Thermodynamic properties of calcium titanates: CaTiO3, Ca4Ti3O10, and Ca3Ti2O7

The chemical potentials of CaO in two-phase fields (TiO₂ + CaTiO₃), (CaTiO₃ + Ca₄Ti₃O₁₀), and (Ca₄Ti₃O₁₀ + Ca₃Ti₂O₇) of the pseudo-binary system (CaO + TiO₂) have been measured in the temperature range (900 to 1250) K, relative to pure CaO as the reference state, using solid-state galvanic cells incorporating single crystal CaF₂ as the solid electrolyte. The cells were operated under pure oxygen at ambient pressure. The standard Gibbs free energies of formation of calcium titanates, CaTiO₃, Ca₄Ti₃O₁₀, and Ca₃Ti₂O₇, from their component binary oxides were derived from the reversible e.m.f.s. The results can be summarised by the following equations: CaO(solid) + TiO₂(solid) → CaTiO₃(solid), ΔG° ± 85/(J · mol^?1) = ?80,140 ? 6.302(T/K); 4CaO(solid) + 3TiO₂(solid) → Ca₄Ti₃O₁₀(solid), ΔG° ± 275/(J · mol^?1) = ?243,473 ? 25.758(T/K); 3CaO(solid) + 2TiO₂(solid) → Ca₃Ti₂O₇(solid), ΔG° ± 185/(J · mol^?1) = ?164,217 ? 16.838(T/K).The reference state for solid TiO₂ is the rutile form. The results of this study are in good agreement with thermodynamic data for CaTiO₃ reported in the literature. For Ca₄Ti₃O₁₀ Gibbs free energy of formation obtained in this study differs significantly from that reported by Taylor and Schmalzried at T = 873 K. For Ca₃Ti₂O₇ experimental measurements are not available in the literature for direct comparison with the results obtained in this study. Nevertheless, the standard entropy for Ca₃Ti₂O₇ at T = 298.15 K estimated from the results of this study using the Neumann–Koop rule is in fair agreement with the value obtained from low-temperature heat capacity measurements.     

Hopping conduction in double layered La2−2xCa1+2xMn2O7 manganite

Double layered La_2−2xCa_1+2xMn₂O₇ manganite has been synthesized using solid state reaction method for different dopant concentration x = 0.0–0.5. Their temperature dependence of resistivity (ρ − T) has been studied in the semiconducting region. The experimental observations were compared with the theoretically simulated temperature dependence of resistivity curves based on nearest neighbour hopping, Efros–Shklovskii variable range hopping, and Mott's 2D and 3D variable range hopping models. From the analysis of these results, Mott's 3D variable range hopping mechanism seemed to be most suitable mechanism describing the semiconducting behaviour of these double layered manganites. Temperature dependent activation energy also supported the Mott's 3D variable range hopping model. The Mott's activation energy was found to vary with the dopant concentration x and it showed a minimum value for x = 0.3.     

Fabrication and magnetic properties of electrospun copper ferrite (CuFe2O4) nanofibers

Tetragonal copper ferrite (CuFe₂O₄) nanofibers were fabricated by electrospinning method using a solution that contained poly(vinyl pyrrolidone) (PVP) and Cu and Fe nitrates as alternative metal sources. The as-spun and calcined CuFe₂O₄/PVP composite samples were characterized by TG-DTA, X-ray diffraction, FT-IR, and SEM, respectively. After calcination of the as-spun CuFe₂O₄/PVP composite nanofibers (fiber size of 89 ± 12 nm in diameter) at 500 °C in air for 2 h, CuFe₂O₄ nanofibers of 66 ± 13 nm in diameter having well-developed tetragonal structure were successfully obtained. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. After calcination at 600 and 700 °C, the nature of nanofibers changed which was possibly due to the reorganization of the CuFe₂O₄ structure at high temperature, and a fiber structure of packed particles or crystallites was prominent. Crystallite size of the nanoparticles contained in nanofibers increases from 7.9 to 23.98 nm with increasing calcination temperature between 500 and 700 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined CuFe₂O₄ samples, having their specific saturation magnetization (M_s) values of 17.73, 20.52, and 23.98 emu/g for the samples calcined at 500, 600, and 700 °C, respectively.     

Neutron powder diffraction study and B-site ordering in microwave dielectric ceramics Ba(Ca1/3Nb2/3)O3

Microwave dielectric powder Ba(Ca_1/3Nb_2/3)O₃ with high B-site cation ordering was synthesized by the molten salt method. Neutron powder diffraction (NPD) and Raman scattering spectra were introduced to investigate the variable ordering degree during the sintering process. It was revealed that the as-synthesized Ba(Ca_1/3Nb_2/3)O₃ powder had a nearly completely ordered structure, and the sintered Ba(Ca_1/3Nb_2/3)O₃ presented a bit higher ordering degree based on the detailed quantitative NPD Rietveld full profile fitting. The complete Raman mode assignment for the Ba(Ca_1/3Nb_2/3)O₃ was presented. The phonon bands could also confirm the order–disorder structural model and increasing ordering degree with the increasing sintering temperature. Final microwave dielectric measurements on sintered samples showed the present compound to tailor other dielectric materials for microwave applications with the dielectric properties of ?_r = 47.11, and Qf = 1389 GHz, and τ_f was about 113 ppm/°C.     

Experimental determination of the liquidus of the binary system (Cu2O + CaO) in air at temperature from (1050 to 1500) °C

The liquidus of the binary system (Cu₂O + CaO) has been experimentally investigated over the temperature range from (1050 to 1500) °C in air. The liquidus has been quantified by the equilibration/quench/analysis technique. Equilibrated specimens were quenched in brine solution of 10 wt% NaCl. Specimens were then examined by Energy-Dispersive X-ray Spectroscopy (EDS) and Electron probe micro-analyzer (EPMA) to quantify liquid chemical compositions. This work reveals that the solubility of CaO in the liquid phase of the binary (Cu₂O + CaO) at higher temperatures is lower than previously reported, and the deviation systematically increases with a corresponding increase in temperature. This is predominant at temperatures between (1300 and 1500) °C where a deviation of up to 5 wt% in CaO solubility is observed. The solubility of Cu₂O in liquid phase for the cuprite saturated section of the binary however is consistent with all the previous authors.     

Thermodynamics of Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4

High-temperature heat capacity measurements were obtained for Cr₂O₃, FeCr₂O₄, ZnCr₂O₄, and CoCr₂O₄ using a differential scanning calorimeter. These data were combined with previously available, overlapping heat capacity data at temperatures up to 400 K and fitted to 5-parameter Maier–Kelley C_p(T) equations. Expressions for molar entropy were then derived by suitable integration of the Maier–Kelley equations in combination with recent S^∘(298) evaluations. Finally, a database of high-temperature equilibrium measurements on the formation of these oxides was constructed and critically evaluated. Gibbs free energies of Cr₂O₃, FeCr₂O₄, and CoCr₂O₄ were referenced by averaging the most reliable results at reference temperatures of (1100, 1400, and 1373) K, respectively, while Gibbs free energies for ZnCr₂O₄ were referenced to the results of Jacob [K.T. Jacob, Thermochim. Acta 15 (1976) 79–87] at T = 1100 K. Thermodynamic extrapolations from the high-temperature reference points to T = 298.15 K by application of the heat capacity correlations gave Δ_fG^∘(298) = (−1049.96, −1339.40, −1428.35, and −1326.75) kJ · mol⁻¹ for Cr₂O₃, FeCr₂O₄, ZnCr₂O₄, and CoCr₂O₄, respectively.     

Raman scattering study of the geometrically frustrated pyrochlore Y2Ru2O7

Geometrically frustrated pyrochlore Y₂Ru₂O₇, which shows a spin-glass-like transition at T_G ∼ 80 K, were investigated by temperature-dependent Raman scattering. Three discernable phonons appear around 315, 410, and 510 cm⁻¹ without any abrupt change in the number of Raman active modes within the temperature range of 10–300 K. Fitting each phonon with Lorentz oscillators, we analyzed the effects of temperature on the phonon frequencies and the linewidths. The temperature-dependence of the mode near 510 cm⁻¹ shows abnormal behavior below T_G, while the other two phonons follow the usual thermal effect of lattice vibration. This behavior can be understood in terms of spin–phonon coupling. Considering the atomic modulations of each phonon mode, it is conjectured that the 510 cm⁻¹ phonon mode is isotropically coupled to the spin degree of freedom, while the other modes are not.     

Solvothermal synthesis and crystal structure of the Mo(VI)-bridged helical chain containing Mo2(V) dimers: (C4H12N2)2[(MoV2O4)(MoVIO4)(C2O4)2]·2H2O

A new molybdenum complex (C₄H₁₂N₂)₂[(Mo^V₂O₄)(Mo^VIO₄)(C₂O₄)₂]·2H₂O, was solvothermally synthesized and characterized by single-crystal X-ray diffraction. The structure of the compound consists of oxalate acid-coordinated mixed-valent [Mo^V₂O₄][Mo^VIO₄] helical chains and protonated piperazine cations. The helical chains are built up from the [Mo^V₂O₄] units and [Mo^VIO₄] tetrahedral. The central axis about helical chain is a 2-fold screw axis. The compound crystallizes in the space group P2₁/n of monoclinic system with a = 11.396(2) Å, b = 14.107(3) Å, c = 15.805(3) Å, β = 102.09(3)°, V = 2484.6(9) Å³, Z = 4. Other characterizations by elemental analysis, IR, and thermal analysis for this compound are also given.     

Heat capacities,third-law entropies and thermodynamic functions of the geometrically frustrated antiferromagnetic spinels GeCo2O4 and GeNi2O4 from T=(0 to 400) K

The molar heat capacities of GeCo₂O₄ and GeNi₂O₄, two geometrically frustrated spinels, have been measured in the temperature range from T=(0.5 to 400) K. Anomalies associated with magnetic ordering occur in the heat capacities of both compounds. The transition in GeCo₂O₄ occurs at T=20.6 K while two peaks are found in the heat capacity of GeNi₂O₄, both within the narrow temperature range between 11.4<(T/K)<12.2. Thermodynamic functions have been generated from smoothed fits of the experimental results. At T=298.15 K the standard molar heat capacities are (143.44 ± 0.14) J · K⁻¹ · mol⁻¹ for GeCo₂O₄ and (130.76 ± 0.13) J · K⁻¹ · mol⁻¹ for GeNi₂O₄. The standard molar entropies at T=298.15 K for GeCo₂O₄ and GeNi₂O₄ are (149.20 ± 0.60) J · K⁻¹ · mol⁻¹ and (131.80 ± 0.53) J · K⁻¹ · mol⁻¹ respectively. Above 100 K, the heat capacity of the cobalt compound is significantly higher than that of the nickel compound. The excess heat capacity can be reasonably modeled by the assumption of a Schottky contribution arising from the thermal excitation of electronic states associated with the CO²⁺ ion in a cubic crystal field. The splittings obtained, 230 cm⁻¹ for the four-fold-degenerate first excited state and 610 cm⁻¹ for the six-fold degenerate second excited state, are significantly lower than those observed in pure CoO.     

Thermodynamic study of the system (LiCl + CaCl2 + H2O)

Solubility isotherms of the ternary system (LiCl + CaCl₂ + H₂O) were elaborately determined at T = (283.15 and 323.15) K. Several thermodynamic models were applied to represent the thermodynamic properties of this system. By comparing the predicted and experimental water activities in the ternary system, an empirical modified BET model was selected to represent the thermodynamic properties of this system. The solubility data determined in this work at T = (283.15 and 323.15) K, as well as those from the literature at other temperatures, were used for the model parameterization. A complete phase diagram of the ternary system was predicted over the temperature range from (273.15 to 323.15) K. Subsequently, the Gibbs free energy of formation of the solid phases CaCl₂ · 4 H₂O_(s), CaCl₂ · 2 H₂O_(s), LiCl · 2H₂O_(s), and LiCl · CaCl₂ · 5H₂O_(s) was estimated and compared with the literature data.     

Thermodynamic properties of Na2Ti6O13 and Na2Ti3O7: electrochemical and calorimetric determination

Standard values of Gibbs free energy, entropy, and enthalpy of Na₂Ti₆O₁₃ and Na₂Ti₃O₇ were determined by evaluating emf-measurements of thermodynamically defined solid state electrochemical cells based on a Na–β″-alumina electrolyte. The central part of the anodic half cell consisted of Na₂CO₃, while two appropriate coexisting phases of the ternary system Na–Ti–O are used as cathodic materials. The cell was placed in an atmosphere containing CO₂ and O₂. By combining the results of emf-measurements in the temperature range of 573⩽T/K⩽1023 and of adiabatic calorimetric measurements of the heat capacities in the low-temperature region 15⩽T/K⩽300, the thermodynamic data were determined for a wide temperature range of 15⩽T/K⩽1100. The standard molar enthalpy of formation and standard molar entropy at T=298.15 K as determined by emf-measurements are Δ_fH_m⁰=(−6277.9±6.5) kJ · mol⁻¹ and S_m⁰=(404.6±5.3) J · mol⁻¹ · K⁻¹ for Na₂Ti₆O₁₃ and Δ_fH_m⁰=(−3459.2±3.8) kJ · mol⁻¹ and S_m⁰=(227.8±3.7) J · mol⁻¹ · K⁻¹ for Na₂Ti₃O₇. The standard molar entropy at T=298.15 K obtained from low-temperature calorimetry is S_m⁰=399.7 J · mol⁻¹ · K⁻¹ and S_m⁰=229.4 J · mol⁻¹ · K⁻¹ for Na₂Ti₆O₁₃ and Na₂Ti₃O₇, respectively. The phase widths with respect to Na₂O content were studied by using a Na₂O-titration technique.     

A Ca substitution study of NaV2O4: High-pressure synthesis of the Na1−xCaxV2O4 solid solution

Ambient pressure CaV₂O₄ and high-pressure NaV₂O₄ crystallize in the CaFe₂O₄ structure type containing double chains of edge-sharing VO₆ octahedra. Recent measurements on NaV₂O₄ reveal low-dimensional metallicity and evidence of half-metallic ferromagnetism. In contrast, CaV₂O₄ is an antiferromagnetic insulator. To explore the evolution of these ground-state behaviors, we have prepared a series of Ca-doped NaV₂O₄ compounds with the formula Na_1?xCa_xV₂O₄ (x = 0–1) using high-pressure synthesis. Samples at the Na end (x = 0–0.07) show a broad antiferromagnetic transition in the 120–160 K range in accordance with earlier reports. Transport measurements show an insulator–metal transition at x ～ 0.2. Samples with higher Ca concentrations (x = 0.4–0.7) exhibit a metal–insulator transition around 150 K. The results for the Na_1?xCa_xV₂O₄ solid solution is discussed in comparison to existing studies at the Ca- and Na-rich ends.     

The heat capacities and thermodynamic properties of NiAl2O4 and CoAl2O4 measured by adiabatic calorimetry from T = (4 to 400) K

The low-temperature heat capacity of NiAl₂O₄ and CoAl₂O₄ was measured between T = (4 and 400) K and thermodynamic functions were derived from the results. The measured heat-capacity curves show sharp anomalies peaking at around T = 7.5 K for NiAl₂O₄ and at T = 9 K for CoAl₂O₄. The exact cause of these anomalies is unknown. From our results, we suggest a standard entropy for NiAl₂O₄ at T = 298.15 K of (97.1 ± 0.2) J · mol^?1 · K^?1 and for CoAl₂O₄ of (100.3 ± 0.2) J · mol^?1 · K^?1.     

Real structure of SrSi2O2N2

SrSi₂O₂N₂ is an important host lattice for Eu²⁺ doped phosphors. Its crystal structure (space group P1, a = 7.0802(2) Å, b = 7.2306(2) Å, c = 7.2554(2) Å, α = 88.767(3)°, β = 84.733(2)°, γ = 75.905(2)° and V = 358.73(2) Å³, Z = 4) is isotypic with EuSi₂O₂N₂: highly condensed silicate layers are separated by Sr²⁺. The samples are characterized by pronounced real structure effects owing to pseudosymmetry of partial structures. Polysynthetic twinning with domains of various sizes is ubiquitous and oriented intergrowth of domains with different orientations has also been observed and analysed in detail by means of electron diffraction and high-resolution electron microscopy. These effects also affect the X-ray powder pattern and were taken into account in a Rietveld refinement.     

Studies of dielectric characteristics of BaBi2Nb2O9 ferroelectrics prepared by chemical precursor decomposition method

BaBiNb₂O₉ (BBN) powders in the nanometer range were prepared by chemical precursor decomposition method (CPD). TG–DTA showed that precursor sample got freed from organic contaminants at 575 °C. XRD showed that a single phase with the layered perovskite structure of BBN was formed after calcining at 600 °C. No intermediate phase was found during heat treatment at and above 600 °C. The crystallite size (D) and the effective strain (η) were found to be 26 nm and 0.000867, respectively, while the particle size obtained from TEM was 28 ± 2 nm. SEM revealed that the average grain size after sintering at 900 °C for 4 h was ∼1.67 μm. A relative density of ∼93% was obtained using a two-step sintering process at moderate pressure. Dielectric and ferroelectric properties were investigated in the temperature range 50–500 °C and frequencies from 1 kHz to 5 MHz. Strong dispersion of the complex relative dielectric constant was observed including typical relaxor features such as shift of permittivity maximum with frequency and broadening of the peak maximum. The high dielectric constant of 545 measured at 100 kHz and other properties of BBN ceramics were compared to that of BBN prepared by other conventional methods and found to be superior.     

Formation of nano-crystalline quartz crystals from ZnO/MgO/Al2O3/TiO2/ZrO2/SiO2 glasses

Glasses with the compositions 50.9 SiO₂ · 20.8 Al₂O₃ · (20.8 ? x) MgO· × ZnO · 3.7 TiO₂ · 3.7 ZrO₂ with x = 0, 2.3, 4.6 and 9.3 were annealed at temperatures in the range from 850 to 1100 °C. Depending on temperature, high- or low-quartz solid solutions, magnesium aluminosilicate, mullit and spinel precipitated. These glass–ceramics exhibit excellent mechanical properties and are potential candidates for applications in micromechanics or as hard disc substrate.The larger the ZnO concentration, the lower is the glass transition temperature. Also microhardnesses and Young’s moduli increased with increasing ZnO concentration. The nucleation temperature was of minor importance. To achieve good mechanical properties, the initially formed high-quartz phase must transform to the corresponding low-quartz phase. This occurs if the quartz phase contains only minor MgO or ZnO concentrations, which can be achieved by increasing the annealing times or temperature. Then MgO, ZnO and Al₂O₃ occur as separate spinel or gahnite phase.     

Effects of NiO addition on the structure and electric properties of Dy3−xNixFe5O12 garnet ferrite

Polycrystalline garnet ferrites Dy_3?xNi_xFe₅O₁₂ with varying Ni substitutions (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) have been prepared by the standard ceramic technique and their crystalline structures were investigated by using X-ray diffraction and IR spectroscopy. The X-ray diffraction analysis showed that all samples have a single cubic garnet phase. The materials prepared are identified by infrared rays which indicate the presence of three absorption bands ν₂, ν₃ and ν₄ which represent the tetrahedral, octahedral and dodecahedral sites respectively which characterize the garnet ferrite.The dielectric constant (?), and dielectric loss (tan δ) of the prepared samples were measured at 1 kHz in the temperature range 300–700 K. The dielectric constant (?), and dielectric loss (tan δ) are functions of temperature.The initial magnetic permeability has been studied at different temperatures. The initial magnetic permeability (μ_i) increases gradually with increasing temperature and then drops suddenly at a certain temperature T_c.     

In situ FT-IR and TPD-MS study of carbon monoxide oxidation over a CeO2/Co3O4 catalyst

The forming of surface species during the adsorption of carbon monoxide (CO) and CO/O₂ on a CeO₂/Co₃O₄ catalyst was investigated by in situ Fourier transform infrared (FT-IR) spectroscopy and temperature programmed desorption-mass spectroscopy (TPD-MS). When CO was adsorbed on the CeO₂/Co₃O₄ catalyst, two types of surface species were distinguishable at room temperature: carbonate and bicarbonate. Surface carbonate was adsorbed on the cerium and cobalt, while the surface bicarbonate absorbed on the CeO₂/Co₃O₄ catalyst at 1611, 1391, 1216 and 830 cm⁻¹. Furthermore, the TPD-MS profiles revealed that the CeO₂/Co₃O₄ catalyst showed a greater amount of CO₂ than CO at 373 K. The CO desorption from the CeO₂/Co₃O₄ catalyst with increasing temperature showed that the order of thermal stability was surface bicarbonate < surface carbonate < interface carbonate species. Interestingly, the residual carbonate species could remain on the interface up to 473 K. The results revealed that surface bicarbonate could promote the conversion of CO into CO₂ for CO oxidation below 50 K.     

Low temperature heat capacity Study of Fe(PO3)3 and Fe2P2O7

The heat capacities of two iron phosphates, Fe(PO₃)₃ and Fe₂P₂O₇, have been measured over the temperature range from (2 to 300) K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). A phase transition related to magnetic ordering has been found in the heat capacity at T = 8.76 K for Fe(PO₃)₃ and T = 18.96 K for Fe₂P₂O₇, which are comparable with literature values from magnetic measurements. By fitting the experimental heat capacity values, the thermodynamic functions, magnetic heat capacities, and magnetic entropies have been determined. Additionally, theoretical fits at low temperatures suggest that Fe₂P₂O₇ has an anisotropic antiferromagnetic contribution to the heat capacity and a large linear term likely caused by oxygen vacancies. Further data fitting in a series over widened temperature regions found that this linear term exists only below 15 K and disappears gradually from (15 to 17) K.     

Fast tunneling Jahn–Teller isomer of the [Mn12O12(O2CC6H4-2-CH3)16(H2O)4] · S single-molecule magnet

The single-crystal X-ray structure of the single-molecule magnet [Mn₁₂O₁₂(O₂CC₆H₄-2-CH₃)₁₆(H₂O)₄] · CH₂Cl₂ · 2H₂O (complex 1) is reported. Complex 1 is a new example of a “Jahn–Teller isomer”, since it has two Mn(III) ions with abnormally oriented Jahn–Teller elongation axes. Complex 1 has a lower activation energy (U_eff = 29 K) for magnetization reversal relative to other reported [Mn₁₂O₁₂] type molecules (e.g., U_eff = 70 K for Mn₁₂Ac). Single-crystal low temperature magnetization measurements are reported that confirm that complex 1 is a single-molecule magnet. High-field electron paramagnetic resonance measurements were performed on a single crystal to give the spin Hamiltonian parameters.