Thermodynamic studies on SrFe12O19(s), SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₂O₄(s), Sr₂Fe₂O₅(s) and Sr₃Fe₂O₆(s) powders and the compounds were characterized by X-ray diffraction analysis. Different solid-state electrochemical cells were used for the measurement of emf as a function of temperature from 970 to 1151 K. The standard molar Gibbs energies of formation of these ternary oxides were calculated as a function of temperature from the emf data and are represented as (SrFe₂O₄, s, T)/kJ mol⁻¹ (±1.7)=−1494.8+0.3754 (T/K) (970?T/K?1151). (Sr₂Fe₂O₅, s, T)/kJ mol⁻¹ (±3.0)=−2119.3+0.4461 (T/K) (970?T/K?1149). (Sr₃Fe₂O₆, s, T)/kJ mol⁻¹ (±7.3)=−2719.8+0.4974 (T/K) (969?T/K?1150).Standard molar heat capacities of these ternary oxides were determined from 310 to 820 K using a heat flux type differential scanning calorimeter (DSC). Based on second law analysis and using the thermodynamic database FactSage software, thermodynamic functions such as Δ_fH°(298.15 K), S°(298.15 K) S°(T), C_p°(T), H°(T), {H°(T)-H°(298.15 K)}, G°(T), free energy function (fef), Δ_fH°(T) and Δ_fG°(T) for these ternary oxides were also calculated from 298 to 1000 K.     

Thermodynamics of BaNd2Fe2O7(s) and BaNdFeO4(s) in the system BaNdFeO

Two compounds, BaNd₂Fe₂O₇(s) and BaNdFeO₄(s) in the quaternary system BaNdFeO were prepared by citrate-nitrate gel combustion route and characterized by X-ray diffraction analysis. Heat capacities of these two oxides were measured in two different temperature ranges: (i) 130-325 K and (ii) 310-845 K, using a heat flux type differential scanning calorimeter. Two different types of solid-state electrochemical cells with CaF₂(s) as the solid electrolyte were employed to measure the e.m.f. as a function of temperature. The standard molar Gibbs energies of formation of these quaternary oxides were calculated as a function of temperature from the e.m.f. data. The standard molar enthalpies of formation from elements at 298.15 K, ΔfHm° (298.15 K) and the standard entropies, Sm° (298.15 K) of these oxides were calculated by the second law method. The values of ΔfHm° (298.15 K) and Sm° (298.15 K) obtained for BaNd₂Fe₂O₇(s) are: −2756.9 kJ mol⁻¹ and 234.0 J K⁻¹ mol⁻¹ whereas those for BaNdFeO₄(s) are: −2061.5 kJ mol⁻¹ and 91.6 J K⁻¹ mol⁻¹, respectively.     

Heat capacity and thermodynamic properties of crystalline ornidazole (C7H10ClN3O3)

The molar heat capacities of 1-(2-hydroxy-3-chloropropyl)-2-methyl-5-nitroimidazole (Ornidazole) (C₇H₁₀ClN₃O₃) with purity of 99.72 mol% were measured with an adiabatic calorimeter in the temperature range between 79 and 380 K. The melting-point temperature, molar enthalpy, Δ_fusH_m, and entropy, Δ_fusS_m, of fusion of this compound were determined to be 358.59±0.04 K, 21.38±0.02 kJ mol⁻¹ and 59.61±0.05 J K⁻¹ mol⁻¹, respectively, from fractional melting experiments. The thermodynamic function data relative to the reference temperature (298.15 K) were calculated based on the heat capacities measurements in the temperature range from 80 to 380 K. The thermal stability of the compound was further investigated by DSC and TG. From the DSC curve an intensive exothermic peak assigned to the thermal decomposition of the compound was observed in the range of 445-590 K with the peak temperature of 505 K. Subsequently, a slow exothermic effect appears when the temperature is higher than 590 K, which is probably due to the further decomposition of the compound. The TG curve indicates the mass loss of the sample starts at about 440 K, which corresponds to the decomposition of the sample.     

Standard molar enthalpies of formation of Ln(C9H6NO)2(C2H3O2) (Ln: Nd, Sm)

Using an on-line solution-reaction isoperibol calorimeter, the standard molar enthalpies of reaction for the general thermochemical reaction: LnCl₃·6H₂O(s) + 2C₉H₇NO(s) + CH₃COONa(s) = Ln(C₉H₆NO)₂(C₂H₃O₂)(s) + NaCl(s) + 2HCl(g) + 6H₂O(l) (Ln: Nd, Sm), were determined at T=298.15 K, as  kJ mol^−l, respectively. From the mentioned standard molar enthalpies of reaction and other auxiliary thermodynamic quantities, the standard molar enthalpies of formation of Ln(C₉H₆NO)₂(C₂H₃O₂)(s) (Ln: Nd, Sm), at T=298.15 K, have been derived to be: −(1494.7±3.3) and −(1501.5±3.4) kJ mol^−l, respectively.     

Low-temperature heat capacity and thermodynamic properties of crystalline carboxin (C12H13NO2S)

Carboxin was synthesized and its heat capacities were measured with an automated adiabatic calorimeter over the temperature range from 79 to 380 K. The melting point, molar enthalpy (Δ_fusH_m) and entropy (Δ_fusS_m) of fusion of this compound were determined to be 365.29±0.06 K, 28.193±0.09 kJ mol⁻¹ and 77.180±0.02 J mol⁻¹ K⁻¹, respectively. The purity of the compound was determined to be 99.55 mol% by using the fractional melting technique. The thermodynamic functions relative to the reference temperature (298.15 K) were calculated based on the heat capacity measurements in the temperature range between 80 and 360 K. The thermal stability of the compound was further investigated by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. The DSC curve indicates that the sample starts to decompose at ca. 290 °C with the peak temperature at 292.7 °C. The TG-DTG results demonstrate the maximum mass loss rate occurs at 293 °C corresponding to the maximum decomposition rate.     

Thermodynamic properties of BaCeO3 and BaZrO3 at low temperatures

Specific heat capacities (C_p) of polycrystalline samples of BaCeO₃ and BaZrO₃ have been measured from about 1.6 K up to room temperature by means of adiabatic calorimetry. We provide corrected experimental data for the heat capacity of BaCeO₃ in the range T < 10 K and, for the first time, contribute experimental data below 53 K for BaZrO₃. Applying Debye's T³-law for T → 0 K, thermodynamic functions as molar entropy and enthalpy are derived by integration. We obtain C_p = 114.8 (±1.0) J mol⁻¹ K⁻¹, S° = 145.8 (±0.7) J mol⁻¹ K⁻¹ for BaCeO₃ and C_p = 107.0 (±1.0) J mol⁻¹ K⁻¹, S° = 125.5 (±0.6) J mol⁻¹ K⁻¹ for BaZrO₃ at 298.15 K. These results are in overall agreement with previously reported studies but slightly deviating, in both cases. Evaluations of C_p(T) yield Debye temperatures and identify deviations from the simple Debye-theory due to extra vibrational modes as well as anharmonicity. The anharmonicity turns out to be more pronounced at elevated temperatures for BaCeO₃. The characteristic Debye temperatures determined at T = 0 K are Θ₀ = 365 (±6) K for BaCeO₃ and Θ₀ = 402 (±9) K for BaZrO₃.     

A solution-reaction isoperibol calorimeter and standard molar enthalpies of formation of Ln(hq)2Ac (Ln = La, Pr)

An on-line solution-reaction isoperibol calorimeter has been constructed. The performance of the apparatus was evaluated by measuring the molar enthalpy of solution of KCl in water at 298.15 K. The uncertainty and the inaccurary of the experimental results were within ±0.3% compared with the recommended reference data. Using the calorimeter, the molar enthalpies of reaction for the following two reactions: LaCl₃·7H₂O(s)+2Hhq(s)+NaAc(s)=La(hq)₂Ac(s)+NaCl(s)+2HCl(g)+7H₂O(l) and PrCl₃·6H₂O(s)+2Hhq(s)+NaAc(s)=Pr(hq)₂Ac(s)+NaCl(s)+2HCl(g)+6H₂O(l), were determined at T=298.15 K, as −(78.3±0.6) and −(97.3±0.5) kJ mol^−l, respectively. From the above molar enthalpies of reaction and other auxiliary thermodynamic quantities, the standard molar enthalpies of formation of La(hq)₂Ac and Pr(hq)₂Ac, at T=298.15 K, have been derived to be −(1535.5±0.7) and −(1536.7±0.6) kJ mol^−l, respectively.     

Low-temperature heat-capacity and standard molar enthalpy of formation of copper l-threonate hydrate Cu(C4H6O5)·0.5H2O(s)

The solid copper l-threonate hydrate, Cu(C₄H₆O₅)·0.5H₂O, was synthesized by the reaction of l-threonic acid with copper dihydrocarbonate and characterized by means of chemical and elemental analyses, IR and TG-DTG. Low-temperature heat-capacity of the title compound has been precisely measured with a small sample precise automated adiabatic calorimeter over the temperature range from 77 to 390 K. An obvious process of the dehydration occurred in the temperature range between 353 and 370 K. The peak temperature of the dehydration of the compound has been observed to be 369.304 ± 0.208 K by means of the heat-capacity measurements. The molar enthalpy, Δ_dH_m, of the dehydration of the resulting compound was of 16.490 ± 0.063 kJ mol⁻¹. The experimental molar heat capacities of the solid from 77 to 353 K and the solid from 370 to 390 K have been, respectively, fitted to tow polynomial equations with the reduced temperatures by least square method. The constant-volume energy of combustion of the compound, Δ_cU_m, has been determined as being −1616.15 ± 0.72 kJ mol⁻¹ by an RBC-II precision rotating-bomb combustion calorimeter at 298.15 K. The standard molar enthalpy of formation of the compound, , has been calculated to be −1114.76 ± 0.81 kJ mol⁻¹ from the combination of the data of standard molar enthalpy of combustion of the compound with other auxiliary thermodynamic quantities.     

Cl NQR study of lattice dynamic and magnetic property of a crystalline coordination polymer {CuCA(phz)(H2O)2}n

Copper(II) compounds {CuCA(phz)(H₂O)₂}_n (H₂CA = chloranilic acid, phz = phenazine) having a layer structure of -CuCA(H₂O)₂- polymer chains and phenazine were studied by ³⁵Cl nuclear quadrupole resonance (NQR). The single NQR line observed at 35.635 MHz at 261.5 K increased to 35.918 MHz at 4.2 K. The degree of reduction of electric field gradient due to lattice vibrations was similar to that of chloranilic acid crystal. Temperature dependence of spin-lattice relaxation time, T₁, of the ³⁵Cl NQR signal below 20 K, between 20 and 210 K, and above 210 K, was explained by (1) a decrease of effective electron-spin density caused by antiferromagnetic interaction, (2) a magnetic interaction between Cl nuclear-spin and electron-spins on paramagnetic Cu(II) ions, and (3) an increasing contribution from reorientation of ligand molecules, respectively. The electron spin-exchange parameter ∣J∣ between the neighboring Cu(II) electrons was estimated to be 0.33 cm⁻¹ from the T₁ value of the range 20−210 K. Comparing this value with that of J = −1.84 cm⁻¹ estimated from the magnetic susceptibility, it is suggested that the magnetic dipolar coupling with the electron spins on Cu(II) ions must be the principal mechanism for the ³⁵Cl NQR spin-lattice relaxation of {CuCA(phz)(H₂O)₂}_n but a delocalization of electron spin over the chloranilate ligand has to be taken into account.     

Ferromagnetic ordering in ThSi2 type CeAu0.28Ge1.72

The compound CeAu_0.28Ge_1.72 crystallizes in the ThSi₂ structure type in the tetragonal space group I4₁/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu_0.28Ge_1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μ_B/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K². The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.     

Estimation of Physico-chemical Properties of Ionic Liquid EPReO_4 Using Surface Tension and Density

An ionic liquid (IL) EPReO₄ (N‐ethylpyridinium rheniumate) was prepared. The density and surface tension values of the IL were determined in the temperature range of 293.15–343.15 K. The ionic volume and surface entropy of the IL were estimated by extrapolation, respectively. In terms of Glasser's theory, the standard molar entropy and lattice energy of the IL were estimated, respectively. Using Kabo's and Rebelo's methods, the molar enthalpy values of vaporization of the IL, Δ^g_lH⁰_m (298 K), at 298 K and, Δ^g_lH⁰_m (T_b), at hypothetical normal boiling point were estimated, respectively. According to the interstice model, the thermal expansion coefficient of IL EPReO₄ (α) was calculated and compared with experimental value, finding their magnitude order is in good agreement by 8.98%.     

Theoretical study of intra- and inter-chain magnetic interactions in [Ni(chxn)2Br]Br2

The intra- and the inter-chain magnetic interactions in [Ni(chxn)₂Br]Br₂, which is one of the typical one-dimensional (1-D) MX complexes are examined by using an unrestricted hybrid DFT (UB3LYP) method. Calculated effective exchange integral (J) value along the 1-D chain is 2J_Intra = −4016 K and is close to an experimental result (−3600 K). On the other hand, a very weak anti-ferromagnetic inter-chain interaction through Br⁻ ions is observed. The value is estimated to be 2J_Inter = −2 to −6 K. In addition to the J values, transfer integral (t), on-site Coulomb repulsion (U) and charge transfer energy (E^CT) values along the 1-D chain are also estimated to be 0.46, 2.46 and 0.6 eV, respectively.     

Crystal structure and magnetic properties of Na2Ni(HPO3)2

Na₂Ni(HPO₃)₂, obtained as light yellow-green crystals under mild hydrothermal conditions, crystallizes in the orthorhombic Pnma space-group with lattice parameters: a=11.9886(3), b=5.3671(2), c=9.0764(3) Å, V=584.01 Å³, Z=4. The structure consists of zig-zag chains of NiO₆ octahedra bridged by two HPO₃²⁻ and the chains are further connected through HPO₃²⁻ to four nearest chains to form a three dimensional framework, delimiting intersecting tunnels in which the sodium ions are located. The Na cations reside in the irregular Na(1)O₅, Na-O of 2.276-2.745 Å, and Na(2)O₉, Na-O of 2.342-2.376 Å, environments. The presence of the phosphite monoanion has been further confirmed by IR spectroscopy. Due to the 3D framework of Ni connected by O-P-O bridges, the magnetic susceptibility behaves as a paramagnet above 100 K (C=1.49(2) emu K mol⁻¹, μ_eff=3.45 μ_B, Θ=−39(2) K) and below 6 K, it orders antiferromagnetically as confirmed the sharp drop and the non-Brillouin behavior of the isothermal magnetization at 2 K.     

Structural evolution of Na0.5K0.5NbO3 at high temperatures

Changes in structure and dielectric properties at elevated temperatures have been investigated on single-crystals of sodium potassium niobate, Na_0.5K_0.5NbO₃, grown by the flux method. Single-crystal X-ray diffraction studies revealed that the crystals underwent orthorhombic-tetragonal and tetragonal-cubic phase transitions at 465 and 671 K during heating and 446 and 666 K during cooling, respectively. Both transitions were accompanied by volumetric discontinuities of collapse upon heating and expansion upon cooling, suggesting that the transitions were of the first order. The coordination numbers of an Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal and 4+2 in orthorhombic. An Na atom occupied a slightly different position from the K atom in 12-fold coordination, resulting in fewer coordination numbers of 8+4 in cubic and tetragonal and 7+5 in orthorhombic. The spontaneous polarisation (P_s) estimated from the atom positions and formal charges were approximately 0.29 C m⁻² in orthorhombic and 0.18 C m⁻² in tetragonal. The contribution of the alkaline oxide components to P_s was estimated to be approximately 15% in both ferroelectric forms. The temperature-induced transitions were also confirmed through the dielectric constant and dielectric loss at various frequencies and the differential scanning calorimetry.     

Synthesis and thermochemistry of SrB2O4·4H2O and SrB2O4

Two pure strontium borates SrB₂O₄·4H₂O and SrB₂O₄ have been synthesized and characterized by means of chemical analysis and XRD, FT-IR, DTA-TG techniques. The molar enthalpies of solution of SrB₂O₄·4H₂O and SrB₂O₄ in 1 mol dm⁻³ HCl(aq) were measured to be −(9.92 ± 0.20) kJ mol⁻¹ and −(81.27 ± 0.30) kJ mol⁻¹, respectively. The molar enthalpy of solution of Sr(OH)₂·8H₂O in (HCl + H₃BO₃)(aq) were determined to be −(51.69 ± 0.15) kJ mol⁻¹. With the use of the enthalpy of solution of H₃BO₃ in 1 mol dm⁻³ HCl(aq), and the standard molar enthalpies of formation for Sr(OH)₂·8H₂O(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpies of formation of −(3253.1 ± 1.7) kJ mol⁻¹ for SrB₂O₄·4H₂O, and of −(2038.4 ± 1.7) kJ mol⁻¹ for SrB₂O₄ were obtained.     

Solubility of H2S in 1-(2-hydroxyethyl)-3-methylimidazolium ionic liquids with different anions

The solubility of hydrogen sulfide in a series of 1-(2-hydroxyethyl)-3-methylimidazolium ([HOemim]⁺)-based ionic liquids (ILs) containing different anions, viz. hexafluorophosphate ([PF₆]⁻), trifluoromethanesulfonate ([OTf]⁻), and bis-(trifluoromethyl)sulfonylimide ([Tf₂N]⁻) at temperatures ranging from 303.15 to 353.15 K and pressures of up to about 1.8 MPa was measured by a volumetric based static apparatus. The solubility data were correlated using two models: (1) the Krichevsky–Kasarnovsky equation and (2) the extended Henry's law combined with the Pitzer's virial expansion for the excess Gibbs energy. Henry's law constants (at zero pressure) in mole-fraction and molality scales were obtained at different temperatures by means of these two models. Using the solubility data, the partial molar thermodynamic functions of solution, i.e. Gibbs energy, enthalpy, and entropy were calculated. Comparison showed that the solubility of H₂S is greater than that of CO₂ in the corresponding ILs studied in this work and that the solubility of both gases increases as the number of trifluoromethyl (–CF₃) groups in the anion increases, i.e. the solubility behavior of both gases follows the order [HOemim][Tf₂N] ≥ [HOemim][OTf] > [HOemim][PF₆] > [HOemim][BF₄].     

On the magnetic structure of DyNiO3

The crystallographic structure of DyNiO₃ has been investigated at T=200, 100, and 2 K from high-resolution neutron powder diffraction (NPD) data. We show that the structure is monoclinic, space group P2₁/n, from the metal-insulator transition temperature at T_MI=564 K down to 2 K. The Ni atoms occupy two different sites 2d (Ni1) and 2c (Ni2), whose valences, estimated from bond-valence consideration, are +2.43(1) and +3.44(1) at 2 K, respectively. This is interpreted as the result of a partial charge disproportionation of the type 2Ni³⁺→Ni1^(3−δ)++Ni2^(3+δ)+, with δ≈0.55 at T=2 K. The magnetic structure has been studied from a NPD pattern at T=2 K, well below the establishment of the antiferromagnetic (AFM) ordering at T_N=154 K, as well as from sequential data collected from 16 K down to 2 K. The magnetic order is defined by the propagation vector k=(1/2,0,1/2). Two possible magnetic structures are compatible with the magnetic intensities. In the second solution both Ni sublattices participate in the magnetic order, as well as Dy since it corresponds to a total disproportionation of Ni³⁺ to Ni²⁺ and Ni⁴⁺. In the second solution both Ni sublattices participate in the magnetic order, as well as Dy. The magnetic moments for Ni1 and Ni2 atoms at T=2 K are 1.8 (2) and 0.8 (2) μ_B, respectively. These values are also compatible with a partial charge disproportionation. Dy³⁺ ions exhibit long-range magnetic ordering below 8 K. An abrupt contraction of the unit-cell volume is observed at this temperature, due to a magnetoelastic coupling. The magnetic moment for Dy³⁺ at T=2 K is 7.87 (6) μ_B.     

Thermoelectric properties of p-type pseudo-binary (Ag0.365Sb0.558Te)x-(Bi0.5Sb1.5Te3)1−x (x=0-1.0) alloys prepared by spark plasma sintering

In this paper, pseudo-binary (Ag_0.365Sb_0.558Te)_x-(Bi_0.5Sb_1.5Te₃)_1−x (x=0-1.0) alloys were prepared using spark plasma sintering technique, and the composition-dependent thermoelectric properties were evaluated. Electrical conductivities range from 7.9×10⁴ to 15.6×10⁴ Ω⁻¹ m⁻¹ at temperatures of 507 and 318 K, respectively, being about 3.0 and 8.5 times those of Bi_0.5Sb_1.5Te₃ alloy at the corresponding temperatures. The optimal dimensionless figure of merit (ZT) of the sample with molar fraction x=0.025 reaches 1.1 at 478 K, whereas that of the ternary Bi_0.5Sb_1.5Te₃ alloy is 0.58 near room temperature. The results also reveal that a direct introduction of Ag_0.365Sb_0.558Te in the Bi-Sb-Te system is much more effective to the property improvement than naturally precipitated Ag_0.365Sb_0.558Te in the Ag-doped Ag-Bi-Sb-Te system.     

(p, ρ, T) properties, and apparent molar volumes V? of ZnBr2 in methanol at T = (298.15 to 398.15) K and pressures up to p = 40 MPa

The (p, ρ, T) properties of pure methanol, the (p, ρ, T) properties and apparent molar volumes V_? of ZnBr₂ in methanol at T = (298.15 to 398.15) K and pressures up to p = 40 MPa are reported, and apparent molar volumes have been evaluated. The experimental (p, ρ, T, m) values were described by an equation of state. For the solutions the experiments were carried out at molalities m = (0.05772, 0.37852, 0.71585 and 1.95061) mol · kg⁻¹ of zinc bromide.     

Thermochemistry of NaRb[B4O5(OH)4]·4H2O

The enthalpies of solution of NaRb[B₄O₅(OH)₄]·4H₂O in approximately 1 mol dm⁻³ aqueous hydrochloric acid and of RbCl in aqueous (hydrochloric acid + boric acid + sodium chloride) were determined. From these results and the enthalpy of solution of H₃BO₃ in approximately 1 mol dm⁻³ HCl(aq) and of sodium chloride in aqueous (hydrochloric acid + boric acid), the standard molar enthalpy of formation of −(5128.02 ± 1.94) kJ mol⁻¹ for NaRb[B₄O₅(OH)₄]·4H₂O was obtained from the standard molar enthalpies of formation of NaCl(s), RbCl(s), H₃BO₃(s) and H₂O(l). The standard molar entropy of formation of NaRb[B₄O₅(OH)₄]·4H₂O was calculated from the Gibbs free energy of formation of NaRb[B₄O₅(OH)₄]·4H₂O computed from a group contribution method.