Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

High-pressure calculations of the elastic properties of ZnSx Se 1−x alloy in the virtual-crystal approximation

We report first-principles calculation results on the structural and elastic properties of ZnS _x Se_1?x alloy. The calculations done using density functional theory within the local density approximation and employing the virtual-crystal approximation. It is found that the lattice parameter, the phase transition pressure, and the elastic constants (and their derivative with respect to the pressure) follow a quadratic law in x.     

Vibrational properties of one- and two-mode behaviour in spinel type mixed systems Zn1−xCdxCr2S4

The infrared reflectivity spectra of spinel type mixed crystals Zn_1−xCd_xCr₂S₄ have been measured from 40 to 900 cm⁻¹. Four or five bands were observed at both end members in the region of intermediate composition. The three bands in higher energy show one-mode behaviour and the other two bands show two-mode behaviour. The concentration dependences of these mode frequencies are interpreted by a model proposed already, in which the 2 basic units and the 6 force constants are contained. By comparing the force constants associated with the Zn-site in several spinels, it is suggested that the two-mode behaviour arises from the large difference in the force constants between the two end members. The E_g (260 cm⁻¹) and T_2g (160 cm⁻¹) Raman modes for ZnCr₂S₄ were also observed and used for the determination of the force constants.     

Thermoelectric properties of the Nd2−x Ce x CuO4−y system

The temperature dependence of the Seebeck coefficient (in region 300–900 K) and the lattice constants of Nd_2–x Ce_xCuO_4–y (x=0, 0.05, 0.10, 0.15) with different oxygen content were measured. The value of S is always negative and it decreases with both Ce content and oxygen non-stoichiometry. At a certain concentration of Ce and oxygen in the material, the Seebeck coefficient becomes temperature-independent.The authors would like to thank Professor J. Horák of University of Chemical Technology, Pardubice for helpful discussions.     

Thermoelectric properties of Cr1−xMoxSi2

The thermoelectric properties of Mo-substituted CrSi₂ were studied. Dense polycrystalline samples of Mo-substituted hexagonal C40 phase Cr_1−xMo_xSi₂ (x=0–0.30) were fabricated by arc melting followed by spark plasma sintering. Mo substitution substantially increases the carrier concentration. The lattice thermal conductivity of CrSi₂ at room temperature was reduced from 9.0 to 4.5 W m⁻¹ K⁻¹ by Mo substitution due to enhanced phonon–impurity scattering. The thermoelectric figure of merit, ZT, increases with increasing Mo content because of the reduced lattice thermal conductivity. The maximum ZT value obtained in the present study was 0.23 at 800 K, which was observed for the sample with x=0.30. This value is significantly greater than that of undoped CrSi₂ (ZT=0.13).     

Magnetic properties of solid solutions of the system ErxY1−xFe2

The magnetic properties of the system Er_xY_1–xFe₂ (MgCu₂ structure) in the temperature range 80–800°K are investigated. The magnetization and the coercive force H_c are measured as a function of T. For alloys with x= 0.5–1 there is a magnetic compensation point _c. The composition dependence of _c, the magnetic moment of the alloy and of the iron atoms Fe are determined. An H_c anomaly is observed in _c. In order to explain the results obtained, the presence of two types of magnetically active atoms and three types of exchange interaction are included.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 10, pp. 27–30, October, 1982.     

Specific heat of EuxSr1−xO near the ferromagnetic phase transition

High-resolution measurements of the specific heat,C, near the ferromagnetic phase transition of the diluted ferromagnetic system Eu_xSr_1–xO are reported. Samples with four different concentrations (x=1,x=0.9,x=0.7 andx=0.5) have been studied and the expected phase diagram, i.e. the linear decrease ofT _c with decreasingx, is confirmed. Our specific-heat data of the pure EuO samples yield a critical exponent =–0.12±0.02 in contrast to literature results, but in agreement with the value expected theoretically for this three-dimensional Heisenberg system. The origin of the discrepancies is traced down to differences in data analysis.     

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnS_xSe_1–x polycrystalline thin films were prepared by evaporation in vacuum of 10^–5 Torr. The molecular fractionx varied in the region ox1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficient) were determined in the wavelength range 300–1600nm. A plot representing ₂=f(hv) shows that the ZnS_xSe_1–x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+. The variation in eitherE orE+ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.     

Effect of pressure on electrical properties of SnS x Se1−x single crystals

The single crystals of tin monosulphoselenides in the form of a series SnS _x Se_1?x (where x?=?0, 0.25, 0.50,0.75 and 1) have been grown using the direct vapor transport technique (DVT). The analysis of the X-ray diffraction patterns reveals that all crystals belong to the orthorhombic crystal structure. Hall effect measurements were carried out on grown crystals at room temperature. The optical absorption measurements at room temperature have been carried out for all crystals. The values of the band gap were determined at atmospheric pressure and also calculated at high-pressure. Simultaneous thermoelectric power (TEP) and a.c. resistance measurements up to 8?GPa were carried out. The results of the effect of high-pressure on the electrical resistance of the grown crystals are reported in this paper.     

Neutron diffraction studies of the diluted spinel ferrite ZnxMg0.75−xCu0.25Fe2O4

Neutron diffraction studies have been performed in the temperature range 1050 K⩾T⩾10 K on the spinel system Zn_xMg_0.75−xCu_0.25Fe₂O₄ (x=0.0, 0.25, 0.5 and 0.75) prepared in the ceramic sintering method. Cation distributions in all the four compositions have been determined from the analysis of neutron data. A perturbed ferrimagnetic order has been observed in the compositions with x values lying in the low to intermediate range, where apparently, randomly canted spins and small fluctuating clusters are superimposed on the ferrimagnetic long range order. A complete breakdown of the ferrimagnetic stability occurs at x=0.75 as indicated by the absence of magnetic long range order. A huge diffuse scattering signal appears at the low-Q region broadening the (1 1 1) diffraction line in the low temperature neutron patterns of this composition indicating the formation of large magnetic clusters. A cluster spin glass state is suggestive for this composition.     

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.     

Critical properties of superconducting Ba1−xKxFe2As2

Magnetisation and magneto-resistance measurements have been carried out on superconducting Ba_1?xK_xFe₂As₂ samples with x = 0.40 and 0.50. From high field magnetization hysteresis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The J_C determined from the high field data is >10⁴ A/cm² at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 T. From the variation of the T_C onset with applied field, dH_C2/dT at T_C was obtained to be ?7.708 T/K and ?5.57 T/K in the samples with x = 0.40 and 0.50.     

Effect of Cd content on the structural characteristic and some physical properties of Zn1−x Cd x Se

The present work focuses on the structural, optical, and electrical properties of Zn_1?x Cd _x Se (0.1≤x≤0.25) compounds. The compounds were synthesized by solid state reaction. X-ray diffraction (XRD) patterns confirm that the samples have cubic single phase (zinc-blende) crystal structure with space group F-43m. The crystal structural parameters were refined by the Rietveld method using the FullProf program. It was found that the lattice parameters increase linearly with increasing the Cd content and obeys Vegard’s law. The refined values of the crystallite size and the bond lengths increase with increasing the Cd content. The energy band gap of the samples has been calculated and it was found that it decreased as Cd increased. The conductivity of the samples increases with increasing both of composition parameter x and temperature, and showing semiconducting behavior.     

Sc doping of MgB2: the structural and electronic properties of Mg1−xScxB2

We have investigated the effect of electron doping on the superconducting properties of MgB₂. For the purpose we have synthesized several samples along the Mg_1−xSc_xB₂ section. The X-ray diffraction measurements reveal small changes in the lattice parameters suggesting that the Sc doping could be considered to simply fill the boron σ bands. Radio frequency surface resistivity measurements has been used to obtain the variation of T_c with Sc doping. Increasing the Sc content, the experimental T_c diverges from the T_c predicted by the BCS single band theory showing the key role of interchannel pairing near a shape resonance.     

Optical properties of Li1−xNb1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of LiNbO₃ is reported. Li_1?xNb_1?xEu_2xO₃ solid solutions exist over a very limited range of europium concentrations (x ≤ 0.01). The solid solutions were characterized by X-ray powder diffraction and density measurements. Within the range of compositions of the solid solutions, the optical properties (emission and excitation spectra) of the Eu³⁺ ions have been characterized. The obtained optical data indicate that two equally abundant europium luminescent species exist in the stoichiometric speciments; these correspond to the Eu³⁺ ions occupying the Li⁺ and Nb⁵⁺ sites, in agreement with the replacement mechanism which is inferred from density measurements. Other Eu³⁺ luminescent species which appear to be closely related with lithium deficiency were found to exist in nonstoichiometric samples, in addition to those which are present in the stoichiometric materials.     

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.