Synthesis and Properties of NdMCr2O5 (M = Na, K, Cs) and NdMgCr2O5.5 Chromites

NdMCr₂O₅ (M = Na, K, Cs) and NdMgCr₂O_5.5 are prepared by solid-state reactions between appropriate oxides and carbonates and are shown to have a tetragonal structure. The heat capacity of these chromites, measured from 298.15 to 673 K, exhibits sharp changes attributable to second-order phase transitions. The C _p ⁰(T) data are represented by quadratic best fit equations. The electrical resistivity of the chromites is measured between 303 and 493 K. The results attest to semiconducting behavior of the materials in certain temperature ranges.     

Oxygen potentials,Gibbs' energies and phase relations in the Cu-Cr-O system

Thermodynamic properties of ternary compounds, cuprous and cupric chromites (CuCro₂, CuCr₂O₄), and oxygen potentials corresponding to three three-phase regions in the Cu-Cr-O system have been measured in the temperature range 900 to 1350 K using a solid state galvanic cell incorporating calcia-stabilized zirconia. Cuprous chromite was found to be nearly stoichiometric. The compositions of non-stoichiometric cupric chromite saturated with CuO and Cr₂O₃ have been determined using electron microprobe and energy dispersive X-ray analysis. The results of this study resolve discrepancies in Gibbs' energies of cuprous and cupric chromites reported in the literature. A ternary phase diagram for the Cu-Cr-O system at 1150 K and phase relations in air for the Cu₂O-CuO-Cr₂O₃ system as a function of temperature have been derived based on the new thermodynamic data. The phase diagram given in the literature is found to be inaccurate.     

Heat Capacity and thermodynamic functions of DyMe<Superscript>II</Superscript>Cr<Subscript>2</Subscript>O<Subscript>5.5</Subscript>(Me<Superscript>II</Superscript>-Mg,Ca) in the range from 298.15 to 673 K

The calorimetric method is used to investigate the heat capacity of DyMe^IICr₂O_5.5(Me^II-Mg, Ca) chromites in the range from 298.15 to 673 K. The C _p ⁰ ～ f(T) curves exhibit λ-like effects at 348 and 548 K for DyMgCr₂O_5.5 and at 473 K for DyCaCr₂O_5.5, which apparently relate to second-order phase transitions. The temperature dependences are calculated for thermodynamic functions C _p ⁰ (T), H ⁰(T)-H ⁰(298.15), S ⁰(T), and Φ**(T).     

Synthesis under high-oxygen pressure, magnetic and structural characterization from neutron powder diffraction data of YGa1−xMn1+xO5 (x = 0.23): A comparison with YMn2O5

A new material of nominal stoichiometry YGaMnO₅ has been prepared in polycrystalline form from citrate precursors followed by thermal treatments under high-oxygen pressure. This compound has been characterized from neutron powder diffraction (NPD) data and magnetic measurements. For comparison, the parent compound YMn₂O₅ has also been synthesized and its crystal structure refined by NPD data. The new oxide has an actual stoichiometry YGa_1−xMn_1+xO₅ (x = 0.23), determined by NPD, showing an important cationic disorder between both metal sites; it is orthorhombic, Pbam (SG), and its crystal structure contains chains of Mn⁴⁺O₆ edge-sharing octahedra, linked together by Ga³⁺O₅ pyramids and YO₈ units. With respect to YMn₂O₅, containing axially elongated MnO₅ pyramids due to the Jahn-Teller effect of Mn³⁺ cations, the GaO₅ pyramidal units in YGa_0.77Mn_1.23O₅ are substantially flattened. This compound has a paramagnetic behaviour with two weak anomalies at about 50 K and 350 K. The magnetic structures, studied at 1.4 K and 100 K show a ferromagnetic coupling along the chains of MnO₆ octahedra.     

Magnetization, magnetic susceptibility, and effective magnetic moment of the Fe3+ ions in Bi2Fe4O9

Bi₂Fe₄O₉ with an orthorhombic structure and lattice parameters a = 7.9595 Å, b = 8.4297 Å, c = 5.9912 Å, and V = 401.987 ų has been prepared by solid-state reactions method. Its molar magnetic susceptibility measured as a function of temperature in the range 5–950 K indicates that Bi₂Fe₄O₉ is an antiferromagnet with a Néel temperature of 258 K. In the range 280–750 K, its molar magnetic susceptibility exhibits Curie-Weiss behavior, which allowed us to determine the Weiss constant (Θ = ?1468 K) of this material and the effective magnetic moment of the Fe³⁺ ions $\left( {\mu _{eff}^{Fe^{3 + } } = 6.37\mu _B } \right)$ . Magnetization versus magnetic field data show no magnetic hysteresis, indicating that the Bi₂Fe₄O₉ sample studied exhibits no weak ferromagnetism.     

Synthesis,crystal structure refinement,electrical and magnetic properties of BaV13O18 and SrV13O18

Polycrystalline samples of BaV₁₃O₁₈ and SrV₁₃O₁₈ were prepared by solid-state reaction of BaCO₃, SrCO₃, V₂O₅ and V at 1773–2073 K in flowing Ar. The crystal structures of BaV₁₃O₁₈ (R-3, a_h=12.6293(10) Å, c_h=7.0121(4) Å) and SrV₁₃O₁₈ (a_h=12.5491(7) Å, c_h=6.9878(3) Å) were refined by the Rietveld method using X-ray diffraction data. BaV₁₃O₁₈ exhibited semiconducting behavior with electrical resistivity from 5.8×10⁻³ to 2.7×10⁻³ Ω cm at 100–300 K. Electrical resistivity of SrV₁₃O₁₈ ranged from 1.5×10⁻³ to 1.8×10⁻³ Ω cm, and it increased slightly up to around 250 K and decreased above 250 K with increasing temperature. Negative Seebeck coefficients of both compounds at 100–300 K indicated that electron was the dominant carrier. BaV₁₃O₁₈ and SrV₁₃O₁₈ showed paramagnetism with the effective magnetic moment of 0.11μ_B and 0.15μ_B, respectively, at 10–100 K.     

Multifunctional dual Z-scheme heterostructured Sm2O3-WO3-La2O3 nanocomposite: Enhanced electrochemical,photocatalytic, and antibacterial properties

In this study, heterostructured dual Z-scheme Sm₂O₃-WO₃-La₂O₃ nanocomposite (NC) and bare Sm₂O₃, WO₃, and La₂O₃ nanostructured (NSs) were prepared using a co-precipitation approach. The comparative electrochemical, photocatalytic, and antimicrobial properties of NC and NSs were studied. The XRD spectrum exhibits the formation of bare NSs and NC having cubic-Sm₂O₃, monoclinic-WO₃, and hexagonal-La₂O₃ phases. FESEM results showed mesoporous morphology of NC. The higher electrical conductivity 261.33 mho-cm⁻¹ and lower optical energy bandgap 2.55 eV were obtained for NC. The photodegradation tests exhibited that NC photocatalyst has degraded 99% (methylene blue), 96% (Methyl orange), 99% (safranin-O), 48% (p-nitroaniline), and 98% (methyl red) dyes pollutants in 40 min sunlight radiation and showed strong inhibition activity against E. coli, K. pneumoniae, S. aureus, P. Vulgaris, and P. aeruginosa bacterial strains with zones of inhibition (ZOI) 31 mm, 30 mm, 30 mm, 28 mm, and 31 mm, respectively. Electrochemical studies revealed the excellent capacitive characteristics of NC with a specific capacitance 532F/g at a scan rate 5 mV/s, energy density 48.0278 W h Kg⁻¹, and power density 0.3782 KW Kg⁻¹ at 0.006 A/cm² current density. Furthermore, the present study revealed a novel composition for environmental remediation and energy storage applications.     

Magnetic properties of RE2Mo2O7 pyrochlores

The magnetic properties of RE₂Mo₂O₇ (RE=Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) and of solid solutions of the type (Nd_1?x A _x )₂Mo₂O₇ (A=Yb or Er; 0.05≤x≤0.4) have been studied using an ac mutual inductance bridge. All the compounds were found to exhibit magnetic ordering in the neighborhood of or below 77 K. The data have been analyzed using (i) the available susceptibility data on Y₂Mo₂O₇, (ii) a ferromagnetic coupling among the Mo⁴⁺ ions to obtain the contribution from the RE³⁺ ion, and (iii) the available susceptibility data on RE₂Ti₂O₇ to obtain the contribution from the Mo⁴⁺ ions. It was found that procedure (iii) gave the most satisfactory explanation of the magnetic ordering. The results indicated that (1) the Mo⁴⁺ ions become ordered magnetically, (2) the behavior of the RE³⁺ ions is almost the same as in the isostructural RE₂Ti₂O₇ and RE₂V₂O₇ compounds, and (3) the susceptibility values differ appreciably as we go from Nd³⁺ to Yb³⁺, possibly due to narrowing of the π* Mo-O conduction band.     

Advanced ceramics: Combustion synthesis and properties

Fine-particle ceramic powders such as chromites, manganites, ferrites, cobaltites, aluminas (α-Al₂O₃, Cr³⁺/Al₂O₃, zirconia-toughened alumina, mullite and cordierite), ceria, titania, zirconia (t, m, c and PSZ), dielectric oxides (MTiO₃, PZT and PLZT) as well as highT _c cuprates have been prepared by the combustion of redox compounds or mixtures. The combustion-derived oxide materials are of submicron size with a large surface area and are sinteractive.     

Crystal structure, magnetic and thermal properties of LaFeTeO6

LaFeTeO₆ was prepared by solid state reaction of La₂O₃, Fe₂O₃ and TeO₂ in 1:1:2 molar ratios and characterized by powder X-ray diffraction, thermogravimetry and magnetometry. The detailed crystal structure analysis was carried out by Rietveld refinement. LaFeTeO₆ crystallizes in a trigonal lattice with unit cell parameters: a = 5.2049(1) Å and c = 10.3457(2) Å, V = 242.73(2) Å³. The crystal structure is built from sheets of the edge shared FeO₆ and TeO₆ octahedra stacked along the c-axis. These sheets are connected together by La³⁺ ions. Thermogravimetric analysis of the compound showed it to be thermally stable up to 1323 K and continuous loss of TeO₂ was observed above 1323 K leading to the formation of LaFeO₃. High temperature XRD studies revealed a normal expansion behavior of the compound. Temperature and field dependent magnetization of LaFeTeO₆ showed paramagnetic behavior in the temperature range of 3-300 K. The effective magnetic moment per Fe³⁺ ion (5.14 μB) indicates the high spin d⁵ state of Fe³⁺ ion.     

Characterization of plasma-sprayed titanium aluminates by electron spin resonance spectroscopy

The structures of plasma-sprayed titanium aluminates containing 13 wt.% and 40 wt.% TiO₂ were investigated using electron spin resonance (ESR) and X-ray diffraction. These sprayed materials were identified as mixtures of α-Al₂O₃, γ-Al₂O₃, β-Al₂TiO₅ and rutile phases. An ESR peak observed near g = 1.9 at room temperature and 77 K was assigned to Ti³⁺ ions in octahedral environments. The relations between the halfwidths of the α-Al₂O₃, γ-Al₂O₃ and β-Al₂TiO₅, the linewidth and the g value of the ESR peak suggest that the Ti³⁺ ions are in the β-Al₂TiO₅ lattice. Further, the degree of asymmetry of the environments of the Ti³⁺ ions in the β-Al₂TiO₅ phase depends on the crystallinity of β-Al₂TiO₅.     

Heat capacity and electrophysical properties of GdMeFe2O5(Me — Li, Na, K, Cs)-type ferrites

The heat capacity of GdMeFe₂O₅(Me — Li, Na, K, Cs) is investigated within the temperature range of 298.15–673 K by a calorimetric method. λ-points are revealed in the C _p ^○ (T) curves at 448 K and 598 K in GdLiFe₂O₅, at 473 K and 573 K in GdNaFe₂O₅, at 448 K and 598 K in GdKFe₂O₅, and at 448 in GdCsFe₂O₅ related to a phase transition of the second kind. The temperature dependencies are calculated for the thermodynamic functions C _p ^○ (T), H ^○(T)-H ^○(298.15), S ^○(T), and Φ**(T). The electrophysical characteristics of the synthesized ferrites are studied within the temperature range of 303–493 K: Effects similar to those found in the C _p ^○ (T) curves are also revealed in the log?(T) and logR (T) curves. The compositions are shown to exhibit a semiconductor-type electrical conductivity. At the T _cond points of the ferrites, the semiconductor-type conductivity changes the metal type one and both the capacities and permittivities vary sharply, and that may be related to a ferroelectric phase transition (the Curie and the Neel points). These effects provide a certain possibility to clarify the nature of the λ-points in the C _p ^○ (T) curves.     

Synthesis, Structure, and Electrical Properties of NdMFe2O5 (M = Li, Na, K, Cs) Ferrites

The NdMFe₂O₅ (M = Li, Na, K, Cs) ferrites are prepared by solid-state reactions using mixtures of Nd₂O₃, Fe₂O₃, and M₂CO₃. The ferrites are shown to have a tetragonally distorted perovskite structure (Z= 16) with the following lattice parameters: a = 10.94 Å, c = 13.83 Å,V = 1655.2 ų for NdLiFe₂O₅; a= 10.98 Å, c = 15.10 Å, V = 1820.5 ų for NdNaFe₂O₅; a= 10.96 Å, c = 16.82 Å, V= 2020.5 ų for NdKFe₂O₅; a= 10.93 Å, c= 17.94 Å, V = 2143.2 ų for NdCsFe₂O₅.     

Phase relation and superconductivity in the oxygen nonstoichiometric system,Ba2YCu3Ox

The superconducting Ba₂YCu₃O_x system prepared under various annealing conditions has been characterized by differential thermal analysis, thermogravimetry, powder X-ray diffraction and electrical resistivity measurements. Oxygen content x increases from 5.86 to 7.14 as the annealing temperature varies from 1273 to 473 K. The lattice symmetry is tetragonal for x=5.86 and orthorhombic for x>6.01. The resistivity decreases from an order of 10⁰ to 10⁻⁴ cm with increasing x from 5.86 to 6.86, but tends to increase for x>6.86; the highest Tc of about 88 K(ϱ=0) has been obtained for Ba₂YCu₃O_6.86, where the Cu³⁺/Cu²⁺ ratio reaches 0.72/2.28. A change in x of ±0.1 results in a considerable depression of Tc.     

Proton-induced reduction ofR s,Jc, andT c in YBa2Cu3O7−δ thin films

We have explored the effect of 2-MeVH⁺ irradiation on the superconducting transport properties of thin films of YBa₂Cu₃O_7?δ [T _c, J_c(B=0; 77 K, 4.2 K), andR _s(36 GHz;T)]. The inductively measured critical temperatureT _c changed slowly and uniformly (～2 K per 10¹⁶/cm²) for fluences less than ～3×10¹⁶/cm². Beginning at ～3–4×10¹⁶/cm², the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwaveT _c (as defined by the sharp transition inR _s as a function of temperature) resembled the low-frequency inductiveT _c measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance (～6–10 mΩ) was not affected for fluences up to 5×10¹⁶/cm². We have interpreted the sudden and reproducible reduction in the microwaveT _c transition as a sensitive indicator of disruption in the copper-oxygen chain sublattice and compared the proton-induced change to that observed in oxygen gettering studies of bulk materials.     

A new high-T c superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-x Bi x )1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+δ (123 phase) and Bi₄Sr₄Ca₂Cu₄O_16+δ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+δ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+δ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Thermal Dissociation of RMnO<Subscript>3</Subscript> (R = Dy,Yb, Lu)

The phase equilibria involved in the thermal dissociation of RMnO₃ (R = Dy, Yb, Lu) were studied in the range 973–1173 K by a static method in a vacuum circulation unit and by x-ray diffraction analysis of quenched solid phases. The RMnO₃ manganites were shown to dissociate by the reaction RMnO₃ = 1/2R₂O₃ + MnO + 1/4O₂. The temperature dependences of the equilibrium oxygen pressure and Gibbs energy change in this reaction were determined for the three compounds. The experimental data were used to evaluate the standard thermodynamic functions of formation of RMnO₃ from R₂O₃ and Mn₂O₃: ΔH⁰(T) = ?88.93 kJ/mol, Δ S⁰(T) = 46.56 J/(mol K) for DyMnO₃; ΔH⁰(T) = ?130.95 kJ/mol, Δ S⁰(T) = 86.25 J/(mol K) for YbMnO₃; ΔH⁰(T) = ?142.94 kJ/mol, Δ S⁰(T) = 102.87 J/(mol K) for LuMnO₃.     

Heterogeneous phase equilibria in the Tb-Mn-O system

The heterogeneous phase equilibria involved in the thermal dissociation and hydrogen reduction of TbMnO₃ and TbMn₂O₅ have been studied in the range 973–1173 K by a static method in a circulation unit and by x-ray diffraction analysis of quenched solid phases. The results are represented in the form of oxygen pressure-temperature-composition three-dimensional phase diagrams and their projections onto the oxygen pressure-composition and temperature-composition planes. Phase compatibility triangles and univariant phase equilibria in the Tb-Mn-O system are identified for varied temperature and oxygen pressure. Using thermodynamic analysis of the phase equilibria identified, we have determined the standard enthalpy and entropy of formation of TbMnO₃ and TbMn₂O₅ from elements: ΔH ⁰(T) = ?1488.95 kJ/mol, ΔS ⁰(T) = 242.17 J/(mol K) for TbMnO₃; ΔH ⁰(T) = ?2110.66 kJ/mol, ΔS ⁰(T) = 482.89 J/(mol K) for TbMn₂O₅.     

Crystal structure and magnetic properties of Sr1 − x Sm x Fe12 − x Co x O19 solid solutions

Sr_{1 ? x} Sm _x Fe_{12 ? x} Co _x O₁₉ (0 ≤ x ≤ 0.5) ferrites have been prepared by solid-state reactions in air at 1470 K using mixtures of samarium oxide, ferric oxide, Co₃O₄, and strontium carbonate. X-ray diffraction characterization showed that the samples with x < 0.2 were single-phase, whereas the samples with 0.2 ≤ x ≤ 0.5 contained α-Fe₂O₃ and those with 0.3 ≤ x ≤ 0.5 contained SmFeO₃, CoFe₂O₄, and Sm₂O₃ as well. The highest degree of Sm³⁺ and Co²⁺ substitutions for Sr²⁺ and Fe³⁺ (x) in the SrFe₁₂O₁₉ ferrite at 1470 K was determined to be slightly less than 0.2. This substitution only slightly decreases the a and c parameters of the hexagonal lattice and the Curie temperature (T _C) of the material. At temperatures of 5 and 300 K in magnetic fields of up to 14 T, we obtained magnetic hysteresis loops, which were used to evaluate the spontaneous magnetization (σ₀), specific saturation magnetization (σ_s), and coercive force (_σ H _c) of the ferrites. The experimentally determined 5-K spontaneous magnetization per formula unit (n ₀) of the x = 0.1 ferrite is 20.86μ_B, which coincides with the theoretical value calculated as n ₀ = (8 × 5) ? (3.9 × 5 ? 0.1 × 3) = 20.8μ_B. At 300 K, the n ₀ and _σ H _c of Sr_0.9Sm_0.1Fe_11.9Co_0.1O₁₉ exceed those of SrFe₁₂O₁₉ by 7.7 and 9.9%, respectively.