Dielectric relaxation properties of tysonite-type solid solutions La1−xBaxF3−x

Dielectric relaxation properties of solid solutions La_1?xBa_xF_3?x (x ? 0105) have been studied by thermally stimulated depolanzation current (TSDC)- and a c. dielectric loss (DL) techniques.For x < 30 × 10^?3 the dielectric spectra show a relaxation peak which is ascribed to a simple associate of a substitutional dopant ion and a fluoride ion vacancy (Ba_LaV_F)^x in nearest-neighbour position, the vacancy being confined to the B sublattice For x values of about 1.3 × 10^?2 a relaxation peak appears which is tentatively attributed to a similar type of defect associate with the vacancy now confined to the A sublattice of the tysonite anion array One broad relaxation peak dominates the TSDC and DL spectra over the whole concentration range This peak is due to the relaxation of macroscopic space charge, i e ionic conductivity The low-temperature ion conductivity has been determined for several solid solutions, and extrapolates to the high-temperature conductivity determined previously with impedance spectroscopy Below liquid-nitrogen temperature three relaxations are observed, and ascribed to electronic transitions in cenum impurities. A computer programme has been developed to analyse TSDC relaxation peaks, taking dipole-dipole interactions into account Relaxation parameters and dipole concentrations are presented.     

Crystal and magnetic structure of the CoMn1−xTixSi system

The magnetic and crystallographic nature of the compounds CoMn_1?xTi_xSi (0?x?0.5) is studied by X-ray, neutron diffraction and magnetometric measurements. It is shown that the solid solutions arise for 0?x?0.5, with the crystal structure of the NiTiSi type. All compounds are antiferromagnets. The magnetic moment localized on Mn and Co atoms forms a double spiral in both sites. The dilution of the Mn sublattice by Ti atoms change only slightly the magnetic properties - Néel temperature and parameters of magnetic structure.     

Magnetic properties of some ternary nickel solid solutions

We present the results of the magnetic measurements on Ni_0.7Cu_0.3-xSi_x (0 ? x ? 0.08) alloys between 4.2 and 800 K. The saturation magnetization and the Curie temperatures are linearly dependent on the mean electronic concentration of the diamagnetic matrix. The effective nickel moments vary only little in the considered concentration range. The ratio between the number of magnetic carriers determined from the Curie constant and the saturation data increases as the Curie temperatures decrease. The magnetic behaviour of nickel in these solid solutions is discussed.     

Defect characterization in CeO2−x at elevated temperatures—II: Neutron diffraction

Polycrystalline CeO_2?x(0? x? 0.21) has been examined at 900°C by means of neutron diffraction to characterize the predominant atomic point defects responsible for nonstoichiometry. The stoichiometry was controlled by adjusting the oxygen partial pressure between 1 and 10^?21 atm. For x ? 0.11, good agreement (residual ? 1.0%) is obtained by assuming a random distribution of defects on the oxygen sublattice. Vacancy concentrations obtained from the neutron-data analysis are in agreement with thermogravimetric determinations. Over this region of x, the temperature factors of both the cation and anion increase with an increase in vacancy concentration, which implies greater root-mean-square (r.m.s.) displacement of the atoms. However, for x > 0.11, the analysis of the neutron data indicates a smaller anion vibrational amplitude than in the near-stoichiometric compositions and is accompanied by a shift in the equilibrium position of the oxygen atoms. The effect of anharmonic thermal motion has also been considered, but alone, cannot explain the neutron data. The results of the present study correlate well with the previous X-ray experiment.     

Composition of cementite in the dependence on the temperature. In situ neutron diffraction study and Ab initio calculations

The structure state of carbon eutectoid steel has been studied by the in situ neutron diffraction method in the temperature range from room temperature to 800°C. It has been shown that an increase in the temperature is accompanied by a decrease in the concentration of carbon in cementite, whereas its weight fraction and the parameters of the orthorhombic lattice change slightly. The ab initio calculations of the nonstoichiometric carbides Fe₃C _x (0 < x < 1) indicate that the structure of cementite remains stable upon the appearance of vacancies in the carbon sublattice with a relatively low formation energy. Thus, cementite should be considered as an interstitial phase Fe₃C _x with a wide homogeneity range.     

Effect of cation distribution on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method

Nano particles of Mn_(0.5–x)Ni_xZn_0.5Fe₂O₄ (x=0.0, 0.1, 0.2, 0.3) have been synthesized by chemical co-precipitation method. The lattice constant and distribution of cation in the tetrahedral and octahedral sites have been deduced through X-ray diffraction (XRD) data analysis. The lattice constant (Å) for all Mn/Ni concentration is found to be less than that for the corresponding bulk values. X-ray intensity calculations indicate that there is deviation in the normal cation distribution. Magnetization decreases with increasing Ni concentration except for x=0.3, where it shows increasing trend. This is due to migration of Fe³⁺ ions from B-site to A-site, which reduces the B–B coupling and there by the spin canting in the B sublattice. The Curie temperature was found to decrease with increase in nickel concentration except for x=0.3, where it shows a rise. Coercivity is very low and is found to be inversely proportional to the grain size.     

Determination of the nonequilibrium concentration of vacancies in silicon crystals by measuring the concentration of nickel atoms at lattice sites

A method is proposed for determining the nonequilibrium concentration of vacancies and vacancy complexes in silicon crystals by measuring the concentration of electrically active nickel atoms at the sites of the silicon lattice, [Ni _s ], after the diffusion of nickel at temperatures from 550 to 650°C. It is shown experimentally that, after the diffusion of nickel from the surface into silicon samples with different initial nonequilibrium concentrations of vacancy complexes, [V]_init, formed during crystal growth, the concentration [Ni _s ] in the bulk of a sample to a good degree of accuracy corresponds to the vacancy concentration [V]_init determined by a standard method based on the analysis of the concentration profiles of [Au _s ] after the diffusion of gold from the surface. This method for determining the concentration of vacancies is much simpler than the standard method and allows one to use lower temperatures and a lower thermal budget.     

Exciton absorption spectra of Cs<Subscript>2</Subscript>CdI<Subscript>4</Subscript> and Rb<Subscript>2</Subscript>CdI<Subscript>4</Subscript> ferroelastic solid solutions

The exciton absorption spectra of thin films of (Cs_{1 − x} Rb _x )₂CdI₄ solid solutions have been investigated and the refractive index n(λ) in their transparency window in the concentration range of 0 ≤ x ≤ 1 has been measured. The exciton-band parameters and optical permittivity ɛ_∞(x) have been found to linearly depend on the concentration. It is established that excitons are incorporated into the CdI₂ sublattice of the solid solutions and belong to intermediate-coupling ones. The characteristics of excitons in ferroelastics are compared with the corresponding parameters for CdI₂, RbI, and CsI, which are used as components to synthesize ternary compounds.     

Correlation of defect structure and ionic conductivity in δ-phase solid solutions in the Bi3NbO7–Bi3YO6 system

Defect structure and conductivity behaviour are discussed in the solid solution Bi₃Nb_1−xY_xO_7−x(0.0 ≤ x ≤ 1.0). Investigations were carried out using a combination of ac impedance spectroscopy and powder X-ray and neutron diffraction. Low temperature conductivity and activation energy both increase as a function of x. The former is attributed to an increase in oxide ion vacancy concentration, whereas the latter is due a redistribution of oxide ion vacancies as determined by neutron diffraction measurements. The defect structures at room temperature and 800 °C are presented. Curvature in Arrhenius plots of conductivity throughout the composition range is associated with a temperature dependent redistribution of oxide ions.     

Mechanism for Conversion of the Conductivity Type in Arsenic-Doped p-CdxHg1–xTe Subject to Ionic Etching

Based on an analysis of chemical diffusion of mercury in p-Cd _x Hg_1–x Te:As narrow-band solid solutions, a mechanism for conversion of the conductivity type upon ionic etching is suggested. It is shown that the n–p conversion of the conductivity in this case is due to the formation of a donor complex between arsenic in the Te sublattice and an interstitial Hg atom. Moreover, the electron concentration in the converted layer corresponds to the concentration of the implanted arsenic impurity. The theoretical results are confirmed by the experimental investigation of the electron concentration distribution over the n-layer of a p-Cd _x Hg_1–x Te:As epistructure converted upon ionic etching.     

Magnetic ordering in Co<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>Mg<Subscript>1−<Emphasis Type="Italic">c</Emphasis></Subscript>O solid solutions

The influence of dilution by diamagnetic ions on the magnetic ordering in single-crystal Co _c Mg_1−c O solid solutions was studied by Raman spectroscopy and magneto-optical microscopy in a wide range of temperatures (6 < T < 200 K). Far infrared absorption measurements of antiferromagnetic resonance (AFMR) were also performed for pure CoO. It was found that the domain structure and the contribution from Brillouin zone center magnons to Raman scattering and AFMR disappear well below the Néel temperature, whose value was determined from neutron diffraction and magnetic susceptibility measurements. The text was submitted by the authors in English.     

Magnetic interactions in Pr1−xTbxMn2Ge2

The magnetic moments in the rare earth and Mn sublattices of RMn₂Ge₂, where R is a rare earth element, feature a variety of ordering configurations. In PrMn₂Ge₂ and TbMn₂Ge₂, the interlayer magnetic coupling in the Mn sublattice is, respectively, ferromagnetic and antiferromagnetic below about 350 K. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. In this study, we investigate the variations in the magnetic properties of Pr_1−xTb_xMn₂Ge₂ as a function of rare earth concentration by examining the evolution of the features in the temperature dependence of the magnetization. The results of earlier neutron diffraction and Mössbauer studies on samples with x=0 and 1 are also used for interpreting the magnetization data and to give an account of the competing effects between various magnetic structures in the Mn and rare earth sublattices. The results are summarized in the Pr_1−xTb_xMn₂Ge₂ magnetic phase diagram.     

Conversion-electron Mössbauer study of sputtered Bi3Fe5O12

Temperature dependence of the magnetization measurements and Conversion Electron Mössbauer Spectroscopy (CEMS) have been performed on sputtered Bi₃Fe₅O₁₂ (BiIG), Y₃Fe₅O₁₂ (YIG) and (Bi, Y)IG films. Under the effect of the diamagnetic dilution of the diamagnetic yttrium sublattice the magnetization of BiIG is decreased with respect to YIG. The hyperfine field at the tetrahedral iron sites is increased, indicating a decrease in the intra-sublattice exchange interaction rather than the decrease of the tetrahedral iron moment.     

The concentration metamagnetic transition in Tm1−x TbxCo2 compounds

The Tm_1?x Tb_xCo₂ (0 ≤ x ≤ 1) system was studied by measuring the magnetic susceptibility, electrical resistance, and neutron diffraction. In the compounds with 0 < x ≤ 0.15, an inhomogeneous magnetic state characterized by the existence of large regions (up to 100 Å in size) with short-range ferrimagnetic order was found to occur. The maximum of the residual electrical resistance observed in the compound with x = 0.1 at the magnetic ordering temperature was established to be due to the scattering of conduction electrons by localized spin fluctuations associated with f-d exchange fluctuations caused by the substitution of terbium for thulium. The increase in the terbium concentration to x ≥ 0.15 leads to a sharp increase in the Co sublattice magnetization and the establishment of a long-range ferromagnetic order, which indicates a concentration metamagnetic transition in the band subsystem.     

Influence of quantum fluctuations on the Néel temperature of a dilute two-dimensional anisotropic antiferromagnet

A quantum Monte Carlo procedure is used to calculate the energy, sublattice magnetization, Néel temperature, and the slopes of the S=[1/T _N(x=0)]dT _N(x)/dx curves as functions of the hole concentration and the exchange anisotropy Δ=1−J _x,y/J _z in the Heisenberg model with anisotropic negative interactions between nearest neighbors in a square lattice with dilution among the lattice sites. The slope diverges in the limit Δ→0: S∼ln(6.5/Δ). Fiz. Tverd. Tela (St. Petersburg) 39, 898–900 (May 1997)     

Temperature and field dependence of the magnetic properties of Nd \mathsf{_{2-x}}Ce\mathsf{_{x}}CuO\mathsf{_4}

We have investigated the magnetic ordering of Nd_2-x Ce _x CuO₄ for x = 0, 0.09, 0.13, 0.15 and 0.18 by neutron diffraction at low temperatures down to 33 mK and under magnetic field up to 5 tesla applied along [1,-1,0] crystallographic direction. At zero applied magnetic field Cu magnetic sublattice orders at , 210, 130 and 105 K for x = 0, 0.09, 0.13, 0.15, respectively. No long range magnetic order of the Cu could be detected for x = 0.18. The magnetic order of Nd was found in all samples, with a gradual increase of the polarized magnetic moment with decreasing temperature, saturating around 1 K. Hyperfine induced nuclear polarization of the Nd nuclear spin has been observed below about 400 mK for samples with x = 0, 0.13, 0.15 and 0.18. Field variation of the intensities of the principal and superstructure reflections of Nd_2-x Ce _x CuO₄ at millikelvin temperatures shows a field-induced second-order double-k to single-k phase transition at H _c = 0.75 and 0.56 tesla for samples with x = 0 and 0.15, respectively at T = 50 mK. We have also investigated the polarization of the Nd electronic sublattice due to the field of the Cu sublattice by the element specific X-ray resonant magnetic scattering investigation with synchrotron radiation.Received: 9 April 2004, Published online: 14 December 2004PACS: 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.)T. Chatterji: Has changed his surname from Chattopadhyay to Chatterji.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Effect of doping with nickel ions on the structural state of a zinc oxide crystal

The fine structure of a hexagonal zinc oxide crystal doped with nickel ions of the composition Zn_{1 − x} Ni _x O has been studied using neutron diffraction and magnetic measurements. It is established that even at very low doping levels (x = 0.0004), the crystal undergoes local distortions in basal planes of the initial hexagonal lattice. The local distortions are assumed to be sources of the formation of ferromagnetism in compounds of this class.     

Magnetic phases in quasi-binary (Fe1−x Cox)Ge2 intermetallic compounds

Magnetic neutron diffractometry revealed the existence in (Fe_1−x Co_x)Ge₂ solid solutions (x<0.5) with C16 structure of only two magnetic phases, namely, low-temperature (AFI) and high-temperature (AFII). A third magnetic phase, AFIII, suggested by earlier magnetic measurements, has not been found. The AFI and AFII phases have a commensurate and an incommensurate antiferromagnetic structure with the wave vectors k ₀=2π/a (1,0,0) and k=k ₀+δ k, respectively. The regions of their existence are shown in the magnetic phase diagram. Neutron diffraction measurements yielded the concentration dependence of the average magnetic moment per atom in the antiferromagnetic sublattice of a 3d metal, which, similarly to the dependence of the Néel point on x, was found to be nonlinear. An analysis of these dependences suggests that substitution of cobalt for iron is accompanied, on the one hand, by a decrease of the local spin density on the iron atoms in the nearest environment of a cobalt atom and, on the other hand, by an increase of the effective exchange integral between the nearest-neighbor iron atoms located along the tetragonal axis. Fiz. Tverd. Tela (St. Petersburg) 41, 283–289 (February 1999)