Study of oxidation properties and decomposition kinetics of three-dimensional (3-D) braided carbon fiber

The XRD, SEM, isothermal oxidation-weight loss and non-isothermal thermogravimetry (TG)-differential thermogravimetry (DTG) were used to study the oxidation properties and oxidation decomposition kinetics of three-dimensional (3-D) braided carbon fiber (abbreviated as fiber). The results showed that the non-isothermal oxidation process of fiber exhibited self-catalytic characteristic. The kinetic parameters and oxidation mechanism of fiber were studied through analyzing the TG and DTG data by differential and integral methods. The oxidation mechanism was random nucleation, the kinetic parameters were: lg A=10.299 min⁻¹; E_a=156.29 kJ mol⁻¹.     

X-ray diffraction study on the thermal expansion behavior of cellulose Iβ and its high-temperature phase

Oriented films of cellulose prepared from algal cellulose were hydrothermally treated to convert them into highly crystalline cellulose Iβ. The lateral thermal expansion behavior of the prepared cellulose Iβ films was investigated using X-ray diffraction at temperatures from 20 to 300 °C. Cellulose Iβ was transformed into the high-temperature phase when the temperature was above 230 °C, allowing the lateral thermal expansion coefficient of cellulose Iβ and its high-temperature phase to be measured. For cellulose Iβ, the thermal expansion coefficients (TECs) of the a- and b-axes were α_a = 9.8 × 10⁻⁵ °C⁻¹ and α_b = 1.2 × 10⁻⁵ °C⁻¹, respectively. This anisotropic thermal expansion behavior in the lateral direction is ascribed to the crystal structure and to the hydrogen-bonding system of cellulose Iβ. For the high-temperature phase, the anisotropy was more conspicuous, and the TECs of the a- and b-axes were α_a = 19.8 × 10⁻⁵ °C⁻¹ and α_b = −1.6 × 10⁻⁵ °C⁻¹, respectively. Synchrotron X-ray fiber diffraction diagrams of the high-temperature phase were also recorded at 250 °C. The cellulose high-temperature phase is composed of a two-chain monoclinic unit cell, a = 0.819 nm, b = 0.818 nm, c (fiber repeat) = 1.037 nm, and γ = 96.4°, with space group = P2₁. The volume of this cell is 4.6% larger than that of cellulose Iβ at 30 °C.     

Formation, sintering, and electrical conductivity of perovskite Sm(Cr1−xMgx)O3 (0?x?0.23) prepared by the hydrazine method

The presence of SmCrO₄ is experimentally established. In Mg²⁺-substituted SmCrO₃, single-phase perovskite Sm(Cr_1−xMg_x)O₃, where x=0-0.23, are formed at ∼830°C by decomposition of Sm(Cr_1−xMg_x)O₄ which crystallizes at 530-570°C from amorphous materials prepared by the hydrazine method. Sm(Cr_1−xMg_x)O₃ solid solution powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700°C in air. The relative densities, grain sizes, and electrical conductivities increase with increased Mg²⁺ content. Sm(Cr_0.77Mg_0.23)O₃ materials exhibit an excellent direct current electrical conductivity of 2.2×10³ S m⁻¹ at 1000°C.     

Kinetics of thermal degradation applied to starches from different botanical origins by non-isothermal procedures

Amylose content, crystallinity, morphology and the kinetic of thermal degradation to starches from different botanical origins are described based on XRD, SEM, DSC and TG/DTG curves. Applying the non-isothermal isoconversional Wall-Flynn-Ozawa method on the TG/DTG curves average activation energy (0.10 ≤ α ≤ 0.70) E = 144.1 ± 9.8, 171.6 ± 14.6, 158.3 ± 7.4 and 159.4 ± 15 kJ mol⁻¹ could be obtained for thermal degradation of corn, rice, potato and cassava starches, respectively. From E values and the generalized time θ, the Sesták-Berggren (SB) in which f(α) = α^m(1 − α)ⁿ seems to be most suitable kinetic model in describing physicogeometrically the thermal degradation for the samples regardless of its botanical origins. The determination of the kinetic exponents m and n allows to obtain the pre-exponential factor (0.2 ≤ α ≤ 0.8) ln A = 8.8, 10.4, 9.2 and 8.9 min⁻¹ for corn, rice, potato and cassava starches, respectively. There were not significant differences between values of the kinetic triplet of the starches, indicating that, despite structural differences, these had little influence on the thermal degradation process of the starches.     

Phase stability of BSCF in low oxygen partial pressures

In situ X-ray diffraction has been used to investigate the phase stability of barium strontium cobalt iron oxide (BSCF) with the formula Ba_0.5Sr_0.5Fe_1−xCo_xO_3−δ (x=0, 0.2, 0.4, 0.6, 0.8, and 1). The thermal decomposition processes in both low partial pressures of oxygen (air −10⁻⁵ atm pO₂) and in reducing conditions have been detailed. BSCF manifests excellent stability down to 10⁻⁵ atm pO₂; however, it decomposes through a complex series of oxides under reducing conditions. Increasing the cobalt content results in a decrease in the temperature range of stability of the material under 4% H₂ in N₂, with the initial decomposition taking place at 375, 425, 550, 600, 650 and 675 °C, for x=1, 0.8, 0.6, 0.4, 0.2 and 0, respectively. Further, the thermal expansion is a strong function of the oxygen activity and Co content. The x=0, 1 end member, BSC, undergoes a phase transition from rhombohedral to cubic symmetry at ∼800 °C under 10⁻⁵ atm pO₂, resulting in an ideal perovskite with a=3.9892(3) Å at room temperature.     

Sol-gel synthesis and characterization of LiNO3-Al2O3 composite solid electrolyte

Composite solid electrolytes in the system (1 − x)LiNO₃-xAl₂O₃, with x = 0.0-0.5 were synthesized by sol-gel method. The synthesis carried out at low temperature resulted in voluminous and fluffy products. The obtained materials were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy/energy dispersive X-ray, Fourier transform infrared spectroscopy and AC impedance spectroscopy. Structural analysis of the samples showed base centred cell type of point lattice of LiNO₃ for the composite samples with x = 0.1-0.2 and body centred cell for the sample with x = 0.3. A trace amount of α-LiAlO₂ crystal phase was also present in these composite samples. The thermal analysis showed that the samples were in a stable phase between 48 °C and 230-260 °C. Morphological analysis indicated the presence of amorphous phase and particles with sizes ranging from micro to nanometre scale for the composite sample with x = 0.1. The conductivities of the composites were in the order of 10⁻³ and 10⁻² S cm⁻¹ at room temperature and 150 °C, respectively.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Thermal degradation mechanism of poly(ethylene succinate) and poly(butylene succinate): Comparative study

Two aliphatic polyesters that consisted from succinic acid, ethylene glycol and butylene glycol, —poly(ethylene succinate) (PESu) and poly(butylene succinate) (PBSu)—, were prepared by melt polycondensation process in a glass batch reactor. These polyesters were characterized by DSC, ¹H NMR and molecular weight distribution. Their number average molecular weight is almost identical in both polyesters, close to 7000 g/mol, as well as their carboxyl end groups (80 eq/10⁶ g). From TG and Differential TG (DTG) thermograms it was found that the decomposition step appears at a temperature 399 °C for PBSu and 413 °C for PESu. This is an indication that PESu is more stable than PBSu and that chemical structure plays an important role in the thermal decomposition process. In both polyesters degradation takes place in two stages, the first that corresponds to a very small mass loss, and the second at elevated temperatures being the main degradation stage. The two stages are attributed to different decomposition mechanisms as is verified from the values of activation energy determined with iso-conversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures, is auto-catalysis with activation energy E = 128 and E = 182 kJ/mol and reaction order n = 0.75 and 1.84 for PBSu and PESu, respectively. The second mechanism is nth-order reaction with E = 189 and 256 kJ/mol and reaction order n = 0.68 and 0.96 for PBSu and PESu, respectively, as they were calculated from the fitting of experimental results.     

La3TaO7 derivatives with Weberite structure type: Possible electrolytes for solid oxide fuel cells and high temperature electrolysers

In this study, with the aim to enhance the ionic conduction of known structures by defect chemistry, the La₂O₃-Ta₂O₅ system was considered with a focus on the La₃TaO₇ phase whose structure is of Weberite type. In order to predict possible preferential substitution sites and substitution elements, atomistic simulation was used as a first approach. A solid solution La_3−xSr_xTaO_7−x/2 was confirmed by X-ray diffraction and Raman spectroscopy; it extends for a substitution ratio up to x = 0.15. Whereas La₃TaO₇ is a poor oxide ion conductor (σ_700 °C = 2 × 10⁻⁵S.cm⁻¹), at 700 °C, its ionic conductivity is increased by more than one order of magnitude when 3.3% molar strontium is introduced in the structure (σ_700 °C = 2 × 10⁻⁴S.cm⁻¹).     

Synthesis, crystal structure and thermal behavior of Sr3B2SiO8 borosilicate

Single crystals of Sr₃B₂SiO₈ were obtained by solid-state reaction of stoichiometric mixture at 1200 °C. The crystal structure of the compound has been solved by direct methods and refined to R1=0.064 (wR=0.133). It is orthorhombic, Pnma, a=12.361(4), b=3.927(1), c=5.419(1) Å, V=263.05(11) Å³. The structure contains zigzag pseudo-chains running along the b axis and built up from corner sharing (Si,B)−O polyhedra. Boron and silicon are statistically distributed over one site with their coordination strongly disordered. Sr atoms are located between the chains providing three-dimensional linkage of the structure.The formation of Sr₃B₂SiO₈ has been studied using annealing series in air at 900-1200 °C. According powder XRD, the probe contains pure Sr₃B₂SiO₈ over 1100 °C. The compound is not stable below 900 °C. In the pseudobinary Sr₂B₂O₅-Sr₃B₂SiO₈ system a new series of solid solutions Sr_3−xB₂Si_1−xO_8−3x (x=0-0.9) have been crystallized from melt. The thermal behavior of Sr₃B₂SiO₈ was investigated using powder high-temperature X-ray diffraction (HTXRD) in the temperature range 20-900 °C. The anisotropic character of thermal expansion has been observed: α_a= −1.3, α_b=23.5, α_c=13.9, and α_V=36.1×10⁻⁶ °C⁻¹ (25 °C); α_a= −1.3, α_b=23.2, α_c=5.2, and α_V=27.1×10⁻⁶ °C⁻¹ (650 °C). Maximal thermal expansion of the structure along of the chain direction [0 1 0] is caused by the partial straightening of chain zigzag. Hinge mechanism of thermal expansion is discussed.     

Formation and structural refinements of tunneled intergrowth phases in the Ga2O3-In2O3-SnO2-TiO2 system

Over 100 samples were prepared as (Ga,In)₄(Sn,Ti)_n−4O_2n−2, n=6, 7, and 9 by solid-state reaction at 1400 °C and characterized by X-ray diffraction. Nominally phase-pure beta-gallia-rutile intergrowths were observed in samples prepared with n=9 (0.17?x?0.35 and 0?y?0.4) as well as in a few samples prepared with n=6 and 7. Rietveld analysis of neutron time-of-flight powder diffraction data were conducted for three phase-pure samples. The n=6 phase Ga_3.24In_0.76Sn_1.6Ti_0.4O₁₀ is monoclinic, P2/m, with Z=2 and a=11.5934(3) Å, b=3.12529(9) Å, c=10.6549(3) Å, β=99.146(1)°. The n=7 phase Ga_3.24In_0.76Sn_2.4Ti_0.6O₁₂ is monoclinic, C2/m, with Z=2 and a=14.2644(1) Å, b=3.12751(2) Å, c=10.6251(8) Å, β=108.405(1)°. The n=9 phase Ga_3.16In_0.84Sn₄TiO₁₆ is monoclinic, C2/m, with Z=2 a=18.1754(2) Å, b=3.13388(3) Å, c=10.60671(9) Å, β=102.657(1)°. All of the structures are similar in that they possess distorted hexagonal tunnels parallel to the [010] vector.     

Tin(II)-EDTA complex: kinetic of thermal decomposition by non-isothermal procedures

Tin on the oxide form, alone or doped with others metals, has been extensively used as gas sensor, thus, this work reports on the preparation and kinetic parameters regarding the thermal decomposition of Sn(II)-ethylenediaminetetraacetate as precursor to SnO₂. Thus, the acquaintance with the kinetic model regarding the thermal decomposition of the tin complex may leave the door open to foresee, whether it is possible to get thin film of SnO₂ using Sn(II)-EDTA as precursor besides the influence of dopants added.The Sn(II)-EDTA soluble complex was prepared in aqueous medium by adding of tin(II) chloride acid solution to equimolar amount of ammonium salt from EDTA under N₂ atmosphere and temperature of 50 °C arising the pH∼4. The compound was crystallized in ethanol at low-temperature and filtered to eliminate the chloride ions, obtaining the heptacoordinated chelate with the composition H₂SnH₂O(CH₂N(CH₂COO)₂)₂·0.5H₂O.Results from TG, DTG and DSC curves under inert and oxidizing atmospheres indicate the presence of water coordinated to the metal and that the ethylenediamine fraction is thermally more stable than carboxylate groups. The final residue from thermal decomposition was the SnO₂ characterized by X-ray as a tetragonal rutile phase.Applying the isoconversional Wall-Flynn-Ozawa method on the DSC curves, average activation energy: E_a=183.7±2.7 and 218.9±2.1 kJ mol⁻¹, and pre-exponential factor: and 19.10±0.27 min⁻¹, at 95% confidence level, could be obtained, regarding the loss of coordinated water and thermal decomposition of the carboxylate groups, respectively. The E_a and log A also could be obtained applying isoconventional Wall-Flynn method on the TG curves.From E_a and log A values, Dollimore and Malék procedures could be applied suggesting R3 (contracting volume) and SB (two-parameter model) as the kinetic model to the loss of coordinated water (177-244 °C) and thermal decomposition of the carboxylate groups (283-315 °C), respectively. Simulated and experimental normalized DTG and DSC curves besides analysis of residuals check these kinetic models.     

Isothermal and polythermal kinetics of depolymerization of C60 polymers

Isothermal depolymerization of the two polymers of C₆₀, i.e. of 1D orthorhombic phase (O) and of “dimer state” (DS) have been studied by means of Infra-red spectroscopy in the temperature ranges 383-423 and 453-503 K, respectively. Differential Scanning Calorimetry (DSC) has been used to obtained depolymerization polytherms for O-phase and DS. Standard set of reaction models have been applied to describe depolymerization behavior of O-phase and DS. The choice of the reaction models was based primarily on the isotherms. Several models however demonstrated almost equal goodness of fit and were statistically indistinguishable. In this case we looked for simpler/more realistic mechanistic model of the reaction. For DS the first-order expression (Mampel equation) with the activation energy E_a = 134 ± 7 kJ mol⁻¹ and preexponential factor ln(A/s⁻¹) = 30.6 ± 2.1, fitted the isothermal data. This activation energy was nearly the same as the activation energy of the solid-state reaction of dimerization of C₆₀ reported in the literature. This made the enthalpy of depolymerization close to zero in accord with the DSC data on depolymerization of DS. Mampel equation gave the best fit to the polythermal data with E_a = 153 kJ mol⁻¹ and preexponential factor ln(A/s⁻¹) = 35.8. For O-phase two reasonable reaction models, i.e. Mampel equation and “contracting spheres” model equally fitted to the isothermal data with E_a = 196 ± 2 and 194 ± 8 kJ mol⁻¹, respectively and ln(A/s⁻¹) = 39.1 ± 0.5 and 37.4 ± 0.2, respectively and to polythermal data with E_a = 163 and 170 kJ mol⁻¹, respectively and ln(A/s⁻¹) = 32.5 and 29.5, respectively.     

Series of compositions Bi2(M′xM1−x)4O9 (M′, M=Al, Ga, Fe; 0≤x≤1) with mullite-type crystal structure: Synthesis, characterization and O/O exchange experiment

Series of compositions Bi₂(M′_xM_1−x)₄O₉ with x=0.0, 0.1,…, 1.0 and M′/M=Ga/Al, Fe/Al and Fe/Ga were synthesized by dissolving appropriate amounts of corresponding metal nitrate hydrates in glycerine, followed by gelation, calcination and final heating at 800 °C for 24 h. The new compositions with M′/M=Ga/Al form solid-solution series, which are isotypes to the two other series M′/M=Fe/Al and Fe/Ga. The XRD data analysis yielded in all cases a linear dependence of the lattice parameters related on x. Rietveld structure refinements of the XRD patterns of the new compounds, Bi₂(Ga_xAl_1−x)₄O₉ reveal a preferential occupation of Ga in tetrahedral site (4 h). The IR absorption spectra measured between 50 and 4000 cm⁻¹ of all systems show systematic shifts in peak positions related to the degree of substitution. Samples treated in ¹⁸O₂ atmosphere (16 h at 800 °C, 200 mbar, 95% ¹⁸O₂) for ¹⁸O/¹⁶O isotope exchange experiments show a well-separated IR absorption peak related to the M-¹⁸O_c-M vibration, where O_c denotes the common oxygen of two tetrahedral type MO₄ units. The intensity ratio of M-¹⁸O_c/M-¹⁶O_c IR absorption peaks and the average crystal sizes were used to estimate the tracer diffusion coefficients of polycrystalline Bi₂Al₄O₉ (D=2×10⁻²² m²s⁻¹), Bi₂Fe₄O₉ (D=5×10⁻²¹ m²s⁻¹), Bi₂(Ga/Al)₄O₉ (D=2×10⁻²¹ m²s⁻¹) and Bi₂Ga₄O₉ (D=2×10⁻²⁰ m²s⁻¹).     

Heat capacity, enthalpy and entropy of ternary bismuth tantalum oxides

Heat capacity and enthalpy increments of ternary bismuth tantalum oxides Bi₄Ta₂O₁₁, Bi₇Ta₃O₁₈ and Bi₃TaO₇ were measured by the relaxation time method (2-280 K), DSC (265-353 K) and drop calorimetry (622-1322 K). Temperature dependencies of the molar heat capacity in the form C_pm=445.8+0.005451T−7.489×10⁶/T² J K⁻¹ mol⁻¹, C_pm=699.0+0.05276T−9.956×10⁶/T² J K⁻¹ mol⁻¹ and C_pm=251.6+0.06705T−3.237×10⁶/T² J K⁻¹ mol⁻¹ for Bi₃TaO₇, Bi₄Ta₂O₁₁ and for Bi₇Ta₃O₁₈, respectively, were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S°_m(298.15 K)=449.6±2.3 J K⁻¹ mol⁻¹ for Bi₄Ta₂O₁₁, S°_m(298.15 K)=743.0±3.8 J K⁻¹ mol⁻¹ for Bi₇Ta₃O₁₈ and S°_m(298.15 K)=304.3±1.6 J K⁻¹ mol⁻¹ for Bi₃TaO₇, were evaluated from the low-temperature heat capacity measurements.     

Quaternary selenostannates Na2−xGa2−xSn1+xSe6 and AGaSnSe4 (A=K, Rb, and Cs) through rapid cooling of melts. Kinetics versus thermodynamics in the polymorphism of AGaSnSe4

The quaternary alkali-metal gallium selenostannates, Na_2−xGa_2−xSn_1+xSe₆ and AGaSnSe₄ (A=K, Rb, and Cs), were synthesized by reacting alkali-metal selenide, Ga, Sn, and Se with a flame melting-rapid cooling method. Na_2−xGa_2−xSn_1+xSe₆ crystallizes in the non-centrosymmetric space group C2 with cell constants a=13.308(3) Å, b=7.594(2) Å, c=13.842(3) Å, β=118.730(4)°, V=1226.7(5) Å³. α-KGaSnSe₄ crystallizes in the tetragonal space group I4/mcm with a=8.186(5) Å and c=6.403(5) Å, V=429.1(5) Å³. β-KGaSnSe₄ crystallizes in the space group P2₁/c with cell constants a=7.490(2) Å, b=12.578(3) Å, c=18.306(5) Å, β=98.653(5)°, V=1705.0(8) Å³. The unit cell of isostructural RbGaSnSe₄ is a=7.567(2) Å, b=12.656(3) Å, c=18.277(4) Å, β=95.924(4)°, V=1741.1(7) Å³. CsGaSnSe₄ crystallizes in the orthorhombic space group Pmcn with a=7.679(2) Å, b=12.655(3) Å, c=18.278(5) Å, V=1776.1(8) Å³. The structure of Na_2−xGa_2−xSn_1+xSe₆ consists of a polar three-dimensional network of trimeric (Sn,Ga)₃Se₉ units with Na atoms located in tunnels. The AGaSnSe₄ possess layered structures. The compounds show nearly the same Raman spectral features, except for Na_2−xGa_2−xSn_1+xSe₆. Optical band gaps, determined from UV-Vis spectroscopy, range from 1.50 eV in Na_2−xGa_2−xSn_1+xSe₆ to 1.97 eV in CsGaSnSe₄. Cooling of the melts of KGaSnSe₄ and RbGaSnSe₄ produces only kinetically stable products. The thermodynamically stable product is accessible under extended annealing, which leads to the so-called γ-form (BaGa₂S₄-type) of these compounds.     

Optical properties of green-blue-emitting Ca-α-Sialon:Ce,Li phosphors for white light-emitting diodes (LEDs)

Ce³⁺,Li⁺-codoped Ca-α-Sialon phosphors with the formula [Ca_(1−2x)Ce_xLi_x]_m/2Si_12−(m+n)Al_m+nO_nN_16−n (0≤x≤0.25, 0.5≤m≤3.5, and 0.16≤n≤2.0) have been synthesized by gas pressure sintering (GPS). The effects of the activator concentration and the overall composition of host lattice on the phase evolution, morphology, and optical properties were investigated. The single-phase Ca-α-Sialon:Ce³⁺,Li⁺ can be synthesized at x<0.1, 1.0≤m≤2.5, and n≤1.2. The synthesized powders exhibit a relatively dispersive and uniform morphology. Under the near UV excitation, the bright green-blue emission centered at 500-518 nm is observed. The photoluminescence can be tailored by controlling Ce³⁺ concentration and the overall composition of the α-Sialon host lattice. With increasing the Ce concentration and m value both excitation and emission bands show a red shift, which perfectly matches with the near-UV LEDs in the range of 360-410 nm. The strongest luminescence is achieved at x=0.08-0.1, m=2.0-2.5, and n=1.0. Simultaneously, the highest quantum efficiency and better thermal stability are also present.     

Revision of iron(III)–citrate speciation in aqueous solution. Voltammetric and spectrophotometric studies

A detailed study of iron (III)–citrate speciation in aqueous solution (θ = 25 °C, I_c = 0.7 mol L⁻¹) was carried out by voltammetric and UV–vis spectrophotometric measurements and the obtained data were used for reconciled characterization of iron (III)–citrate complexes. Four different redox processes were registered in the voltammograms: at 0.1 V (pH = 5.5) which corresponded to the reduction of iron(III)–monocitrate species (Fe:cit = 1:1), at about −0.1 V (pH = 5.5) that was related to the reduction of FeL₂⁵⁻, FeL₂H⁴⁻ and FeL₂H₂³⁻ complexes, at −0.28 V (pH = 5.5) which corresponded to the reduction of polynuclear iron(III)–citrate complex(es), and at −0.4 V (pH = 7.5) which was probably a consequence of Fe(cit)₂(OH)_x species reduction. Reversible redox process at −0.1 V allowed for the determination of iron(III)–citrate species and their stability constants by analyzing E_p vs. pH and E_p vs. [L⁴⁻] dependence. The UV–vis spectra recorded at varied pH revealed four different spectrally active species: FeLH (log β = 25.69), FeL₂H₂³⁻ (log β = 48.06), FeL₂H⁴⁻ (log β = 44.60), and FeL₂⁵⁻ (log β = 38.85). The stability constants obtained by spectrophotometry were in agreement with those determined electrochemically. The UV–vis spectra recorded at various citrate concentrations (pH = 2.0) supported the results of spectrophotometric–potentiometric titration.     

Synthesis, crystal structure, phase transition and thermal behaviour of a new dabcodiium hexaaquanickel(II) bis(sulphate), (C6H14N2)[Ni(H2O)6](SO4)2

A new dabcodiium-templated nickel sulphate, (C₆H₁₄N₂)[Ni(H₂O)₆](SO₄)₂, has been synthesised and characterised by single-crystal X-ray diffraction at 20 and −173 °C, differential scanning calorimetry (DSC), thermogravimetry (TG) and temperature-dependent X-ray powder diffraction (TDXD). The high temperature phase crystallises in the monoclinic space group P2₁/n with the unit-cell parameters: a = 7.0000(1), b = 12.3342(2), c = 9.9940(2) Å; β = 90.661(1)°, V = 862.82(3) Å³ and Z = 2. The low temperature phase crystallises in the monoclinic space group P2₁/a with the unit-cell parameters: a = 12.0216(1), b = 12.3559(1), c = 12.2193(1) Å; β = 109.989(1)°, V = 1705.69(2) Å³ and Z = 4. The crystal structure of the HT-phase consists of Ni²⁺ cations octahedrally coordinated by six water molecules, sulphate tetrahedra and disordered dabcodiium cations linked together by hydrogen bonds. It undergoes a reversible phase transition (PT) of the second order at −53.7/−54.6 °C on heating-cooling runs. Below the PT temperature, the structure is fully ordered. The thermal decomposition of the precursor proceeds through three stages giving rise to the nickel oxide.