Immiscibility in the Fe3O4-FeCr2O4 spinel binary

A recent thermodynamic model of mixing in spinel binaries, based on changes in cation disordering (x) between tetrahedral and octahedral sites [Am. Mineral. 68 (1983) 18, 69 (1984) 733], is investigated for applicability to the Fe₃O₄-FeCr₂O₄ system under conditions where incomplete mixing occurs. Poor agreement with measured consolute solution temperature and solvus [N. Jb. Miner. Abh. 111 (1969) 184] is attributed to neglect of: (1) ordering of magnetic moments of cations in the tetrahedral sublattice antiparallel to the moments of those in the octahedral sublattice and (2) pair-wise electron hopping between octahedral site Fe³⁺ and Fe²⁺ ions. Disordering free energies (ΔG_D), from which free energies of mixing are calculated, are modeled by

     

Magnetic contribution to heat capacity and entropy of nickel ferrite (NiFe2O4)

The heat capacity of nickel ferrite was measured as a function of temperature from 50 to 1200 °C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 °C, the expected Curie temperature, T_C. The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a λ-transition (antiferromagnetic-paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based IR absorption spectrum, while the second was modeled using a spin wave analysis that provided a T^3/2 dependency in the low-temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperatures, as first suggested by Grimes [15]. The λ-transition was fitted to an Inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T_C). Exponential equality (m=n=7) was observed below and above T_C, indicating symmetry about the Curie temperature. Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe₂O₄, A=Fe, Co) revealed nearly the same exponential equality, i.e., m=n=5.     

Immiscibility in the NiFe2O4-NiCr2O4 spinel binary

The solid solution behavior of the Ni(Fe_1−nCr_n)₂O₄ spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe₂O₃ and Cr₂O₃ reactants, mixed to give NiCrFeO₄, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, T_cs). A solvus exists below T_cs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.     

Stiffening of nanoscale anatase Ti0.9Zr0.1O2 upon multiple compression cycles

The compression behavior of nanoscale Zr-doped anatase was studied by means of a diamond anvil cell experiment with alternating cycles of compression and decompression in the stability field of anatase (up to 13 GPa). We found that multiple cycles of compression lead to stiffening of the material: Precompressed samples of nanoanatase Ti_0.9Zr_0.1O₂ have a higher bulk modulus (K₀=249(9) and 266(6) GPa) compared with the sample when compressed for the first time (K₀=211(7) GPa). Upon compression, the crystallite size remains the same and the crystalline areas are free of defects. After the experiment, the crystallites are surrounded by amorphous rims, confirming the theoretical prediction by Pischedda et al. [Ultrastability and enhanced stiffness of similar to 6 nm TiO₂ nanoanatase and eventual pressure-induced disorder on the nanometer scale, Phys. Rev. Let. 96 (2006) 035509] for nanoscale anatase, but yielding much lower pressures (12 GPa) for the onset of partial amorphization.     

Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb₂Co₂V₃O₁₂ and NaPb₂Ni₂V₃O₁₂ have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb₂Co₂V₃O₁₂) and 12.666 Å (NaPb₂Ni₂V₃O₁₂), respectively. The magnetic susceptibility of NaPb₂Ni₂V₃O₁₂ shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μ_eff of NaPb₂Ni₂V₃O₁₂ is 3.14 μ_B due to Ni²⁺ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb₂Co₂V₃O₁₂, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb₂Co₂V₃O₁₂.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo₄O₇ has been investigated. It is shown that the oxides CaBaCo₄O_7+δ and CaBaCo_4−xO_7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with T_C~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co³⁺ cations, with respect to the ideal formula CaBaCo₂²⁺Co₂³⁺O_7. This excess Co³⁺ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.     

Van Vleck paramagnetism of the thulium garnet Tm3Al5O12

The magnetic susceptibility of the garnet-type single crystal Tm₃Al₅O₁₂ exhibits the typical Van Vleck temperature independent paramagnetism below ≈8 K. The temperature dependence of the susceptibility over the range 2.0-300 K has been analyzed on the assumption that the cubic crystal-electric-field dominates the energy level on ³H₆ (J=6) ground multiplet for Tm³⁺ ion having 12-electrons in 4f shell. The ground state of the ³H₆ is nonmagnetic with Γ₂ singlet, avoiding the Kramers doublet. The energy separation between Γ₂ and the first excited state Γ⁽²⁾₅ triplet is evaluated to be 68.0 K. The whole energy interval Δ between Γ₂ and the highest state Γ₁ in ³H₆ is estimated to be 339.5 K.     

The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7

The Ruddlesden–Popper (RP) phase compounds (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu, Sm, Nd and La) were prepared, and their transport and thermoelectric properties were investigated. The results indicate that high-T electrical resistivity ρ (300 K<T<1000 K) increases monotonically with temperature and basically has a relation ρ∝T^M, with M varying from 0.91 to 1.92 at temperatures T>~650 K, suggesting acoustic phonon scattering is dominant. At low temperatures (5 K<T<300 K), ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Nd and La) decreases monotonously with decreasing temperature, whereas ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu and Sm) decreases first, and then increases instead as T decreases to a critical temperature T_c. Moreover, electrical conductivity σ∝T^1/2 holds at lower temperatures, indicating that the electron–electron interaction caused by the presence of disorder dominates the transport process at the low temperatures. Besides, experiments show that at T<~400 K the lattice thermal conductivity of the doped compounds basically decreases with increase of the atomic mass of dopants. Generally, the figure of merit (ZT) at 1000 K increases first, and then decreases with the increase of the dopants' ionic radius, and the largest ZT is achieved in (Sr_0.95Gd_0.05)₃Ti₂O₇ mainly owing to its lower lattice thermal conductivity.     

A Ni/YSZ composite containing Ce0.9Ca0.1O2−δ particles as an anode for SOFCs

A composite material (hereafter referred to as NYC) containing Ni, Y₂O₃-stabilized ZrO₂ (YSZ) and Ce_0.9Ca_0.1O_2−δ (CC10) particles was prepared and used as the anode of solid oxide fuel cells (SOFCs). The performance of NYC was better than that of conventional Ni/YSZ anodes in terms of anodic overpotential and interface impedance. The additional CC10 particles improved the anode properties. XRD results suggest that a solid solution of YSZ and CC10 was produced. From impedance measurements, it is concluded that the solid solution exhibits substantial electronic conduction. Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ anodes exhibited the best properties over the experimental temperature range. A SOFC with an anode of Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ provided the maximum power density and current density. Addition of CC10 with an average particle size of 0.3 μm was more advantageous than that with an average size of 3 μm.     

Magnetic properties of compounds in the system CaBaCo4−x−yZnxAlyO7 (x=0,1,2, y=0,1)

The magnetic properties of four compounds in the series CaBaCo_4−x−yZn_xAl_yO₇ (x=0,1,2, y=0,1) were investigated. Using AC-susceptibility and DC-magnetometry, magnetic transitions (T_fs) were found for all four compositions in the range 50-3 K. The data from the AC measurements proved to be frequency dependent: T_f increases with higher frequencies. An energy-loss in the magnetic coupling, indicated as contributions in the imaginary part of the magnetic susceptibility (χ″), was seen for every compound and its maximum appeared just below the maximum χ′. Modelling the data with Arrhenius-, Vogel-Fulcher-, and the power-law made it possible to relate the four compounds to spin-glass materials. The Casimir-du Pré relation was used to extract average relaxation times at T_f. The DC magnetisations clearly show differences between field-cooled and zero-field-cooled measurements. None of the compounds exhibit any metamagnetic properties up to 8 T. A new method is presented to calculate the saturation fields using DC data. Relaxation measurements on three compounds indicate that the systems relax very fast, in contrast to spin-glasses. Aging does not affect the fast relaxations. The compounds are interpreted as disordered anti-ferromagnets with spin-glass features.     

Phase stability in the LnCa2Mn2O7 (Ln=Pr, Nd, Sm and Gd) Ruddlesden-Popper series

The synthesis of the Ruddlesden-Popper series, LnCa₂Mn₂O₇, (Ln=Pr, Nd, Sm and Gd) is described and their structure and electronic properties investigated. The reduction in size of the A-site cation causes an increase in the distortion of their orthorhombic structures (space group Cmcm). All of these compounds form with a perovskite impurity, the amount of which increases on reduction of the cation size. The synthesis temperature also alters the amount of perovskite impurity in the phase, but only to a lower limit, implying the perovskite phase is intrinsic to the material and that a phase equilibrium exists between the layered Ruddlesden-Popper and perovskite phases, which is controlled by the cation size. The magnetic susceptibility show transitions characteristic of the perovskite phase, therefore little direct information can be obtained about the Ruddlesden-Popper phases, except that ferromagnetism is not observed in any of these materials.     

Effect of chromium substitution in Ca4Mn3O10

Ca₄Mn_3−xCr_xO₁₀ compounds were synthesized in order to investigate the role of an isoelectronic substitution in the layered manganite. Induced structural changes are mainly described as a distortion of the two types of octahedra in the n=3 RP structure. The results indicate that Cr³⁺ is not the only significant valence state for chromium ions. Electrical and magnetic characterization allow to conclude that chromium does not favour the double exchange mechanism in these compounds.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Crystallization kinetics study of cerium titanate CeTi2O6

Cerium titanate CeTi₂O₆ has been investigated recently for its photocatalytic activity and as a safe analogue to actinide-containing brannerite-like titanates (UTi₂O₆, PuTi₂O₆, e.g.) which are intensively studied because of their use for storing nuclear waste. In this paper we report on the monoclinic phase CeTi₂O₆ obtained from the Ti–Ce oxide mixture prepared by a reverse micelles directed sol–gel method and subsequently annealed. The kinetics of the isothermal crystallization process is investigated by means of Johnson–Mehl–Avrami–Kolmogorov equation. The effective activation energy of the formation of CeTi₂O₆ particles, which is an important parameter for its synthesis, is estimated.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Effect of Bi substitution on the magnetic and dielectric properties of epitaxially grown BaFe0.3Zr0.7O3−δ thin films on SrTiO3 substrates

The structural, dielectric and magnetic properties of single crystalline Ba_1−xBi_xFe_0.3Zr_0.7O_3−δ (x=0.0-0.29) thin films have been studied. The pseudotetragonal epitaxial thin films were obtained by pulsed laser-beam deposition (PLD) on (0 0 1) SrTiO₃ (STO) single-crystal substrates. The Bi substitution for the Ba ions up to an extent of x=0.18 caused a slight improvement in the leakage current properties, as well as an enhancement of the apparent dielectric constant. The saturation magnetization of the films was significantly decreased following Bi substitution. These changes were thought to be related to the increase in oxygen deficiencies in the films. The effect of the Bi substitution on the dielectric and magnetic properties was analyzed in conjunction with the change in valence value of the Fe ions.     

Preparation and characterization of Cd2Nb2O7 thin films on Si substrates

Thin Cd₂Nb₂O₇ films were grown on single-crystal p-type SiO₂/Si substrates by the metallo-organic decomposition (MOD) technique. The films were investigated by X-ray diffraction, X-ray energy-dispersive spectroscopy, and field emission scanning electron microscopy, and showed a single phase (cubic pyrochlore), a crack-free spherical grain structure, and nanoparticles with a mean size of about 68 nm. A Cauchy model was also used in order to obtain the thickness and index of refraction of the stack layers (transparent layer/SiO₂/Si) by spectroscopic ellipsometry (SE). The dielectric constant (K) of the films was calculated to be about 25 from the capacitance-voltage (C-V) measurements.