Electrochemical properties of LaCePr-oxide/K2WO4 composite electrolyte for low-temperature SOFCs

In this study, we introduced tungstate into solid oxide fuel cells (SOFCs) for the first time by using a La/Pr-doped CeO₂ (LCP)/K₂WO₄ composite as the electrolyte, which exhibited remarkably enhanced grain boundary conduction compared to that of single-phase LCP. The composition dependence of the electrical conductivity was investigated. As a result, the composite with 10 wt% K₂WO₄ was proven to be the optimum ratio, revealing a significantly higher ionic conductivity than LCP, along with a negligible electronic conductivity. The fuel cell using the LCP/K₂WO₄ electrolyte displayed an encouraging performance of 500 mW cm^− 2 at 550 °C. These findings indicate that the LCP/K₂WO₄ composite is a promising electrolyte for low-temperature SOFCs.     

Structural characterization,phase transition and dielectric properties of 4-cyanopyridynium perchlorate monohydrate: [(4-CNC5H4NH)][ClO4]·H2O

Crystal structure of 4-cyanopyridynium perchlorate monohydrate ([(4-CNC₅H₄NH)][ClO₄]·H₂O) has been determined at 293 and 240 K as orthorhombic space group, Pnma and monoclinic space group, P2₁/c, respectively, by means of single crystal X-ray diffraction. At room temperature the perchlorate anion reveals significant disorder, which is realized by the splitting of two oxygen atoms into four sites. DSC, dilatometric and dielectric spectroscopy techniques show that the crystal undergoes phase transition at 286/288 K (on cooling/heating scans). [(4-CNC₅H₄NH)][ClO₄]·H₂O appears to be an insulator with relatively high activation energy of the order of 100 kJ/mol. The phase transition in the title crystal is believed to be related to the dynamics of the perchlorate anion.     

Phase transitions and thermodynamic properties of (NH4)3VO2F4 cryolite

Calorimetric measurements performed in a wide temperature range on (NH₄)₃VO₂F₄ have shown the presence of four heat capacity anomalies at T₁ = 438 K, T₂ = 244 K, T₃ = 210.2 K, T₄ = 205.1 K associated with the first order phase transitions. In accordance with the permittivity behavior, the structural transformations are of nonferroelectric nature. Pressure dependence of the phase transition temperatures has been studied by DTA under pressure. The entropy of phase transitions is analyzed mainly in the framework of the orientational disordering of NH₄⁺ and VO₂F₄^3? ions in a cubic phase.     

High pressure polymorphs of LiCoPO4 and LiCoAsO4

Olivine-LiCoXO₄ (X = P, As) compounds might transform to the denser spinel-type and Na₂CrO₄-type structures under pressure. In this work, the relative energetic stability of the three polymorphs and the pressure of the possible polymorphic transformations are investigated combining experiments and first principles calculations. Olivine-LiCoAsO₄ is predicted to transform to the Na₂CrO₄-like structure at 0.4 GPa and to the spinel structure at 5.8 GPa (0 K). Quenching HP/HT experiments show that olivine-LiCoAsO₄ treated at 6 GPa/1173 K transforms to the spinel-like structure. Computational results indicate that olivine-LiCoPO₄ transforms to the Na₂CrO₄-like form at around 4 GPa (0 K), the latter being the stable form till very high pressures (21.6 GPa). In good agreement with this, olivine-LiCoPO₄ when subjected to 6 GPa/1173 K and 15 GPa/1173 K is converted to the Na₂CrO₄-type polymorph. Crystallographic data of the new compound LiCoPO₄ within the Na₂CrO₄ structural type are provided.     

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.     

On the phase behaviour of polyethoxylated sorbitan (Tween) surfactants in the presence of potassium inorganic salts

The salting out potential of potassium-based inorganic salts was assessed in aqueous solutions of two non-ionic surfactants from the Tween family. New solubility data of the systems {surfactant (Tween 20/Tween 80) + inorganic salt (K₃PO₄/K₂CO₃/K₂HPO₄/K₂S₂O₃/K₂SO₃) + H₂O} were experimentally ascertained at T = 298.15 K and these data were correlated by means of several three and four parameters empirical equations. Tie-line data were determined for the aqueous ternary systems and Ohtmer-Tobias and Bancroft equations have been proposed to correlate these data. The phase segregation effect of the proposed salts was investigated and compared with the sequence indicated by the Hofmeister series and the molar Gibbs energy of hydration (Δ_hydG) data.     

(Liquid + solid) metastable equilibria in quinary system Li2CO3 + Na2CO3 + K2CO3 + Li2B4O7 + Na2B4O7 + K2B4O7 + H2O at T=288 K for Zhabuye salt lake

An experimental study on metastable equilibria at T=288 K in the quinary system Li₂CO₃ + Na₂CO₃ + K₂CO₃ + Li₂B₄O₇ + Na₂B₄O₇ + K₂B₄O₇ + H₂O was done by isothermal evaporation method. Metastable equilibrium solubilities and densities of the solution were determined experimentally. According to the experimental data, the metastable equilibrium phase diagram under the condition saturated with Li₂CO₃ was plotted, in which there are four invariant points; nine univariant curves; six fields of crystallization: K₂CO₃ · 3/2H₂O, K₂B₄O₇ · 5H₂O, Li₂B₂O₄ · 16H₂O, Na₂B₂O₄ · 8H₂O, Na₂CO₃ · 10H₂O, NaKCO₃ · 6H₂O. Some differences were found between the stable phase diagram at T=298 K and the metastable one at T=288 K.     

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO₂F are carried out up to 19 GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4 GPa with rhombohedral (R-3c) structure above 4 GPa, but the structure between 0.7 GPa and 4 GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8 GPa and gets completed at 4.4 GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19 GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4 GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO₂F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4 GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4 GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO₂F. Disorder-induced first order Raman modes at 176, 212, 381 and 485 cm⁻¹ soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91 ppm K⁻¹) is in fair agreement with the reported values (−1 to +1 ppm K⁻¹). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.     

Thermodynamic evaluation and optimization of the (NaNO3 + KNO3 + Na2SO4 + K2SO4) system

A complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (NaNO₃ + KNO₃ + Na₂SO₄ + K₂SO₄) ternary reciprocal system, and optimised model parameters have been found. The model parameters obtained for the four binary common-ion subsystems (i.e. (NaNO₃ + Na₂SO₄), (KNO₃ + K₂SO₄), (NaNO₃ + KNO₃) and (Na₂SO₄ + K₂SO₄)) are used to predict thermodynamic properties and phase equilibria for the entire system. The Modified Quasichemical Model in the Quadruplet Approximation for short-range ordering was used for the molten salt phase, and the Compound Energy Formalism was used for the various solid solutions.     

Using Raman spectroscopy to understand the origin of the phase transition observed in the crystalline sulfur based amino acid l-methionine

We present the Raman spectra of l-methionine (C₅H₁₁NO₂S) monocrystals obtained in the spectral region ranging from 3200 to 50 cm⁻¹ at temperatures from 20 to 375 K. We investigated the dynamics of the different functional groups in l-methionine and related their behaviour to the structural transition previously reported at about 307 K. Additionally, on cooling, changes in the intensities of some Raman bands were associated with conformational changes of at least one of the two l-methionine conformers in the monoclinic unit cell in the interval 160–140 K. Thermal analysis and DFT calculations provide further support to the interpretation of the Raman results.     

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.     

Vibrational spectroscopic study of syngenite formed during the treatment of liquid manure with sulphuric acid

Syngenite (K₂Ca(SO₄)₂·H₂O), formed during treatment of manure with sulphuric acid, was studied by infrared, near-infrared (NIR) and Raman spectroscopy. C_s site symmetry was determined for the two sulphate groups in syngenite (P2₁/m), so all bands are both infrared and Raman active. The split ν₁ (two Raman+two infrared bands) was observed at 981 and 1000 cm⁻¹. The split ν₂ (four Raman+four infrared bands) was observed in the Raman spectrum at 424, 441, 471 and 491 cm⁻¹. In the infrared spectrum, only one band was observed at 439 cm⁻¹. From the split ν₃ (six Raman+six infrared) bands three 298 K Raman bands were observed at 1117, 1138 and 1166 cm⁻¹. Cooling to 77 K resulted in four bands at 1119, 1136, 1144 and 1167 cm⁻¹. In the infrared spectrum, five bands were observed at 1110, 1125, 1136, 1148 and 1193 cm⁻¹. From the split ν₄ (six infrared+six Raman bands) four bands were observed in the infrared spectrum at 604, 617, 644 and 657 cm⁻¹. The 298 K Raman spectrum showed one band at 641 cm⁻¹, while at 77 K four bands were observed at 607, 621, 634 and 643 cm⁻¹. Crystal water is observed in the infrared spectrum by the OH-liberation mode at 754 cm⁻¹, OH-bending mode at 1631 cm⁻¹, OH-stretching modes at 3248 (symmetric) and 3377 cm⁻¹ (antisymmetric) and a combination band at 3510 cm⁻¹ of the H-bonded OH-mode plus the OH-stretching mode. The near-infrared spectrum gave information about the crystal water resulting in overtone and combination bands of OH-liberation, OH-bending and OH-stretching modes.     

Morphology controllable synthesis and luminescence properties of NaLa(WO4)2:Eu microcrystals

NaLa(WO₄)₂:Eu microcrystals with shapes of four-arris shuttle, quadrangled star, and quadrangled dendrite were hydrothermally synthesized at 180 °C for 16 h. The concentration of the reactants and cetyltrimethyl ammonium bromide (CTAB) influenced the morphologies of the products. As La(NO₃)₃, Na₂WO₄ and CTAB was 0.375, 1.0, and 1.0 mmol, respectively, four-arris shuttle was obtained. As the concentration of the reactants doubled and the amount of CTAB ranged from 0.4 to 2.0 mmol, quadrangled dendrite, quadrangled star and four-arris shuttle were prepared, respectively. Luminescence intensity measurement of the three morphologies of NaLa(WO₄)₂:Eu showed that quadrangled dendrite was the strongest and four-arris shuttle was the lowest.     

Crystal structure and phase transitions of sodium potassium niobate perovskites

This paper presents the crystal structure and the phase transitions of K_xNa_1?xNbO₃ (0.4 ≤ x ≤ 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100–800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° ≤ β ≤ 90.34°). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic–tetragonal and tetragonal–cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.     

Raman spectroscopy investigation on (Pb1−xLax)(Zr0.90Ti0.10)1−x/4O3 ceramic system

A Raman spectroscopy study at room temperature was carried out on (Pb_1−xLa_x)(Zr_0.90Ti_0.10)_1−x/4O₃ ceramics (x = 2, 3, 4 at%). The results were analyzed considering the x-ray patterns at room temperature showing a mixture of two phases: a rombohedral-ferroelectric phase and an orthorhombic-antiferroelectric, increasing the% of the second one with the lanthanum concentration. For x = 3 at%, the analysis was also carried out in a wide temperature range. Two anomalies were evaluated, one around 363 K, which has been associated to a ferroelectric-antiferroelectric phase transition; the second one around 430 K, which has been associated to a transition from an incommensurable state to a ferroelectric phase.     

Raman investigation of the order-disorder phase transitions in the 2[N(C3H7)4]SbCl4 compound

The Raman spectra of bis (tetrapropylammonium tetrachloroantimonate (III)) 2[(C₃H₇)₄N]SbCl₄ compound single crystals were studied in the wavenumber range from 3500 to 50 cm⁻¹ for temperatures between 300 and 415 K. Two phase transitions occurring at 343 (T_tr1) and 363 K (T_tr2) were observed and characterized. The strong evolutions of the Raman shift, half-widths and intensity of many lines associated with the organic cations were observed with discontinuities in the vicinity of the two phase transitions. The most important changes were noticed for the band at 307 cm⁻¹ (at room temperature) assignable to the torsion of CH₃ groups of the cations. The spectral characteristics of this band was analyzed and consistently described in the framework of an order–disorder model for the two phase transitions. They allowed us to obtain information relative to the activation energy, the correlation length, and the critical exponent of the mechanism. The decrease of the estimated activation energies for the band 307 cm⁻¹ with the increase in temperature has been interpreted in terms of a change in the reorientation motion of cations. The temperature dependence of the reduced peak intensity allowed for the determination of the critical exponents and evolution of the correlation length on approaching the transition.     

Vibrational spectra and structure of mixed alkali borotungstate glasses: Evidence of mixed alkali effect

Mixed alkali borotungstate glasses with xLi₂O–(30 − x)Na₂O–10WO₃–60B₂O₃ (0 ≤ x ≤ 30) composition were prepared by melt quench technique. FT-IR and Raman spectroscopic studies were employed to investigate the structure of all the prepared glasses. Acting as complementary techniques, both IR and Raman measurements revealed that the network structure of the present glasses mainly based on BO₃ and BO₄ units placed in different structural groups. Raman spectra confirm the IR results regarding the presence of tungsten ions mainly as WO₆ groups. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system using FTIR and Raman studies.     

The system LaPO4–CaKPO4

A previously unknown system LaPO₄–CaKPO₄ has been investigated using the X-ray diffraction and thermoanalytical methods (DTA, TG, and DTG). Its phase diagram is proposed. The system contains the phosphate CaKLa(PO₄)₂ which melts incongruently at 1480 ± 20 °C. This compound exhibits a polymorphic transformation at 1120 °C. Two forms of CaKLa(PO₄)₂ have been obtained: a hexagonal and a monoclinic one, and some of their calculated structural data are presented.     

Gas sensing selectivity of hexagonal and monoclinic WO3 to H2S

Hexagonal and monoclinic tungsten oxide (h- and m-WO₃) samples were produced by annealing hexagonal ammonium tungsten bronze, (NH₄)_0.07(NH₃)_0.04(H₂O)_0.09WO_2.95 at 470 and at 600 °C, respectively. Their structure, composition and morphology were analyzed by XRD, Raman, XPS, ¹H-MAS NMR and SEM. In order to study the effect of crystal structure on the gas sensitivity of tungsten oxides, h- and m-WO₃ were tested as gas sensors to CH₄, CO, H₂, NO and H₂S (1000 and 10 ppm) at 200 °C. Monoclinic WO₃ responded to all gases, but its gas sensing signal was two magnitudes greater to 10 ppm H₂S than to other gases, and it also detected H₂S even at 25 °C. Hexagonal WO₃ responded only to 10 ppm H₂S. Its sensitivity was smaller compared to m-WO₃, however, the response time of h-WO₃ was significantly faster. The gas sensing tests showed that while m-WO₃ had relative selectivity to H₂S in the presence CH₄, CO, H₂, NO; h-WO₃ had absolute selectivity to H₂S in the presence these gases.