Dielectric and magnetic properties of Bi1−xYxFeO3 ceramics

Y doped BiFeO₃ polycrystalline ceramics were prepared by sol–gel method. Crystal structure examined by X-ray diffraction indicates that the samples were single-phase and crystallize in rhombohedral structure. An anomaly in the dielectric constant and dielectric loss in the vicinity of the antiferromagnetic Neel temperature (T_N) was observed. Saturated magnetization loops were observed for all sample with saturated magnetization M_s=0.678 emu/g and remnant magnetization M_r=0.084 emu/g for x=0.3.     

Dielectric and electromechanical properties of (1−x)PMN-xPZT ferroelectric ceramics

The dielectric and electromechanical characteristics of the (1?x)PMN-xPZT ferroelectric ceramics have been obtained at different temperatures, amplitudes, and frequencies of the measuring field and at different bias field strengths. It is shown that this ferroelectric ceramics at low and infralow frequencies possesses pronounced relaxor properties in a certain temperature range and ferroelectric properties in other temperature range. The temperature and amplitude ranges have been determined, in which the permittivity ?′ either only decreases or first increases and then decreases with an increase in the measuring field amplitude E ₀. The temperature ranges of existing the phases similar to the superparaelectric phase, dipole glass phase, and ferroelectric phase are evaluated from the temperature dependences of the coercive field E _c (T) and the remanent polarization P _r (T) and also from the reverse dependences of ?* and the electromechanical characteristics. The PZT concentration in the PMN-PZT system is determined, at which the electrostrictive constant M ₁₁ is maximum. It is demonstrated that, in the neighborhood of the temperature at a maximum of ?′, the strain S ₃ is quadratic in the field E ₌; that is, S ₃=M ₁₁ E ².     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Elastic and thermal properties of Sr1−xYxCoO3

The elastic and thermal properties of Sr_1?xY_xCoO₃ (0 ≤ x ≤ 0.5) have been investigated, probably for the first time, by using modified rigid ion model. We present the elastic constants (C_11, C_12, C₄₄) and other elastic properties like bulk modulus (B), Young's modulus (Y), shear modulus (G), Poisson's ratio (σ), Lame's parameter (μ, λ), transverse, longitudinal and average wave velocity (υ_t, υ_l and υ_m) and anisotropy parameter (A). Besides, we have reported the thermodynamic properties molecular force constant (f), Reststrahlen frequency (υ), cohesive energy (φ), Debye temperature (θ_D) and Gruneisen parameter (γ). We have also computed the variation of specific heat (C) and volume thermal expansion coefficient (α). The computed results on the elastic and thermodynamic properties are the first report on them. This model is capable of explaining the Cauchy's discrepancy, elastic and thermal properties successfully.     

Synthesis and superconductivity properties of Ba1−xKxBiO3

Ba_1?xK_xBiO₃ with x from 0.315 to 0.6 were successfully synthesized by molten salts method and characterized by XRD and magnetic susceptibility measurements. It is found that Ba_1?xK_xBiO₃ powders could directly be precipitated from KOH melts. Superconductivity has been observed in all samples and the highest superconducting transition temperature was found to be T_c = 30.6 K with x = 0.4. The lattice constant linearly depended on the potassium content in accord with the equation of a = 4.3548–0.1743x, and the decrease of the mole ratio of Bi³⁺/Bi⁵⁺ resulted in the increase of the potassium content, which suggested the disproportionation of Bi valence.     

Dielectric studies of phase transitions in the ferroelectric CdTiO3 and the Sr1−x CdxTiO3 solid solution

Temperature dependences of the permittivity and of the dielectric hysteresis loops in ceramic samples of nominally pure CdTiO₃ and a Sr_1?x Cd_xTiO₃ solid solution were studied. At 76.5±0.5 K, CdTiO₃ was established to undergo a ferroelectric phase transition close to the tricritical point. The temperature dependence of spontaneous polarization of CdTiO₃ is described within the Landau theory of phase transitions with the critical order parameter exponent ≈0.25. The phase diagram of the Sr_1?x Cd_xTiO₃ solid solution was drawn in (T, x) coordinates, and the critical concentration x _c =0.002, above which an induced polar state sets in the solid solution, was determined.     

Defect chemistry and electrical properties of La1−xSrxCoO3−δ IV. Electrical properties

This paper considers electrical properties of La_1−xSr_xCoO_3−δ in terms of defect models, such as random defect model and the cluster model. It is shown that the experimental data of the electrical conductivity may be explained in terms of the random defect model rather than the cluster model.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

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.     

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.     

Electrophysical properties of Pb1−xGdxTe

The solid solution system PbTe-GdTe has been investigated. Polycrystalline samples have been formed by the direct synthesis method. Metallographie and X-ray diffractometric analysis was performed. The microhardness was investigated and the Hall coefficient was measured over the temperature range 77–300 K. The experimental results show the donor action of the Gd and that the samples are single phase.     

Structural and superconducting properties of Eu1−xPrxBaSrCu3O7−δ

We have investigated the structural and superconducting properties of Eu_1−xPr_xBaSrCu₃O_7−δ by X-ray diffraction analysis, AC and DC magnetization and DC resistivity measurements. The parent compound crystallizes in the orthorhombic symmetry which transforms to tetragonal symmetry at x=0.2. Superconducting transition temperature T_c decreases monotonically with increasing x which approaches zero at x=0.6. The enhancement in critical concentration x_cr observed in Eu_1−xPr_xBa₂Cu₃O_7−δ on replacing 50 at.% Ba by Sr is attributed to the resulting local structural changes.     

Optical properties of Hg1−xCdxTe

Optical properties of Hg_1−xCd_xTe are summarized in this study. Based on Penn-like models, the Moss relation and the Wemple and DiDomenico approach, calculations of energy gap, plasmon energy, Fermi energy, oscillator strength and electronic polarizability have been made. Comparisons are made with the data available in the literature. Details of the dependency of the properties on composition are presented.     

Dielectric and piezoelectric properties of (1−x)Ba0.7Sr0.3TiO3−xBa0.7Ca0.3TiO3 perovskites

Dielectric and piezoelectric properties of (1?x)Ba_0.7Sr_0.3TiO₃?xBa_0.7Ca_0.3TiO₃ (BST?xBCT) (x=0.2–0.9) perovskite ceramics have been investigated. BCT has fully incorporated into BST lattice, forming a complete perovskite solid solution, whose lattice constant χ decreases almost linearly with increase in x from 0.2 to 0.4, while showing an anomalous expansion at 0.4<x≤0.6. This, together with the deviation of tetragonal–orthorhombic phase transition temperature (T_O–T) from the linear relation T_O–T (K)=?103.7x+239.3 at x=0.5, suggests that a small amount of Ca²⁺ has substituted for Ti⁴⁺. Curie temperature T_C increases linearly with increase in x from 0.2 to 0.9, which is mainly contributed to the increase of the Ba/Sr ratio. The calculated degree of relaxation (γ) is in the range of 1.41–1.53, indicating that the BST–xBCT ceramics are ferroelectric materials with diffuse phase transition. Strain and piezoelectric constant (d₃₃) decrease with increasing x, whereas planar electromechanical coefficient (k_p) reaches a maximum (17.0%) at x=0.6.     

Photoconductivity and electrical properties of epitaxial Pb1 − xCdxSe films

Photosensitive monocrystalline films Pb_{1 − x}Cd_xSe (x⩽ 0.03) of low carrier concentration (n = 0.3 ÷ 4 × 10¹⁷ cm⁻³ have been grown by hot wall epitaxy method on cleaved (111)BaF₂ or (100)KCl substrates.Photoconductive thresholds determined by spectral response measurements (at temperatures 4.2, 77 and 300 K) were in good agreement with the fundamental absorption edges and showed a large increase in the energy gap E_g with Cd content. Photoconductivity in the impurity range (hv < E_g) was observed.The temperature dependence of the Hall coefficient and the mobility between 4.2 and 300 K have been studied. Temperature freeze out of the free carriers was observed.Discussion of the carrier scattering mechanisms and impurity photoconductivity phenomena is presented.     

Room-temperature ferromagnetism and optical properties of Zn1−xMnxO nanoparticles

The room-temperature ferromagnetism is observed in Zn_0.98Mn_0.02O nanoparticles, which is related to the host-lattice defects induced by doping Mn. The ferromagnetism in Zn_0.95Mn_0.05O nanoparticles can be suppressed by Mn clusters. The additional peak at 519 cm^?1 is observed in Raman scattering spectra of the Zn_1?xMn_xO nanoparticles associated with intrinsic host-lattice defects, which become activated due to the Mn doping. The decrease in band gap and the weak intensity of absorption peak in the nanoparticles may be due to the sp–d interaction between transition metal and Zn anions.