共查询到20条相似文献,搜索用时 14 毫秒
1.
J.P. Mercurio J. Etourneau R. Naslain P. Hagenmuller J.B. Goodenough 《Journal of solid state chemistry》1974,9(1):37-47
The electric and magnetic properties of the LaxEu1?xB6 solid solutions have been investigated. A sharp evolution from semiconducting to metallic character is observed between x = 0 and x = 0.01. Evolution with x of the atomic volume, conductivity, Seebeck coefficient and magnetic properties are discussed in terms of a band model in which the conduction band is the 5d or 6s band of the rare-earth-atom sublattice. It is argued that chemical inhomogeneities produce a finite range of compositions over which the Seebeck coefficient changes from metallic character toward small-polaron character with decreasing x, whereas the metal-to-semiconductor transition appears more sharply defined. 相似文献
2.
Polycrystalline perovskite cobalt oxides Sr1-xRxCoO3 (R=Y and Ho; 0?x?1) were prepared by high-pressure/high-temperature technique. X-ray powder patterns of the Y-system indicated cubic perovskite form for 0?x?0.5, and orthorhombic perovskite form for x=0.8 and 1.0, while coexisting of the two phases for x=0.6. The cubic perovskite samples had metallic electric resistivities while the orthorhombic ones with semiconducting or insulating nature. The parent compound SrCoO3 showed a ferromagnetic transition at 266 K. With the Y substitution, the transition temperature increased slightly to ∼275 K at x=0.1, then decreased rapidly to ∼60 K for x=0.6. The YCoO3 (x=1) sample showed non-magnetic behavior. The Ho-substituted system showed quite similar structural, transport and magnetic properties to those of the Y-system. 相似文献
3.
Ulrich Häussermann Magnus BoströmPer Viklund Östen RappTherese Björnängen 《Journal of solid state chemistry》2002,165(1):94-99
The intermetallic compounds FeGa3 and RuGa3 were prepared from the elements using a Ga flux and their structures were refined from single-crystal X-ray data. Both compounds crystallize with the FeGa3 structure type (tetragonal, space group P42/mnm, Z=4). Electrical resistivity measurements revealed a semiconducting behavior for FeGa3 and RuGa3, which is in contrast to the good metallic conductivity observed for the isotypic compound CoGa3. The origin of the different electronic properties of these materials was investigated by first-principle calculations. It was found that in compounds adopting the FeGa3 structure type the transition metal atoms and Ga atoms interact strongly. This opens a d-p hybridization bandgap with a size of about 0.31 eV in the density of states at the Fermi level for 17-electron compounds (i.e., FeGa3 and RuGa3). The electronic structure of CoGa3 (an 18-electron compound) displays rigid band behavior with respect to FeGa3. As a consequence, the Fermi level in CoGa3 becomes located above the d-p hybridization gap which explains its metallic conductivity. 相似文献
4.
Fumihiko Nakamura Norifumi Ohba Hiroyuki Matsubara Takuya Hashimoto 《Thermochimica Acta》2005,435(2):222-229
The phase transition behavior of perovskite-type compounds, La1−xSrxCrO3, was investigated by differential scanning calorimetry (DSC), dilatometry, dc magnetic susceptibility measurement and X-ray diffraction analysis. Both second-order magnetic phase transition from antiferromagnetic to paramagnetic and first-order structural phase transition from orthorhombic to rhombohedral were observed in the DSC or dilatometric curve of every specimen. The temperatures of both these magnetic and structural phase transitions decreased linearly with an increase in Sr content. The structural phase transition temperature of La1−xSrxCrO3 with x less than 0.11 is higher than the magnetic phase transition temperature; however, a larger decrease in structural phase transition temperature than in magnetic phase transition temperature was observed with an increase in Sr content, resulting in a structural phase transition temperature lower than the magnetic phase transition temperature for La1−xSrxCrO3 with x of more than 0.12. It was also observed that the heat of absorption of the structural phase transition decreased with an increase in x. In the dependence of dc magnetic susceptibility on temperature, variations by not only magnetic but also structural phase transitions were observed. It was also revealed that thermal expansion coefficient is affected not only by structural phase transition but also magnetic phase transition. Magnetic and structural phase diagram of La1−xSrxCrO3, suggesting the existence of two Sr contents and temperatures at which triple phases coexist, was proposed. 相似文献
5.
C. Tenailleau A. Pring Y. Liu S. Tarantino M.A. Carpenter 《Journal of solid state chemistry》2005,178(3):882-891
Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba1−xLax)2In2O5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken. 相似文献
6.
Iron doped semiconducting nanoparticles Sn1−xFexO2 with x=0, 0.001, 0.002, 0.003, 0.004, 0.01 and 0.03 were prepared by a sol-gel method. The X-ray diffraction, Transmission Electron Microscopy measurements confirm the rutile structure with no impurity phase. The three characteristic lines of electron spin resonance (ESR) are observed in the doped samples for all compositions, which is a clear evidence for rhombic Fe3+ in rutile phase. The line width of ESR increases with increase in Fe concentration due to induced disorder. The spin-pumping effect is observed at temperatures below 250 K for the samples with x=0.01 and 0.03. However, based on the Curie-Weiss susceptibility, iron is in paramagnetic state and is subject to weak antiferromagnetic interaction. Blue shift in the optical band gap is observed with increase in the Fe content. 相似文献
7.
The solid solutions Hf(S2?xTex)1?y and Hf(Se2?xTex)1?y were prepared to examine changes in the electronic structure in a narrow composition range near the metal-insulator transition. Powder X-ray diffraction analysis and resistivity measurements are presented. The x3 dependence of the hexagonal c parameter for the sulfur solutions is due to large packing mismatches in the layers. In the selenium solutions, anomalous behavior is observed in the composition dependence of the a and c hexagonal lattice parameters in the range 0.05 < x < 0.10, y ~ 0. Between these concentration limits, both lattice parameters show positive deviations from smooth behavior and the diffraction linewidths broaden. The temperature dependence of the resistivity suggests that the Hf(S2?xTex)1?y solutions have an energy of activation for conduction at room temperature for ; the sample of composition has a metal-insulator transition as a function of temperature below room temperature. The band gap goes to zero with composition for Hf(Se2?xTex)1?y in the range x = 0.080 to x = 0.095. Nonstoichiometry, phase separation, and changes from covalent to metallic bonding explain the structural and electronic changes observed in the seleno-telluride system near the metal-insulator transition. 相似文献
8.
Carlos Moure Dionisio GutierrezOctavio Peña Pedro Duran 《Journal of solid state chemistry》2002,163(2):377-384
The structural properties of the YMexMn1−xO3 (Me=Cu, Ni, Co) pseudobinary oxides have been studied by X-ray diffraction and electrical measurements. The powders were prepared by solid state reaction between the corresponding oxides. The incorporation in solid solution of small divalent cations, Cu2+, Ni2+, and Co2+, substituting for Mn in the hexagonal YMnO3 compound, leads to a phase transition in which a perovskite-type structure is formed. The amount of substituting cation necessary for such a transition depends on the cation nature and, to a small extent, on the ionic radius. The phase transition depends strongly on the progressive substitution of the Jahn-Teller Mn3+ cation and therefore of the cooperative Jahn-Teller interaction weakness. The steric influence plays a secondary role, as is shown by the very small variation of the tolerance factor, t, as a function of the cation content. The solid solutions with perovskite-type structure show semiconducting behavior. The conductivity mechanism is of a thermally activated small polaron hopping. 相似文献
9.
The high-temperature metal-insulator transition in Cr-doped V2O3, (V1?xCrx)2O3, was investigated by TEM at a composition x = 0.006. Boundaries between the metallic phase and the insulator phase were observed between room temperature and 150°C by both heating and cooling and their crystallographic features were investigated. Boundaries are low-index planes and their orientations are such that the change of dimension by the transition in the direction parallel to the specimen surface is zero. This indicates that the orientation of the boundary is determined by the condition of minimum strain energy. Misfit dislocations parallel to the specimen surface were found to exist under this condition. The motion of the boundaries is fast, but the boundaries can be trapped by defects or at those places where the area of boundaries becomes minimum. The observation is consistent with the existence of the temperature hysteresis of the transition and the discontinuous change of the resistivity-temperature curve of this transition. 相似文献
10.
Samuel Chen James E. Hahn Catherine E. Rice William R. Robinson 《Journal of solid state chemistry》1982,44(2):192-200
The crystal structures of five samples of (TixV1?x)2O3 (0.011 ≤ x ≤ 0.077) and seven samples of (CrxV1?x)2O3 (both metallic and insulating phases, 0 ≤ x ≤ 0.05) were determined from X-ray diffraction data collected from single crystals. These compounds are isomorphous with α-alumina. The cell dimensions change such that the a axes increase and the c axes decrease with increasing Ti or Cr. In the CrV2O3 system, from 0 to 1.25% Cr doping, changes in structure parallel those observed in the TiV2O3 system. These changes are consistent with a slight weakening of the bonding metal-metal interactions in the basal plane, leading to an increase in the metal-metal distances coupled with changes which maintain constant metal-oxygen distances. A discontinuity appears at about 1.25% Cr as the transition from metal to insulating behavior occurs with increasing Cr content. No change in crystal symmetry accompanies this transformation. It appears that the metal-metal bonding interactions are retained even in the insulating phase of Cr-doped V2O3. A comparison of the structural variation in the Cr- and Ti-doped systems suggests that the change from metallic to insulating behavior cannot be a structure effect. These changes are, however, consistent with the band model proposed by others for these systems. 相似文献
11.
Band structure calculations at the level of LMTO-ASA provide insight into the electronic structure of BaV10O15 and the origin of the structural phase transition. A crystal orbital Hamiltonian population/integrated crystal orbital Hamiltonian population analysis provides evidence that the crystallographic phase transition is driven by V-V bond formation. As well, the energy bands near the Fermi level are very narrow, <1 eV, consistent with the fact that the observed insulating behavior can be due to electron localization via either Mott-Hubbard correlation and/or Anderson disorder. The partial solid solution, BaV10−xTixO15, was examined to study the effect of Ti-doping at the V sites on the structure and electronic transport properties. In spite of the non-existence of “BaTi10O15”, the limiting x=8, as indicated by a monotonic increase in the cell volume and systematic changes in properties. This limit may be due to the difficulty of stabilizing Ti2+ in this structure. For x=0.5 both the first order structural phase transition and the magnetic transition at 40 K are quenched. The samples obey the Curie-Weiss law to x=3 with nearly spin only effective moments along with θ values which range from −1090 K (x=0.5) to −1629 K (x=3). For x>3 a very large, ∼2×10−3 emu/mol, temperature independent (TIP) contribution dominates. Conductivity measurements on sintered, polycrystalline samples show semiconducting behavior for all compositions. Activation energies for Mott hopping derived from high temperature data range from ∼0.1 eV for x=0-1 and fall to a plateau of 0.06 eV for x=3-7. Low temperature data for x=3, 5 and 7 show evidence for Mott variable range hoping (VRH) with a T1/4 law and in one case between 5 and 17 K, a Efros-Shklovskii correlated hopping, T1/2 law, was seen, in sharp contrast to BaV10O15 where only the E-S law was observed up to 75 K. Seebeck coefficients are small (<35 μV/K), positive, roughly TIP and increase with increasing x up to x=5. This may point to a Heikes hopping of holes but a simple single carrier model is impossible. The compositions for x>3 are remarkable in that local moment behavior is lost, yet a metallic state is not reached. The failure of this system to be driven metallic even at such high doping levels is not fully understood but it seems clear that disorder induced carrier localization plays a major role. 相似文献
12.
Structural, magnetic and transport properties of La0.6−xPrxSr0.4MnO3 with x=0.0, 0.03, 0.06, 0.18, 0.3, 0.42, 0.54 and 0.6 are studied. The system exhibits a rhombohedrally distorted perovskite structure for x?0.3. A rhombohedral-orthorhombic (Pnma) structure transition is detected in the doping range from x=0.42 to 0.6. The structure refinement by Rietveld analysis of the X-ray powder diffraction data shows that the average distance Mn-O increases in the rhombohedral phases and decreases in the orthorhombic phases. Results show that the Curie temperature decreases from 374 to 310 K when 〈rA〉 varies from 1.254 to 1.231 Å. Electrical measurements show that all samples exhibit a metallic to semiconducting transition with increasing temperature. Meanwhile, the size of the resistivity ρ increases near TC. This phenomenon is interpreted as a gradual bending of the Mn-O-Mn bond angle, with decreasing 〈rA〉, which causes the narrowing of the electronic bandwidth and the effect of the A-site variance σ2. 相似文献
13.
The phase diagrams of the spinel systems Cd1?xCuxCr2S4, Cd1?xCuxCr2Se4, and Mn1?xCuxCr2S4 have been studied on the basis of X-ray powder photographs of quenched samples and high-temperature X-ray diffraction patterns. At room temperature the mutual solid solubilities of the metallic copper and the semiconducting cadmium and manganese spinels are only small (x < 0.05 and >0.95). The interchangeability, however, increases largely with increasing temperature. Complete series of mixed crystals, as in the Zn1?xCuxCr2X4 (X = S, Se) systems, however, are not formed. The solid solutions with x > 0.07 and <0.95, x > 0.095 and <0.90, and x > 0.36 and <0.87, respectively, formed at higher temperatures cannot be quenched to room temperature without decomposition. The unit cell dimensions of the spinel solid solutions studied obviously do not obey Vegard's rule. 相似文献
14.
Najmeddine Abdelmoula Hamadi Khemakhem Mario Maglione 《Journal of solid state chemistry》2006,179(12):4011-4019
The perovskite-type oxides Ba1−xLax(1−y)/2Euxy/2Nax/2TiO3 (0?x?0.5 and xy=0.04) were synthesized and characterized by X-ray diffraction as well as dielectric measurements and Raman spectroscopy. The crystal structure of these ceramics has been determined by the Rietveld refinement powder X-ray diffraction data at room temperature. These compounds crystallize at room temperature in tetragonal space group P4mm for 0?x?0.1 and in the cubic group for 0.2?x?0.5. The phase transition temperature TC (or Tm) decreases as x content increases. The degree of diffuseness of the phase transition is more pronounced for higher x content, implying the existence of a composition-induced diffuse phase transition of the ceramics with x?0.1. The evolution of the Raman spectra was studied as a function of various compositions at room temperature. The polarization state was checked by pyroelectric measurements. 相似文献
15.
C. Autret-Lambert B. Pignon I. Monot-Laffez L. Ammor J.M. Bassat 《Journal of solid state chemistry》2006,179(6):1698-1706
X-ray powder diffraction and resistivity measurements were performed on Bi2Sr2CaCu2O8+δ ceramics substituted by Y and Zn for Ca and Cu sites, respectively. X-ray diffraction patterns show an incommensurate modulated structure along the b-axis. The structural refinements were carried out using the four-dimensional space group Bbmb(0β1)0 0 0. From the X-ray peak profiles analysis, an anisotropic line-shape broadening was observed. The use of the “Williamson and Hall” method allows distinguishing the origin of broadening as mainly due to microstrains. A large transition from a metallic to semiconductor behaviour is observed on the resistivity curves at x≈0.4 for Bi2Sr2Ca1−xYxCu2O8+δ and at x′≈0.36 for Bi2Sr2Ca1−xYxCu1.94Zn0.06O8+δ, which can be also correlated to the defects. Oppositely to the metallic behaviour, which satisfies the Mathiessen's rule, the semiconducting one can be modelled by a variable range hopping process. 相似文献
16.
Andrew P. Grosvenor Ronald G. Cavell Robert I.R. Blyth 《Journal of solid state chemistry》2007,180(10):2670-2681
The ternary hafnium silicon arsenide, Hf(SixAs1−x)As, has been synthesized with a phase width of 0.5?x?0.7. Single-crystal X-ray diffraction studies on Hf(Si0.5As0.5)As showed that it adopts the ZrSiS-type structure (Pearson symbol tP6, space group P4/nmm, Z=2, a=3.6410(5) Å, c=8.155(1) Å). Physical property measurements indicated that it is metallic and Pauli paramagnetic. The electronic structure of Hf(Si0.5As0.5)As was investigated by examining plate-shaped crystals with laboratory-based X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoemission spectroscopy (PES). The Si 2p and As 3d XPS binding energies were consistent with assignments of anionic Si1− and As1-. However, the Hf charge could not be determined by analysis of the Hf 4f binding energy because of electron delocalization in the 5d band. To examine these charge assignments further, the valence band spectrum obtained by XPS and PES was interpreted with the aid of TB-LMTO band structure calculations. By collecting the PES spectra at different excitation energies to vary the photoionization cross-sections, the contributions from different elements to the valence band spectrum could be isolated. Fitting the XPS valence band spectrum to these elemental components resulted in charges that confirm that the formulation of the product is Hf2+[(Si0.5As0.5)As]2−. 相似文献
17.
《Solid State Sciences》2012,14(9):1273-1276
In this study, ab initio calculation results of electronic structure and elastic properties of NiCoCrGa quaternary Heusler compound are presented. Plane wave pseudopotential method is used with spin-polarized Generalized Gradient Approximation (σ-GGA) scheme of the Density Functional Theory (DFT). Static elastic constants of the cubic system satisfy mechanical stability criteria. The cubic phase of the system remains stable under tetragonal distortion. The spin-polarized electronic band structures and density of electronic states indicate a metallic band structure for majority spins, while minority spin structure has semiconducting character. This situation displays a slightly disturbed half-metallic behavior with high-spin polarization ratio (P = 0.961) at Fermi level EF. Two electronic bands of minority spins resulting from d-states of cobalt atom cross Fermi level at Γ-point. This situation gives a finite but very low density of states at EF. The material can be classified as a new half-metallic ferromagnet for spintronic applications. 相似文献
18.
Patrick Dougier Didier Deglane Paul Hagenmuller 《Journal of solid state chemistry》1976,19(2):135-142
The crystallographic, electrical, and magnetic properties of the system La1?xCaxVO3 have been investigated as a function of x and temperature. An insulator ? metal transition similar to that observed in the system La1?xSrxVO3 appears for : It may be described as a Mott-Anderson transition in an impurity band. 相似文献
19.
Fride Vullum 《Journal of solid state chemistry》2008,181(10):2580-2585
The solid solubility between LaNbO4 and LaTaO4 was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb1−xTaxO4 where 0.4?x?0.8. Single-phase LaNb1−xTaxO4 (0?x?0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb1−xTaxO4 (0?x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb1−xTaxO4 (0.8?x?1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO4 could also be transformed to monoclinic LaTaO4 at ambient temperature by applying a uniaxial pressure of 150-170 MPa, reflecting the lower molar volume of monoclinic LaTaO4. 相似文献
20.
C. Tenailleau J. Rodriguez-Carvajal P. Lacorre 《Journal of solid state chemistry》2003,174(2):431-440
The crystal structure of new molybdenum-doped vanadium sesquioxides (V1−xMox)2O3 (0?x?0.20) has been studied at low temperature (10 K) and up to room temperature, through neutron and X-ray powder diffraction. The transition from insulating I- to metallic M-type phases, either by doping or thermally driven, is accompanied by an abrupt decrease of all interatomic distances. Within each structural type however, at 10 K, the effect of doping is essentially the same as at room temperature: it increases cation-oxygen distances, and decreases cation-cation distances, making the cationic coordination octahedra more regular. Thermal effects differ for each phase type: all interatomic distances normally increase in the M-type phase (but with different octahedral modifications depending on doping), but they decrease or remain constant in the I-type phase. This produces an unusual negative thermal expansion coefficient up to 5% at low temperature for the doped compounds. 相似文献