On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La_{1? x} Sr _x MnO₃ perovskite manganite materials with different compositions (x?=?0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T _c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.     

Thermoelectric power of Ba(Fe1−x Co x )2As2 (0 ≤ x ≤ 0.05) and Ba(Fe1−x Rh x )2As2 (0 ≤ x ≤ 0.171)

Temperature-dependent, in-plane, thermoelectric power data are presented for single crystals of Ba(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.05) and Ba(Fe_1?x Rh _x )₂As₂ (0?≤?x?≤?0.171). Given that previous thermoelectric power and angle resolved photoemission spectroscopy studies of Ba(Fe_1?x Co _x )₂As₂ delineated a rather large Co-concentration range for Lifshitz transitions to occur, and the underdoped side of the phase diagram is poorly explored, new measurements of thermoelectric power on tightly spaced concentrations of Co, 0?≤?x?≤?0.05, were carried out. The data suggest evidence of a Lifshitz transition, but instead of a discontinuous jump in thermoelectric power in the range 0?≤?x?≤?0.05, a more gradual evolution in the S(T) plots as x is increased was observed. The thermoelectric power data of Ba(Fe_1?x Rh _x )₂As₂ show very similar behavior to that of Co substituted BaFe₂As₂. The previously outlined T–x phase diagrams for both systems are further confirmed by these thermoelectric power data.     

Structural,dielectric and Raman scattering studies of (1− x )PbMg1/3Nb2/3O3– x PbTiO3 single crystals (0 ≤ x ≤ 0.38)

The X-ray powder diffraction, dielectric and Raman studies of the (1???x) PbMg_1/3Nb_2/3O₃–xPbTiO₃ (0?≤?x?≤?0.38) single crystals were performed in a wide temperature range. The results of these complementary studies revealed that with the increase in PT content, the relaxor behaviour transforms into the ferroelectric one and a sequence of different structural phase transitions occur. On the base of the structural and dielectric results the phase diagram was determined. In addition, these results were used as a base for the group theory analysis and interpretation of the observed Raman spectra. The Raman spectra were deconvoluted using the independent damped oscillator model. From this model the temperature dependencies of the wavenumbers, reduced intensities and line widths were determined.     

Temperature-dependent sound velocities,attenuation and elastic moduli anomalies in Pr1− x Sr x MnO3 perovskite manganite materials at 0.28 ≤ X ≤ 0.41

Pr_{1? x} Sr _x MnO₃ perovskite manganite materials with different compositions of x (x?=?0.28, 0.33, 0.35, 0.38 and 0.41) have been prepared using solid-state reaction technique. The ultrasonic longitudinal and shear velocities and longitudinal attenuation have been measured employing through transmission method operated at a fundamental frequency of 5?MHz over a wide range of temperatures. In all the compositions, the temperature-dependent sound velocities, attenuation and elastic constants show interesting anomalies at Curie temperature (T _C) and Neel temperature (T _N). The observed anomalies at T _C and T _N have been discussed in terms of phase transition, strength of linear magnetostriction effect and competition between paramagnetism and ferromagnetism.     

Cluster Method Calculation of the Curie Temperature and Exchange Parameters for the Magnetocaloric Compounds MnFeAs x P1 − x (0.25 ≤ x ≤ 0.65) and Hexagonal MnFeAs

A wealth of experimental and theoretical data on the crystallographic and magnetic properties of the magnetocaloric compounds MnFeAs _x P_1???x (0.25?≤?x?≤?0.65) and MnFeAs has become available in the last decade. By analyzing the data and treating the spin interactions with Callen’s cluster expansion method, we extrapolate first-principle results for the exchange-coupling constants of MnFeAs to the P-substituted compounds and find Curie temperatures that agree, within 5 % deviation, with experiment. Simulations with different coupling parameters show that T _c is weakly dependent on the Fe–Fe interactions. Analysis of lattice expansion as a function of composition shows that changes in the lattice parameters a and c have opposite effects upon the strength of the magnetic interactions between ions. The results indicate that the cluster expansion method provides reliable estimates of magnetic properties, even for metallic compounds characterized by multiple interactions among ions with distinct magnetic moments.     

Structure and electrochemical performance of LiV x Mn2 − x O4 (0 ≤ x ≤ 0.20) cathode materials for rechargeable lithium ion batteries

LiMn₂O₄ and vanadium-substituted LiV _x Mn_2???x O₄ (x?=?0.05, 0.10 0.15 and 0.20) cathode materials were synthesized by sol–gel method using aqueous solutions of metal nitrates and tartaric acid as chelating agent at 600 °C for 10 h. The structure and electrochemical properties of the synthesized materials were characterized by using X-ray diffraction, SEM, TEM and charge–discharge studies. X-ray powder diffraction analysis was changed in lattice parameters with increasing vanadium content suggesting the occupation of the substituent within LiMn₂O₄ interlayer spacing. TEM and SEM analyses show that LiV_0.15Mn_1.85O₄ has a smaller particle size and more regular morphological structure with narrow size distribution than LiMn₂O₄. It is concluded that the structural stability and cycle life improvement were due to many factors like better crystallinity, smaller particle size and uniform distribution compared to the LiMn₂O₄ cathode material. The LiV_0.15Mn_1.85O₄ cathode material has improved the structural stability and excellent electrochemical performances of the rechargeable lithium ion batteries.     

Optical properties of TlGaxIn1-xSe2-layered mixed crystals (0.5 ≤ x ≤ 1) by spectroscopic ellipsometry,transmission, and reflection measurements

The layered semiconducting TlGa_xIn_1-xSe₂-mixed crystals (0.5?≤?x?≤?1) were studied for the first time by spectroscopic ellipsometry measurements in the 1.2–6.2?eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index, and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The effect of the isomorphic cation substitution (indium for gallium) on critical point energies in TlGa_xIn_1-xSe₂ crystals was established. Moreover, the absorption edge of TlGa_xIn_1-xSe₂ crystals have been studied through the transmission and reflection measurements in the wavelength range of 500–1100?nm. The analysis of absorption data revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps decrease with the increase of indium content in the studied crystals.     

Iron content and temperature dependences of diffuse scattering in Ti–(50–x)Ni–xFe (6 ≤ x ≤ 10) alloys

Diffuse scatterings appearing in electron diffraction patterns of Ti–(50???x)Ni–xFe (x?=?6, 7, 8, 10, in at.%) alloys were investigated. In the alloys, martensitic (R-phase) transformation is suppressed down to 4.2 K, but the electrical resistivity exhibits a local minimum at T _min (210 K, 195 K, 180 K and 140 K for x?=?6, 7, 8, 10, respectively). The following results were obtained for all the alloys. Diffuse scattering appears below T _min and its intensity maximum is located at an incommensurate position of g?+??ζζ0?*, where g is a reciprocal lattice vector of the B2-phase. The value?ζ?at T _min is significantly smaller than 1/3 and increases with decreasing temperature; it decreases with increasing Fe content. The value of?ζ?at T _min agrees with the length of nesting vector obtained by a band calculation, suggesting that the diffuse scattering is caused by the nesting effect of the Fermi surface in the B2-type structure.     

Structural and magnetic properties with reversible magnetocaloric effect in PrSr1–xPbxMn2O6 (0.1 ≤ x ≤ 0.3) double perovskite manganite structures

In this study, the effect of Pb substitution with Sr on structural, magnetic and magnetocaloric properties in Pr(Sr_1–xPb_x)Mn₂O₆ (0.1?≤?x?≤?0.3) double perovskite manganite compounds prepared by the sol–gel method has been reported. Crystallinity has been worked by X-ray diffraction measurement and the Rietveld refinement method, which shows all samples are in monoclinic structure with P21/n space group without impurity phase. Scanning electron microscope analysis shows that all samples constituted from square-shaped grains with distinct grain boundaries, and energy-dispersive X-ray spectroscopy analysis shows that all expected elements are detected and there is no impurity element in the sample. All samples exhibit a magnetic phase transition from ferromagnetic to paramagnetic phase at T_C determined as 274, 282 and 286?K for x?=?0.1 (PSPM-1), 0.2 (PSPM-2) and 0.3 (PSPM-3) in the PrSr_1–xPb_xMn₂O₆ structure, respectively. Isothermal magnetisation curves near T_C are used to determine the nature of the magnetic phase transition and the magnetic entropy change values (?ΔS_M), which are found as 2.78, 3.11 and 3.52?J?kg^?1?K^?1 for PSPM-1, PSPM-2 and PSPM-3 samples, respectively. Relative cooling power (RCP) values of the samples are also determined as 241, 311 and 246?J?kg^?1 for PSPM-1, PSPM-2 and PSPM-3 samples, respectively. ΔS_M and RCP values are also calculated by using the Landau theory and the results are in accordance with the experimental results, which indicate the role of electron condensation energy and magneto-elastic coupling in magnetic entropy change value of the samples.     

A review on the mineral chemistry of the non-stoichiometric iron sulphide,Fe1− x S (0 ≤ x ≤ 0.125): polymorphs,phase relations and transitions,electronic and magnetic structures

Fe_{1? x} S (0?≤?x?≤?0.125), the pyrrhotite group minerals are extremely complex in their chemical and physical properties, which largely attribute to the various non-stoichiometric compositions and myriad superstructures. In this article, a review is focused on the crystal structures, phase relations and transition of the pyrrhotite group minerals.     

Electro-optic effect in SrTiO3 and Sr1–x Ca x TiO3 (x = 0.014)

The results of the investigation of the quadratic electro-optic effect in Sr_1–x Ca _x TiO₃ with x = 0.014 (SCT) and in nominally pure SrTiO₃ (STO) at room temperature in applied direct-current (dc) and alternating-current (ac) electric fields have been presented. It has been shown that the quadratic (in polarization) electro-optic coefficients of STO and SCT crystals coincide within the accuracy of the determination (±5%). It has been found that, in nominally pure STO measured in a dc electric field, there is a relaxation of the electro-optic effect with a relaxation time τ ≈ 30 s due to the formation of a space charge in the sample. No similar effect in SCT has been observed. A possible mechanism for the formation of a space charge in STO and SCT has been discussed.     

High-Pressure Synthesis of SeCu 1− x Zn x O 3 Perovskites

The SeCu 1 m x Zn x O 3 solid solution, with a distorted perovskite-type structure, has been synthesized under high pressures and temperatures. X-ray diffraction analysis indicates that the zinc ions occupy the copper sites, a solid solution being formed. It seems that high-pressure stabilises a small cation such as Se 4+ in the A site of the perovskite structure ABO 3 although the material is better described as formed by selenite anions \rm{SeO}_{3}^{-} and Cu 2+ /Zn 2+ cations.     

Heat capacity of multiferroics Bi1–x Pr x FeO3

Physics of the Solid State - The heat capacity of multiferroics Bi1–x Pr x FeO3 (0 ≤ x ≤ 0.20) has been studied in the temperature range 130–800 K. An insignificant...     

Influence of Cr and Fe on magnetic phase transition and magnetocaloric effect in Co2(Cr1-xFex)Al, (x ≤ 0 ≤ 1) soft magnetic Heusler alloys

The structural measurement indicates that the system possesses A2 to B2 phase transformation at the extent of A2 phase at room temperature. The present system shows first order magneto-structural transformation (FOMST). The substitution of Cr by Fe causes a vital role for an increase in magnetocaloric properties. The change in magnetic entropy (ΔS_M) and relative cooling power (RCP) are evaluated under an applied field of 20 KOe that shows drastic changes near the blocking temperature under the specific temperature regime. In the vicinity of these observed properties, critical exponent parameters such as, β, ? and δ were also observed using field dependence magnetic entropy change.     

Phase diagram of perovskite manganites Sm1 − x Sr x MnO3

Physics of the Solid State - The magnetic and structural phase diagram of perovskite manganites Sm1 ? x Sr x MnO3 (0.16 ≤ x ≤ 0.67) are constructed based on systematic studies by...     

Heat capacity of nanostructured multiferroics BiFe1–x Zn x O3

Physics of the Solid State - The heat capacity of ceramic BiFe1–x Zn x O3 multiferroics has been studied in the temperature range 150–750 K. It is found that the antiferromagnetic...     

Optical constants for Ge30− x Se70Ag x (0 ≤ x ≤ 30 at%) thin films based only on their reflectance spectra

In this paper, different homogenous compositions of Ge_{30? x} Se₇₀Ag _x (0?≤?x?≤?30 at%) thin films were prepared by thermal evaporation. Reflection spectra, R(λ), for the films were measured in the wavelength range 400–2500?nm. A straightforward analysis proposed by Minkov [J. Phys. D: Appl. Phys. 22 (1989) p.1157], based on the maxima and minima of the reflection spectra, allows us to derive the real and imaginary parts of the complex index of refraction and the film thickness of the studied films. Increasing Ag content at the expense of Ge atoms is found to affect the refractive index and the extinction coefficient of the films. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. Optical absorption measurements were used to obtain the fundamental absorption edge as a function of composition. With increasing Ag content, the refractive index increases while the optical band gap decreases. The compositional dependence of the optical band gap for the Ge_{30? x} Se₇₀Ag _x (0?≤?x?≤?30) thin films is discussed in terms of the chemical bond approach.     

LaAg x In1−x

The structures of LaAg _x In_1–x alloys withx=0.75, 0.89 are determined by neutron diffraction on powder samples. The space group isI4/mmm (D _4h ^/17 ). The lattice constants splitting, the order parameter and the mean square vibrational amplitudes of the atoms are given in the temperature range from 20 KT300 K.