Thermal properties of [Cr(NH3)6](BF4)3 studied by adiabatic and relaxation calorimetry

Four (solid–solid) phase transitions were detected in the temperature range of (9 to 300) K in polycrystalline [Cr(NH₃)₆](BF₄)₃ at T_C1 = 240.7 K, T_C2 = 108.0 K, T_C3 = 91.9 K, and T_C4 = 61.3 K by adiabatic calorimetry. The measurements by relaxation calorimetry were followed on lowering temperature from 20 K down to 0.35 K under six different external magnetic field values (9, 7, 5, 3, 1 and 0) T. For non-zero values of applied magnetic field well-defined Schottky anomaly appears. Magnetic heat capacity was calculated assuming the zero-field splitting for the decoupled Cr(III) ions. There is no discrepancy between the observed and calculated values. Isothermal magnetization curve recorded up to 5 T was measured at temperature of 1.8 K.     

Structure and properties of Gd4Pd10In21

Gd₄Pd₁₀In₂₁ was synthesized from the elements in a glassy carbon crucible in an induction furnace and investigated by X-ray powder and single crystal diffraction: C2/m, a = 2293.3(2), b = 444.49(3), c = 1934.3(1) pm, β = 133.00(1)°, wR2 = 0.0496, 1564 F² values, and 106 variable parameters. The five crystallographically independent palladium atoms have all a trigonal prismatic coordination. Together, the palladium and indium atoms build up a three-dimensional [Pd₁₀In₂₁] network in which the gadolinium atoms fill distorted pentagonal channels. Magnetic susceptibility measurements show that Gd₄Pd₁₀In₂₁ orders antiferromagnetically at T_N = 14.2(1) K. Two further magnetic transitions were found at temperatures 11.8 and 6.4 K, respectively. Below T_irr = 10 K, strong irreversibility between ZFC and FC magnetic susceptibilities are observed. The zero-field specific heat measurements show a clear peak around the magnetic transition temperatures T_N. Measured quadrupole interaction constants by ¹⁵⁵Gd Mössbauer spectroscopy give estimations for the V_zz component of the electric field gradient tensor at both gadolinium sites in Gd₄Pd₁₀In₂₁.     

Phase transition and dynamics of NH3 ligands in [Zn(NH3)4](ReO4)2

One phase transition in [Zn(NH₃)₄](ReO₄)₂ at T_c = 393.5 K (on heating) and 392.0 K (on cooling) was found. Thermal stability of this compound was investigated by thermal analysis methods. It decomposes in three main stages. The first two are connected with deamination process, whereas Re₂O₇ evaporates in the last step. The activation energy for NH₃ loss processes was determined from thermogravimetric (TG) measurements. The vibrational and reorientational dynamics of NH₃ ligands in the low-temperature phase was probed by various complementary techniques. It was found that at temperatures close to 150 K, NH⋯O hydrogen bond is formed. Temperature-dependent band shape analysis of properly chosen infrared (IR) band was performed, whose results showed that activation energy for NH₃ reorientational motion (<300 K) is rather small and is approximately equal to 2 kJ mol⁻¹. Neutron and X-ray powder diffraction patterns did not reveal any drastic change in the crystal structure in a wide temperature range.     

The crystal structure of Rb2SeO4 at high temperature

The crystal structure of Rb₂SeO₄ in its high-temperature phase is reported for the first time. Powder diffraction data collected at T = 898 K show that it is hexagonal (a = 6.3428(1) Å, c = 8.5445(1) Å, V = 297.71(1) Å³, space group P6₃/mmc (194), Z = 2) and is isostructural to Tl₂SeO₄, thus belonging to the α-K₂SO₄ structure type. DSC measurements indicate that the phase transition occurs at T = 818 K.     

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.     

New three-dimensional (3,4)-net zincophosphites templated by linear diammonium cations: H3N(CH2)5NH3·Zn3(HPO3)4 and β-H3N(CH2)6NH3·Zn3(HPO3)4

The mild-condition syntheses, single-crystal structures and properties of H₃N(CH₂)₅NH₃·Zn₃(HPO₃)₄ and β-H₃N(CH₂)₆NH₃·Zn₃(HPO₃)₄ are reported. Both are constructed from (3,4)-nets of ZnO₄ tetrahedra and HPO₃ pyramids, sharing vertices to result in three-dimensional anionic open-frameworks. In both materials, the organic species interacts with the framework by way of N–H⋯O bonds. Crystal data: H₃N(CH₂)₅NH₃·Zn₃(HPO₃)₄, M_r = 620.22, orthorhombic, Pccn (No. 56), a = 9.5364 (9) Å, b = 21.8015 (19) Å, c = 9.1118 (7) Å, V = 1894.4 (3) Å³, Z = 4, R(F) = 0.044, wR(F²) = 0.111. β-H₃N(CH₂)₆NH₃·Zn₃(HPO₃)₄, M_r = 634.25, monoclinic, P2₁/n (No. 14), a = 8.7627 (1) Å, b = 13.8117 (2) Å, c = 16.6187 (3) Å, β = 92.680 (1)°, V = 2009.12 (5) Å³, Z = 4, R(F) = 0.072, wR(F²) = 0.187.     

Thermodynamic study on the sublimation of six methylnitrobenzoic acids

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following six compounds: 2-methyl-3-nitrobenzoic acid, between T =  357.16 K and T =  371.16 K; 2-methyl-6-nitrobenzoic acid, between T =  355.16 K and T =  369.16 K; 3-methyl-2-nitrobenzoic acid, between T =  371.16 K and T =  385.14 K; 3-methyl-4-nitrobenzoic acid, between T =  363.21 K and T =  379.16 K; 4-methyl-3-nitrobenzoic acid, between T =  363.10 K and T =  377.18 K; 5-methyl-2-nitrobenzoic acid, between T =  355.18 K and T =  371.08 K. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation were derived by the Clausius–Clapeyron equation and the molar entropies of sublimation at equilibrium pressures were calculated. Using estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds the standard, p^o =  10⁵Pa, molar enthalpies Δ_cr^gH_m^o, entropies Δ_cr^gS_m^oand Gibbs energies Δ_cr^gG_m^oof sublimation at T =  298.15 K, were derived:     

Free NH3 quantum rotations in Hofmann clathrates: structure factors and line widths studied by inelastic neutron scattering

Quantum rotations of NH₃ groups in Hofmann clathrates Ni–Ni–C₆H₆ and Ni–Ni–C₁₂H₁₀ have been studied using inelastic neutron scattering. Calculations of the dynamical structure factor for a free uniaxial quantum rotor reproduce the neutron scattering data with respect to their Q- and T-dependence as well as the relative intensities for the 0 → 1, 0 → 2 and 1 → 2 transitions. Though the effective NH₃ rotation constant is different from the gas phase value, the effective radius of rotation (i.e., the average distance of protons from the rotation axis) is equal or very close to the geometrical value r = 0.94 Å for a NH₃ group. Comparing the experimental data with the calculated dynamical structure factor for the 0 → 3 transition it could be shown, that the corresponding transition line, in contrast to transitions between j = 0,1,2 levels measured so far, has a finite width at T = 0 K.     

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.     

Structural characterization,molecular dynamics,dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]·2H2O

The structure of [C₃N₂H₅]₄[Bi₂Br₁₀]·2H₂O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å³ and Z = 2. The structure consists of discrete binuclear [Bi₂Br₁₀]⁴⁻ anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the ¹H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm⁻¹. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.     

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.     

Thermodynamic study on the sublimation of 3-phenylpropionic acid and of three methoxy-substituted 3-phenylpropionic acids

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following compounds: 3-phenylpropionic acid, between T =  305.17 K and T =  315.17 K; 3-(2-methoxyphenyl)propionic acid, between T =  331.16 K and T =  347.16 K; 3-(4-methoxyphenyl)propionic acid, between T =  341.19 K and T =  357.15 K; 3-(3,4-dimethoxyphenyl)propionic acid, between T =  352.18 K and T =  366.16 K. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation Δ_cr^gH_m^owere derived by the Clausius–Clapeyron equation and the molar entropies of sublimation at equilibrium pressures were calculated. On the basis of estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds the standard, p ^∘  =  10⁵Pa, molar enthalpies, entropies and Gibbs energies of sublimation at T =  298.15 K, were derived:     

The p–T phase diagram for ferroelectric bis-thiourea pyridinium nitrate

The effect of temperature and pressure on physical properties of the ferroelectric bis-thiourea pyridinium nitrate inclusion compound has been studied by dielectric spectroscopy and nuclear magnetic resonance (NMR). At ambient pressure the ferroparaelectric phase transition observed at T₂ = 216 K is continuous in contrast to the nonferroelectric phase transition observed at T₁ = 273 K. Under small pressures, the temperatures of the phase transitions T₁ and T₂ increase with increasing pressure. Starting from about 250 MPa, T₁ temperature decreases with increasing pressure, while T₂ temperature increases with increasing pressure. At 450 MPa and 245 K a triple point is observed. Bis-thiourea pyridinium nitrate undergoes a continuous phase transition from the ferroelectric to paraelectric phase under 450 MPa, while above this pressure the phase transition from the ferroelectric to paraelectric phase is discontinuous. The change in the phase transition character is related to the crystallographic change in the group–subgroup relation between the ferro- and paraelectric phases taking place with increasing pressure.     

High-temperature calorimetry of (La1−xLnx)PO4 solid solutions

The enthalpy increment of the monazite-type solid solutions of LaPO₄ with NdPO₄, EuPO₄ and GdPO₄ has been measured by drop calorimetry at T = 1000 K. The results show deviations (excess enthalpy) from ideal behaviour that have been interpreted in terms of lattice strains resulting from the ion size effects of substitution of La³⁺ by Ln³⁺. For (La_0.5Gd)_0.5PO₄ also the temperature dependence has been determined for T = (515 to 1565) K, indicating that the excess enthalpy decreases with increasing temperature.     

The thermodynamics of formation,molar heat capacity,and thermodynamic functions ofZrTiO4(cr)

As part of an ongoing study of titanate-based ceramic materials for the disposal of surplus weapons-grade plutonium, we report thermodynamic properties of a sample ofzirconium titanate (ZrTiO₄) quenched from a high-temperature synthesis. The standard enthalpy of formationΔ_fH_m^o was obtained by using high-temperature oxide-melt solution calorimetry. The molar heat capacity C_{p, m}was measured fromT =  13 K to T =  400 K in an adiabatic calorimeter and extrapolated toT =  1800 K by using an equation fitted to the low-temperature results. The results atT =  298.15 K areΔ_fH_m^o =  − (2024.1  ±  4.5)kJ · mol ^− 1,Δ₀^TS_m^o =  (116.71  ±  0.31 )J · K ^− 1· mol ^− 1, andΔ_fG_m^o =  − (1915.8  ±  4.5 )kJ · mol ^− 1; the molar entropy includes a contribution of 2 R ln2 to account for the random mixing of Zr^4 + and Ti^4 + on a four-fold crystallographic site. Values for the standard molar Gibbs energies and enthalpies of formation of ZrTiO_4,Δ_fG_m^oandΔ_fH_m^o , and for the free energies and enthalpies for the reaction to form ZrTiO₄(cr) from ZrO₂(cr) and TiO₂(cr), are tabulated over the temperature interval, 0 ⩽(T / K)⩽ 1800. From these results, we conclude that ZrTiO₄is not stable with respect to (ZrO₂ +  TiO₂) at T =  298.15 K, but becomes so at T =  (1250  ±  150) K.     

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.     

Isopiestic determination of the osmotic and activity coefficients of Rb 2SO4 (aq) and Cs 2SO 4(aq) at T = (298.15 and 323.15) K,and representation with an extended ion-interaction (Pitzer) model

Isopiestic vapor-pressure measurements were made for Rb ₂SO ₄(aq) from molalitym =  (0.16886 to 1.5679 )mol · kg ^− 1atT =  298.15 K and from m =  (0.32902 to 1.2282 )mol · kg ^− 1at T =  323.15 K, and for Cs ₂SO₄ (aq) from m =  (0.11213 to 3.10815 )mol · kg ^− 1at T =  298.15 K and fromm =  (0.11872 to 3.5095 )mol · kg ^− 1atT =  323.15 K, with NaCl(aq) as the reference standard. Published thermodynamic information for these systems were reviewed and the isopiestic equilibrium molalities and dilution enthalpies were critically assessed and recalculated in a consistent manner. Values of the four parameters of an extended version of Pitzer`s model for osmotic and activity coefficients with an ionic-strength dependent third virial coefficient were evaluated for both systems at both temperatures, as were those of the usual three-parameter Pitzer model. Similarly, parameters of Pitzer`s model for the relative apparent molar enthalpies of dilution were evaluated at T =  298.15 K for both Rb ₂SO ₄(aq) and Cs ₂SO ₄(aq) for the more restricted range of m⩽ 0.101 mol · kg ^− 1. Values of the thermodynamic solubility product K_s(Rb₂ SO ₄, cr, 298.15 K )  =  (0.1392  ±  0.0154) and the CODATA compatible standard molar Gibbs free energy of formationΔ_fG_m^o (Rb ₂SO ₄, cr, 298.15 K )  =  − (1316.91  ±  0.59)kJ · mol ^− 1, standard molar enthalpy of formationΔ_fH_m^o (Rb ₂SO ₄, cr, 298.15 K )  =  − (1435.07  ±  0.60)kJ · mol ^− 1, and standard molar entropy S _m^o(Rb₂ SO ₄, cr, 298.15 K )  =  (199.60  ±  2.88)J · K ^− 1· mol ^− 1were derived. A sample of one of the lots of Rb ₂SO ₄(s) used for part of our isopiestic measurements was analyzed by ion chromatography, and was found to be contaminated with potassium and cesium in amounts that significantly exceeded the claims of the supplier. In contrast, analysis by ion chromatography of a lot of Cs ₂SO ₄(s) used for some of our experiments showed it was highly pure.     

Synthesis,structure and solid state NMR analysis of a new templated titanium(III/IV) fluorophosphate

The title compound MIL-131 (MIL stands for Material from Institut Lavoisier) was prepared hydrothermally (4 days, 473 K, autogenous pressure) in the presence of an organic base (N((CH₂)₂NH₂)₃). The structure of MIL-131 or Ti^IIITi^IV(OH)F₄(HPO₄)·(PO₄)·(N((CH₂)₂NH₃)₃) has been determined ab initio from X-Ray synchrotron powder diffraction data using simulated annealing methods and was refined in the triclinic space group P-1 (no. 2). MIL-131 exhibits a one-dimensional structure built up from inorganic chains of corner sharing TiO₅(OH) titanium(III) octahedra and PO₄ and HPO₄ phosphate tetrahedra, related to TiO₂F₄ titanium octahedra. Protonated triamine cations are located between the inorganic motifs, and interact strongly with the mineral network through hydrogen bondings both with terminal fluorine atoms and hydroxo or oxo groups. Multinuclear solid state NMR has allowed a clear attribution of the protons, fluoride, and phosphate groups environment within the framework of MIL-131. The large values of chemical shift anisotropy together with the absence of any ¹³C NMR response confirmed the presence of paramagnetic titanium(III) species deduced from the crystal structure. Finally, 2D MAS ¹H-³¹P CP-HETCOR NMR correlation experiment gives some insight on the nature of the intra-framework hydrogen bonding.Crystal data for MIL-131: a = 14.109(1) Å, b = 8.462(3) Å, c = 7.179(1) Å, α = 93.772(1)°, β = 96.566(2)°, γ = 98.004(1)°, V = 840.36(2) ų, z = 2.     

Combined experimental and computational thermochemistry of isomers of chloronitroanilines

The standard (p^° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of 4-chloro-3-nitroaniline and 5-chloro-2-nitroaniline, in the condensed phase, were derived from their standard molar energies of combustion, in oxygen, to yield CO₂(g), N₂(g), and HCl · 600H₂O(l), measured by rotating bomb combustion calorimetry. From the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen effusion technique, their standard molar enthalpies of sublimation, at T = 298.15 K, were derived by means of the Clausius–Clapeyron equation. The Calvet microcalorimetry was also used to measure the standard molar enthalpies of sublimation of these compounds, at T = 298.15 K. The combination of the standard molar enthalpies of formation in the condensed phases and the standard molar enthalpies of sublimation yielded the standard molar enthalpies of formation in the gaseous phase at T = 298.15 K for each isomer. Further, the standard (p^° = 0.1 MPa) molar enthalpies, entropies and Gibbs free energies of sublimation, at T = 298.15 K, were also derived.The standard molar enthalpies of formation, in the gaseous phase of all the chloronitroaniline isomers were also estimated by the Cox scheme and by the use of computational thermochemistry and compared with the available experimental values.