Np Mössbauer Investigations of (U1−x Np x )2Rh2Sn

Surprisingly, Np₂Rh₂Sn does not order magnetically whereas the uranium counterpart U₂Rh₂Sn orders antiferromagnetically at 24 K with a 5f moment μ _U ≈0.38μ _B . We have investigated the magnetic and electronic properties of (U_1−x Np _x )₂Rh₂Sn solid solutions. For x=0.25 and 0.5, the ordering temperature decreases to 11 K whereas the Np-rich compound (x=0.75) shows the onset of magnetic order around T≈6 K. The average Np magnetic moment amounts to 0.84 μ _B ,0.83μ _B and 0.25 μ _B respectively. The isomer shift slightly decreases, from −9.6 mm/s to −10.4 mm/s (versus NpAl₂) as x increases. The values of the quadrupole interaction parameter in the ordered and paramagnetic state suggest that Np moments are parallel to c for x=0.25 and then rotate to the basal plane for higher x. This revised version was published online in September 2006 with corrections to the Cover Date.     

Vortex structure in BaFe2(As1 − x P x )2 single crystals

Single crystals of iron-containing Ba(FeAs)₂ superconductors doped with phosphorus have been studied. The vortex structure has been studied by means of the decoration method. Large domains of the triangular vortex structures have been observed in the phosphorus-doped single crystals of iron-containing superconductors. The effect of doping on pinning in iron-containing superconductors has been discussed.     

p(x) and Tc(x) characteristics in the Y 1−b(Ca)bBa2Cu3O6+x cuprate family

A model has been proposed to calculate the $p\left(x\right)$ and $T_c\left(x\right)$ dependences in the Y _1?b(Ca)_bBa₂Cu₃O_6+x high-$T_c$ cuprate family and applied to $b=0$, $b=0.1$, and $b=0.2$ cases, for which experimental data exist in the literature. The results obtained imply that the Ca efficiency to provide holes is independent of the basal plane oxygen concentration, which is consistent with a view that electrons from CuO₂ layers would go primarily to Ca since it is twice closer than oxygen (in addition, the chain oxygen is screened by a layer made up of Ba and O(4) ions). It is shown that, in fully oxygenized compounds ($x=1$) the average efficiency, $\chi$, of a chain oxygen to attract an electron from the two nearby layers is reduced by the Ca insertion, though not because the charge transfer mechanism is in itself weakened by Ca, but because a part of electrons that are otherwise available in CuO${}_2$ layers has already been removed by the substitution of Y ³⁺ with Ca²⁺. It has been found that the $b$-dependence of the average oxygen doping efficiency can be fairly accurately described by the following relation: $\chi \left(b\right)=0.39\times (1?0.78b)$. The calculated $p\left(x\right)$ and $T_c\left(x\right)$ dependences are in very good agreement with the available experimental data.     

Effect of Zn impurity in K0.8Fe(2-δ-x)Zn(x)Se2

A series of K(0.8)Fe(2-δ-x)Zn(x)Se(2) single-crystal samples with nominal compositions 0?≤?x?≤?0.05 were grown and their physical properties were measured in order to study the effect of Zn impurity. It is found that the Zn impurity (x?≤?0.02) does not affect the superconducting transition temperature T(c) significantly. Meanwhile the hump in resistivity which corresponds to the transition from the insulating to metallic phase quickly shifts towards low temperatures. The results imply that there should be a phase separation in this system and Zn impurity causes the enhancement of the insulating phase. The negligible effect of Zn impurity on T(c) suggests an s-wave pairing in the superconducting phase. Meanwhile there is a possibility that the Zn impurity may selectively enter into the insulting phase.     

A theoretical study of the non-adiabatic charge transfer process Ar2+ (3 P) + He(1 S) → Ar+(2 P) + He+(2 S)

CI calculation with a large basis have been used to calculate the two lowest ³Π adiabatic potential energy curves for the title reaction. These potentials have been transformed to diabatic potentials by employing a recipe based on the CI coefficients. Quantum mechanical close coupling calculations in the diabatic basis have produced total and differential cross sections which are in good agreement with experimental data. Full quantum mechanical and Landau-Zener calculations of the total cross section are in fair agreement with recent experimental measures and by small changes to the diabatic potentials can be brought into essentially exact agreement.     

Correlation between the magnetic and electrical properties of the (VS)x(Fe2O3)2−x oxysulfide system

The correlation between the magnetic and electrical properties of the (VS)_x(Fe₂O₃)_2?x (0.9<x<1.25) oxysulfide solid solutions has been studied. The crossover of conductivity from the semimetallic to semiconducting type is accompanied by changes in the magnetic susceptibility, which are characteristic of the transition from delocalized to localized electrons. For x=1.25, a region of the ferromagnetic ordering has been established in the temperature range 90–120 K.     

Preparation and properties of substances in the series Cd2−xAg2xP2Se6, 0⩽x⩽2

Cd₂P₂Se₆ - Ag₄P₂Se₆ system was investigated and Cd_2−xAg_2xP₂Se₆ mixed crystals have been found for 0<x<0.3. Their conductivity has been found to have ionic character. Silver ion conductivity values are reported. From diffuse reflectance data, the bandgaps for Cd₂P₂Se₆ and Ag₄P₂Se₆ have been estimated as equal to 2.4 eV and 1.8 eV, respectively.