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1.
The effect of substitution at the Fe site in the CuFeO2 delafossite is known to induce a magnetic structure responsible for the appearance of electric polarization. A CuFe1−xRhxO2 series of ceramic samples is studied in order to determine the composition range exhibiting such a polar state. It is found that for the CuFe1−xRhxO2 (x≤0.15) solid solution, the Néel temperature TN2 decreases monotonously with x from 11.5 K to 5.9 K, for x=0.00 to 0.15, respectively, and that the dielectric peak and the polarization transition temperatures coincide with TN2. In contrast, the dielectric peak and polarization magnitudes go through an optimum for CuFe0.92Rh0.08O2 (x=0.08). These results demonstrate that, compared to other substituting elements, the Rh3+ for Fe3+ substitution in CuFeO2 allows to extend significantly the ferroelectric region in the (x, T) phase diagram in connection with the slower TN2 decrease.  相似文献   

2.
The phase diagram of the SrCo0.8Fe0.2O3−δ compound has been determined at high temperatures (823?T?1223 K) and in the oxygen partial pressure range (10−5?pO2?1 atm) by thermogravimetric measurements of the equilibrium pO2, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo0.8Fe0.2O3−δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo0.8Fe0.2O2.5 stabilizes within a small range around 3−δ=2.5 at temperatures below 1073 K. Equilibrium pO2 measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO2 with decreasing temperature. This behavior is discussed considering the evolution from coherent intergrowth interfaces with elastic strain energy to incoherent interfaces without elastic strain energy as T decreases. The thermodynamic quantities hO2oxide and sO2oxide for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity (σ) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of σ with the oxygen content is non-linear and the activation energy varies from 0.4 to 0.28 eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model.  相似文献   

3.
The new compounds Ce2/3−xRh2O4 (x∼0.11-0.14) and CeRh2O5 have been prepared. Their structures were determined from single crystal X-ray diffraction data. Electrical and magnetic properties were also evaluated. Based on the structural analysis and physical properties, oxidation states for CeRh2O5 can be assigned as Ce4+Rh3+2O5. A small variation in x was detected for Ce2/3−xRh2O4 indicating a formula ranging from Ce3.64+0.55Rh3+2O4 to Ce3.81+0.525Rh3+2O4.  相似文献   

4.
A complete solid solution was found between isostructural Pb3Mn7O15 and Pb3Rh7O15. Single-crystals of two members of the solid solution Pb3Rh7−xMnxO15 (x=1.07 and 2.26) were grown and their crystal structures were determined. The Verwey-type transition for Pb3Rh7O15 at 185 K remains with a 3% substitution of Mn for Rh but disappears with 4% substitution of Mn for Rh. The magnetic ordering temperature found for Pb3Mn7O15 at about 70 K is maintained at a 43% substitution of Rh for Mn but has disappeared for 57% substitution of Rh for Mn. The unit cell volume of this layered structure contracts with increasing x for Pb3Rh7−xMnxO15 phases, but the structure actually expands in the direction perpendicular to the layers due to increased separation between the layers.  相似文献   

5.
The thermodynamic properties of the Fe3O4ZnFe2O4 spinel solid solution were determined at 900°C by the use of the solid electrolyte galvanic cell Fe2O3 + Fe3O4|O2?|Fe2O3 + ZnxFe3?xO4The activity values obtained exhibit slight negative deviation from the ideal solution model. An analysis of the free energy of mixing of the spinel solid solution provided information on the distribution of cations between the tetrahedral and octahedral sites of the spinel lattice. This is the basis for the estimation of the free energy of formation of pure zinc ferrite from oxides. ΔG0ZnFe2O4 = ?2740 ? 1.6 T cal mole?1  相似文献   

6.
A sub-solidus phase evolution study was done in CeO2-Sc2O3 and CeO2-Lu2O3 systems under slow-cooled conditions from 1400 °C. Long-range order probing of X-ray diffraction technique is utilized in conjunction with the ability of Raman spectroscopy to detect the changes in local co-ordination. Lu2O3 showed solubility of 30 mol% in CeO2, thus forming an anion deficient fluorite-type (F-type) solid solution, whereas Sc2O3 did not show any discernible solubility. A biphasic region (F+C) was unequivocally detected by Raman spectroscopy in Ce1−xLuxO2−x/2 (0.4?x?0.9) and in Ce1−xScxO2−x/2 (0.1?x?0.9) systems. Raman spectroscopy was valuable in studying these systems since oxygen vacancies are created on doping RE2O3 into ceria and Raman spectroscopy is very much sensitive to oxygen polarizability and local coordination. Back scattered images collected on representative compositions support the above-mentioned results.  相似文献   

7.
Phase relations in the system NiAl2O4Ni2SiO4 were studied in the pressure range 1.5 ~ 13.0 GPa and in the temperature range 800 ~ 1450°C. Two new phases, IV and V, were found in regions of pressure higher than 4 GPa. Phase V disproportionates into a mixture of Ni2SiO4-spinel, NiO, and Al2O3 at approximately 9.5 GPa and 1100°C. Phases III, IV, and V form a solid solution in some compositional range: phases IV and V have a composition around NiAl2O4·Ni2SiO4, whereas phase III spreads from NiAl2O4·Ni2SiO4 to the NiAl2O4-rich side. All the phases I ~ V are structurally considered to be spinel derivatives, “spinelloids,” with three kinds of tetrahedral groups; isolated tetrahedra TO4, linked ones T2O7, and triply linked ones T3O10. The ratios of isolated tetrahedra to linked ones are large in the higher-pressure phases and small in the lower-pressure phases. The difference of compositional range of phase III from that of phases IV and V is possibly explained by the avoidance of linked tetrahedra such as O3AlOAlO3.  相似文献   

8.
Investigations on the Crystallization of Rhodium(III) Oxo Compounds – Chemical Vapour Transport of Rh2O3 using Chlorine Rh2O3,s migrates in chemical transport experiments with chlorine as transport agent from the higher (T2) to the lower (T1) temperature of a gradient (ΔT = 100°) due to endothermal reactions (900°C < T ≤ 1050°C; T = 0,5 · (T2 + T1)). Under the conditions of transport experiments RhCl3,s is observed in most experiments as equilibrium solid besides the sesquioxide. The transport rates for Rh2O3,s and the sublimation rates for RhCl3,s grow with increasing temperature T . The composition of the equilibrium solids, the rates of migration and the sequence of deposition (1. RhCl3,s, 2. Rh2O3,s) is well reproduced by thermodynamic model calculations. As a result of this calculations the transport behaviour of the system Rh2O3,s/Cl2 is determined by the two equilibria The influence of RhCl2,g and RhCl4,g on the transport behaviour of Rh2O3,s as well as the possible occurence of RhOCl2,g in the equilibrium gas phase will be discussed. Predictions of the transport behaviour of ternary rhodium(III) oxo compounds will be made.  相似文献   

9.
The relaxor ferroelectric Pb2Bi4Ti5O18 has been studied by Rietveld refinement of powder neutron diffraction data collected at temperatures of 100, 250 and 400 °C. Our refinements are compatible with the ‘average’ crystal structure of Pb2Bi4Ti5O18 undergoing the phase transition sequence F2mmI4mmI4/mmm as a function of increasing temperature, with the latter phase being observed above the known ferroelectric Curie temperature, Tm, and the intermediate phase consistent with a previously observed dielectric anomaly around 207 °C. The results are, however, in conflict with both observation of a symmetry lowering (to space group B2eb) in the lowest temperature phase, observed by electron diffraction, and also with electrical property measurements, which suggest both a- and c-axis polarisation up to Tm. Nevertheless, these crystallographic results are consistent with the observation of relaxor behaviour in this material, and underline the importance of considering ‘long-range’ versus ‘local’ structural effects in relaxor materials.  相似文献   

10.
The temperature dependence of the paramagnetic susceptibility χm(T) taken in 2500 Oe, the resistivity ρ(T), and the thermoelectric power α(T) of DyBaCo2O5+x, which has Ba and Dy ordered into alternate (001) planes of an oxygen-deficient perovskite, have revealed a phase segregation in the compositional range 0.3?x<0.5. Orthorhombic DyBaCo2O5.51 has, in addition, oxygen vacancies ordered into alternate rows of the DyO0.51 (001) planes; a cold-pressed polycrystalline sample exhibits a first-order insulator-metal transition at TIM=320 K, a Curie temperature TC=300 K, and a broadened metamagnetic transition temperature TM≈265 K in 2500 Oe. A ferromagnetic M-H hysteresis curve fails to saturate at 5 T, and a minority ferromagnetic phase below TM has a volume fraction that decreases with decreasing temperature, vanishing below 50 K. Oxygen vacancies in the DyBaCo2O5.5 phase suppress the metallic state; interstitial oxygen does not. A thermoelectric power α(T)>0 of DyBaCo2O5.51 changing continuously across TIM is interpreted to manifest a metallic minority phase crossing a percolation threshold; α(T) also provides evidence for a progressive excitation of higher-spin Co(III) with increasing temperature from below 50 K to above TIM. A previous model of the RBaCo2O5.5 phase is extended to account for the Ising spin configuration below TC, the magnetic order in the presence of higher-spin octahedral-site Co(III), and the α(T) data.  相似文献   

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