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.     

High temperature characteristics of ZnO-based MOS-FETs with a photochemical vapor deposition SiO2 gate dielectric

The authors report the fabrication of ZnO-based metal-oxide-semiconductor field effect transistors (MOSFETs) with a high quality SiO₂ gate dielectric by photochemical vapor deposition (photo-CVD) on a sapphire substrate. Compared with ZnO-based metal-semiconductor FETs (MESFETs), it was found that the gate leakage current was decreased to more than two orders of magnitude by inserting the photo-CVD SiO₂ gate dielectric between ZnO and gate metal. Besides, it was also found that the fabricated ZnO MOSFETs can achieve normal operation of FET, even operated at 150 °C. This could be attributed to the high quality of photo-CVD SiO₂ layer. With a 2 μm gate length, the saturated I_ds and maximum transconductance (G_m) were 61.1 mA/mm and 10.2 mS/mm for ZnO-based MOSFETs measured at room temperature, while 45.7 mA/mm and 7.67 mS/mm for that measured at 150 °C, respectively.     

Dilithium zirconium hexafluoride Li2ZrF6 at high pressures: A new monoclinic phase

Dilithium zirconium hexafluoride, Li₂ZrF₆ (, Z=1), is studied at high pressures using synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell at room temperature. At atmospheric conditions, it has a structure with all the cations octahedrally coordinated to fluorine atoms. Above 10 GPa it transforms reversibly to a new polymorph (C2/c, Z=4), in which the coordination polyhedron of the Zr atoms is a distorted square antiprism, while the Li atoms are in the octahedral coordination. The LiF₆ octahedra form layers parallel to (100) that are connected by zig-zag chains of the edge-sharing Zr polyhedra running in the [001] direction. The relative change in volumes per one formula unit for both polymorphs is 6% at 11.8 GPa. The relations to other A₂BX₆-type structures are discussed.     

Young's modulus surface and Poisson's ratio curve for cubic metals

The general expressions for the compliance , Young's modulus E(h k l) and Poisson's ratio υ(h k l, θ) along arbitrary loading direction [h k l] are given for cubic crystals. The representation surface for which the length of the radius vector in the [h k l] direction equals to E(h k l) and representation curve for which the length of the radius vector with angle θ deviated from the reference directions , and equals to υ(100, θ), υ(110, θ) and υ(111, θ) for example, are constructed for six FCC metals Ag, Al, Au, Cu, Ni, Pb and seven BCC metals Cr, Fe, Mo, Nb, Ta, V, W.     

Antiferromagnetic transition and electrical conductivity in α-Gd2S3

Magnetic susceptibility and electrical resistivity of α-Gd₂S₃ with an orthorhombic structure (space group: Pnma) have been measured for powder and single-crystal samples. While the magnetic susceptibility of powder sample exhibits a broad peak having a maximum at 4.2 K, the susceptibility for a single crystal with an applied magnetic field along the b-axis demonstrates a sharp drop below 10 K. Nevertheless, the susceptibility with the field perpendicular to the b-axis keeps increasing with decreasing temperature even below 10 K. The electrical resistivity ρ for the powder sample of 4.2×10³ Ω cm around room temperature increases with decreasing temperature and shows a slight discontinuity at about 65 K. In both regions above and below 65 K, is proportional to T^−1/4 with respective coefficients, which is associated with Mott variable-range hopping conductivity. The resistivity of a single crystal along the b-axis is considerably smaller than the value for the powder sample as 0.35 Ω cm at room temperature, and its temperature dependence is fairly weak. While cooling, the resistivity first decreases down to 240 K and then keeps the value independent of the temperature down to 140 K, and subsequently rises gently below 140 K.     

Effects of lattice and local dynamics in EPR spectra and electron spin relaxation of vibronic Cu(imidazole)6 complexes in Zn(imidazole)6Cl2·4H2O crystals

Cu(im)₆ complexes in Zn(im)₆Cl₂·4H₂O exhibit a strong Jahn-Teller effect which is static below 100 K and the complex in localized in the two low-energy potential wells. We have reinvestigated electron paramagnetic resonance (EPR) spectra in the temperature range 4.2-300 K and determined the deformation directions produced by the Jahn-Teller effect, energy difference 11 cm⁻¹ between the wells and energy 300 cm⁻¹ of the third potential well. The electron spin relaxation was measured by electron spin echo (ESE) method in the temperature range of 4.2-45 K for single crystal and powder samples. The spin-lattice relaxation is dominated by a local mode of vibration with energy 11 cm⁻¹ at low temperatures. We suppose that this mode is due to reorientations (jumps) of the Cu(im)₆ complex between the two lowest energy potential wells. At intermediate temperatures (15-35 K), the T₁ relaxation is determined by the two-phonon Raman processes in acoustic phonon spectrum with Debye temperature Θ_D=167 K, whereas at higher temperatures the relaxation is governed by the optical phonon of energy 266 cm⁻¹. The ESE dephasing is produced by an instantaneous diffusion below 15 K with the temperature-independent phase memory time , then it grows exponentially with temperature with an activation energy of 97 cm⁻¹. This is the energy of the first excited vibronic level. The thermal population of this level leads to a transition from anisotropic to isotropic EPR spectrum observed around 90 K. FT-ESE gives ESEEM spectrum dominated by quadrupole peaks from non-coordinating ¹⁴N atom of the imidazole rings and the peak from double quantum transition ν_dq. We show that the amplitude of the ν_dq transition can be used to determine the number of non-coordinating nitrogen atoms.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.     

On the high-resistivity contacts to YBa2Cu3O7 thin films, electrical behavior in the regime of high temperatures and high-bias voltages

We prepared in-situ Au contacts on high-quality epitaxial YBa₂Cu₃O₇ (YBCO) films. Very high specific contact resistivity values up to ∼10⁻² Ω cm² at 4.2 K were obtained on 12×5 μm² contact areas. This resistivity value decreased by two orders of magnitude as the temperature was raised to room temperature. In the temperature range T<200 K, the contacts showed non-ohmic behavior suggesting the presence of a well-defined insulating native Y-Ba-Cu-O barrier between the two electrodes. The electrical transport in this barrier layer was analyzed in the limit of high temperatures and high voltages to follow Mott's variable-range hopping conduction mechanism with physically reasonable parameters describing the localized states in the barrier. The high-resistivity contacts were tested successfully in quasiparticles injection experiments where the critical current I_c of the YBCO microbridge could be strongly suppressed on injection of an additional current through the contact into the superconducting channel.     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

Optical and electroluminescent properties of 3,4,6-triphenyl-α-pyrone

The optical and electroluminescent properties of 3,4,6-triphenyl-α-pyrone (α-pyrone), a new blue fluoresce dye, are investigated using films prepared by wet and dry process and organic light-emitting diodes (OLEDs) fabricated with an α-pyrone-emitting layer. The optical properties of α-pyrone are found to be affected by its crystallinity. In the fabrication of OLEDs, wet processing (spin coating) is shown to be more suitable for preparation of the α-pyrone layer than dry processing (thermal evaporation). The best device performance is obtained for a device prepared using poly (n-vinylcarbazole) as the dye host, and a bathocuproine/tris-(8-hydroxyquinoline)aluminum bilayer as a hole-blocking and carrier-injection layer. The maximum luminance of this device is 3000 cd/m² at a current density of 0.2 A/cm², with a current efficiency of 1.8 cd/A at 0.02 A/cm².     

Photocatalytic O2 evolution performances of Cd1+xIn2−2xSnxO4 (x=0.1, 0.3, 0.5, 0.7, 1.0) conducting oxides

The conducting oxides solid solutions of Cd_1+xIn_2−2xSn_xO₄ (x=0.1, 0.3, 0.5, 0.7, 1.0) were prepared via a solid state reaction method. The band gaps were estimated to be 2.4 eV for x=1.0, 2.5 eV for x=0.7, 2.6 eV for x=0.5, 2.7 eV for x=0.3 and 2.8 eV for x=0.1. Oxygen could be evolved over Cd₂SnO₄ under the irradiation of Xe-lamp or even visible light (λ>420 nm), while the others could only work in the UV-light range. Raman showed the cation distribution in Cd₂SnO₄ is ordered, while that in the others is disordered. The cations distribution was proposed to be the cause of the difference in photocatalytic O₂-evolution activities.     

Thermally stimulated current measurements in undoped Ga3InSe4 single crystals

The trap levels in nominally undoped Ga₃InSe₄ crystals were investigated in the temperature range of 10-300 K using the thermally stimulated currents technique. The study of trap levels was accomplished by the measurements of current flowing along the c-axis of the crystal. During the experiments we utilized a constant heating rate of 0.8 K/s. Experimental evidence is found for one hole trapping center in the crystal with activation energy of 62 meV. The analysis of the experimental TSC curve gave reasonable results under the model that assumes slow retrapping. The capture cross-section of the trap was determined as 1.0×10⁻²⁵ cm² with concentration of 1.4×10¹⁷ cm⁻³.     

Heat capacity and magnetic phase transitions of rare-earth orthoferrite HoFeO3

Heat capacity of two rare-earth orthoferrites HoFeO₃ and LuFeO₃ were measured between 1.8 and 200 K. A distinctly large and two small heat capacity anomalies were detected for HoFeO₃ under zero magnetic field around 3.3, 53 and 58 K, respectively. The low-temperature anomaly with a peak at 3.3 K is due to the ordering of Ho³⁺ ions and the estimated magnetic entropy for this transition was favorably compared with the expected (R ln 2). Application of magnetic field significantly affects the positions and the magnitudes of the anomaly at 3.3 K. Energies of low-lying levels of the lowest J-term of Ho³⁺ ion were roughly estimated through analysis of the Schottky heat capacity.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.     

Temperature and pressure effects on the spin state of ferric ions in the [Fe(sal2-trien)][Ni(dmit)2] spin crossover complex

Thermal and pressure effects have been investigated on the [Fe(sal₂-trien)][Ni(dmit)₂] spin crossover complex by means of Mössbauer spectroscopic, calorimetric, X-ray diffraction and magnetic susceptibility measurements. The complex displays a complete thermal spin transition between the and spin states of Fe^III near 245 K with a hysteresis loop of ca. 30 K. This transition is characterised by a change of the enthalpy, ΔH_HL=7 kJ/mol, entropy, ΔS_HL=29 J/Kmol, and the unit cell volume, ΔV_HL=15.4 Å³. Under hydrostatic pressures up to 5.7 kbar the thermal transition shifts to higher temperatures by ca. 16 K/kbar. Interestingly, at a low applied pressure of 500 bar the hysteresis loop becomes wider (ca. 61 K) and the transition is blocked at ∼50% upon cooling, indicating a possible (irreversible) structural phase transition under pressure.     

Metal-insulator transition and variable-range hopping conductivity of n-CdSb in magnetic field

Resistivity, ρ, of a II-V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to and at temperatures . The low-temperature resistivity ρ(T) increasing with T in the range of B<4 T is found to have an upturn around B∼4 T and strong activated behavior at further increase of B. These observations give evidence for magnetic-field-induced metal-insulator transition (MIT). In the insulating side of the MIT, Mott variable-range hopping (VRH) conductivity with two types of asymptotic behavior, ln ρ (T, B)∼T^−3/4B² and ln ρ (T, B)∼(B/T)^1/3, is established in low and high magnetic fields, respectively. The VRH conductivity is analyzed using a model of the near-edge electron energy spectrum established by investigations of the Hall effect. The VRH conductivity is shown to take place over the band tail states of one out of two impurity bands, which for T=0 and B=0 lie above the conduction band edge.     

Investigations on Pr1.6Sr0.4NiO4-YSZ-Ag composite cathode for solid oxide fuel cells

Ag-network was successfully deposited by VA-EP (vacuum assisted electroless plating) method on Pr_1.6Sr_0.4NiO₄-YSZ cathode to form (1−x) wt% Pr_1.6Sr_0.4NiO₄−x wt% YSZ-Ag (x=0, 10, 20, 30, 40) (abbr. PYx-Ag) composite cathode. XRD results suggested that there was a good chemical stability between Pr_1.6Sr_0.4NiO₄ and YSZ at temperatures below 1050 °C. PY20-Ag cathode exhibited higher exchange current density, lower overpotential and ASR (Area Specific Resistance) than PY20 cathode. At 650 °C, the ASR of PY20-Ag cathode was 2.5 Ω cm², which was only about 42% of that of PY20, 5.9 Ω cm². PY20-Ag can be a promising candidate for SOFC cathode.     

Relationships between lattice energy and electronic polarizability of AB crystals

The correlations between the electronic polarizability, determined from Clausius-Mosotti equation based on dielectric constant ε, and the lattice energy density u have been established for A^NB^8-N crystals, such as the systems of rock salt crystals (group I-VII, II-VI) and tetrahedral coordinated crystals (group II-VI, III-V). For the A^NB^8-N crystals systems, our present conclusions suggest that lattice energy density u decreases exponentially with increasing electronic polarizability, and the normal mathematical expression between lattice energy density u and electronic polarizability is u = pα^q, p and q depend on the type of crystals. For the same cation binary A^NB^8-N crystals systems, curve fitting equations have been obtained, and the relevant squares of the correlation coefficient R² are larger than 0.99, which show all lattice energy density u are in good exponential relation with electronic polarizability. These empirical equations will give more information on calculating lattice energy or electronic polarizability. New data of lattice energy have been calculated on the above equation u = pα^q, and a good linear trend in the calculating values along with the Zhang’s values has been obtained.