Energetics of cobalt phosphate frameworks: α, β, and red NaCoPO4

Thermal behavior, relative stability, and enthalpy of formation of α (pink phase), β (blue phase), and red NaCoPO₄ are studied by differential scanning calorimetry, X-ray diffraction, and high-temperature oxide melt drop solution calorimetry. Red NaCoPO₄ with cobalt in trigonal bipyramidal coordination is metastable, irreversibly changing to α NaCoPO₄ at 827 K with an enthalpy of phase transition of −17.4±6.9 kJ mol⁻¹. α NaCoPO₄ with cobalt in octahedral coordination is the most stable phase at room temperature. It undergoes a reversible phase transition to the β phase (cobalt in tetrahedra) at 1006 K with an enthalpy of phase transition of 17.6±1.3 kJ mol⁻¹. Enthalpy of formation from oxides of α, β, and red NaCoPO₄ are −349.7±2.3, −332.1±2.5, and −332.3±7.2 kJ mol⁻¹; standard enthalpy of formation of α, β, and red NaCoPO₄ are −1547.5±2.7, −1529.9±2.8, and −1530.0±7.3 kJ mol⁻¹, respectively. The more exothermic enthalpy of formation from oxides of β NaCoPO₄ compared to a structurally related aluminosilicate, NaAlSiO₄ nepheline, results from the stronger acid-base interaction of oxides in β NaCoPO₄ (Na₂O, CoO, P₂O₅) than in NaAlSiO₄ nepheline (Na₂O, Al₂O₃, SiO₂).     

LiMO2 (M=Mn, Fe, and Co): Energetics, polymorphism and phase transformation

LiMO₂ materials (M=Mn, Fe, and Co) with different structures were synthesized and their enthalpies of formation from oxides (Li₂O and M₂O₃, M=Mn and Fe), or from oxides (Li₂O and CoO) plus oxygen at 25 °C were determined by high-temperature oxide melt solution calorimetry. The relative stability of the polymorphs of the compound LiMO₂ was established based on their enthalpies of formation. Phase transformations in LiFeO₂ were investigated by differential scanning calorimetry and high-temperature oxide melt solution calorimetry. The phase transition enthalpies at 25 °C for β→α, γ→β, and γ→α are 4.9±0.7, 4.3±0.8 and , respectively. Thus the γ phase (ordered cations) is the stable form of LiFeO₂ at room temperature, the α phase (disordered cations) is stable at high temperature and the β phase may have a stability field at intermediate temperatures.     

Energetics of copper diphosphates – Cu2P2O7 and Cu3(P2O6OH)2

Differential scanning calorimetry and high temperature oxide melt solution calorimetry are used to study enthalpy of phase transition and enthalpies of formation of Cu₂P₂O₇ and Cu₃(P₂O₆OH)₂. α-Cu₂P₂O₇ is reversibly transformed to β-Cu₂P₂O₇ at 338–363 K with an enthalpy of phase transition of 0.15 ± 0.03 kJ mol⁻¹. Enthalpies of formation from oxides of α-Cu₂P₂O₇ and Cu₃(P₂O₆OH)₂ are −279.0 ± 1.4 kJ mol⁻¹ and −538.8 ± 2.7 kJ mol⁻¹, and their standard enthalpies of formation (enthalpy of formation from elements) are −2096.1 ± 4.3 kJ mol⁻¹ and −4302.7 ± 6.7 kJ mol⁻¹, respectively. The presence of hydrogen in diphosphate groups changes the geometry of Cu(II) and decreases acid–base interaction between oxide components in Cu₃(P₂O₆OH)₂, thus decreasing its thermodynamic stability.     

Synthesis, heat capacity and enthalpy of formation of [Ho2(l-Glu)2(H2O)8](ClO4)4·H2O

A complex of holmium perchlorate coordinated with l-glutamic acid, [Ho₂(l-Glu)₂(H₂O)₈](ClO₄)₄·H₂O, was prepared with a purity of 98.96%. The compound was characterized by chemical, elemental and thermal analysis. Heat capacities of the compound were determined by automated adiabatic calorimetry from 78 to 370 K. The dehydration temperature is 350 K. The dehydration enthalpy and entropy are 16.34 kJ mol⁻¹ and 16.67 J K⁻¹ mol⁻¹, respectively. The standard enthalpy of formation is −6474.6 kJ mol⁻¹ from reaction calorimetry at 298.15 K.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

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.     

Phase transition between two anhydrous modifications of NaHSO4 mediated by heat and water

The phase transition between the two anhydrous modifications of NaHSO₄ (α and β) was studied using Raman spectroscopy and differential scanning calorimetry. These measurements indicate that β-NaHSO₄ is a metastable phase and readily undergoes phase transition to thermodynamically stable α-NaHSO₄ with an exothermic enthalpy change of 3.5 kJ/mol. Both thermal (temperatures >434 K) and chemical (exposure to H₂O) pathways were identified for this transition. The transition is irreversible, and α-NaHSO₄ is an intermediate phase between β-NaHSO₄ and NaHSO₄·H₂O. The possible mechanism of the phase transition is discussed.     

Thermodynamics of CoAl2O4-CoGa2O4 solid solutions

CoAl₂O₄, CoGa₂O₄, and their solid solution Co(Ga_zAl_1−z)₂O₄ have been studied using high temperature oxide melt solution calorimetry in molten 2PbO·B₂O₃ at 973 K. There is an approximately linear correlation between lattice parameters, enthalpy of formation from oxides, and the Ga content. The experimental enthalpy of mixing is zero within experimental error. The cation distribution parameters are calculated using the O’Neill and Navrotsky thermodynamic model. The enthalpies of mixing calculated from these parameters are small and consistent with the calorimetric data. The entropies of mixing are calculated from site occupancies and compared to those for a random mixture of Ga and Al ions on octahedral site with all Co tetrahedral and for a completely random mixture of all cations on both sites. Despite a zero heat of mixing, the solid solution is not ideal in that activities do not obey Raoult's Law because of the more complex entropy of mixing.     

Formation of Co octahedra and tetrahedra in YBaCo4O8.1

High-resolution neutron and synchrotron X-ray powder diffraction experiments were performed, at 300 and 10 K, for the determination of the structure of YBaCo₄O_8.1, which was prepared by controlled oxidation of the Kagomé lattice compound YBaCo₄O₇. Our diffraction data demonstrate that YBaCo₄O_8.1 crystallizes in the orthorhombic Pbc2₁ space group with the formation of a large superstructure (a=12.790 Å, b=10.845 Å, c=10.149 Å), with respect to the parent trigonal YBaCo₄O₇ material. The Co ions occupy both corner-sharing tetrahedral and edge-sharing octahedral sites, in contrast to YBaCo₄O₇, which has only corner-sharing tetrahedra. The octahedral sites form by the addition of two extra oxygen atoms and the drastic displacements of some of the original O atoms relative to the parent. The edge-sharing octahedra form isolated zigzag chains parallel to the c-axis linked to one another via tetrahedra. While found in a few phosphates, silicates and germanates, this motif appears unique to YBaCo₄O_8.1 among mixed-metal oxides. No structural phase transition or long range antiferromagnetic ordering are observed at 10 K.     

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.     

Hydrothermal synthesis, characterization and thermochemistry of Ca2[B2O4(OH)2]·0.5H2O

A pure calcium borate Ca₂[B₂O₄(OH)₂]·0.5H₂O has been synthesized under hydrothermal condition and characterized by XRD, FT-IR and TG as well as by chemical analysis. The molar enthalpy of solution of Ca₂[B₂O₄(OH)₂]·0.5H₂O in HC1·54.582H₂O was determined. From a combination of this result with measured enthalpies of solution of H₃BO₃ in HC1·54.561H₂O and of CaO in (HCl + H₃BO₃) solution, together with the standard molar enthalpies of formation of CaO(s), H₃BO₃(s) and H₂O(l), the standard molar enthalpy of formation of −(3172.5 ± 2.5) kJ mol⁻¹ of Ca₂[B₂O₄(OH)₂]·0.5H₂O was obtained.     

Thermochemistry of dicesium calcium tetraborate octahydrate

The enthalpies of solution of Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) in approximately 1 mol dm⁻³ aqueous hydrochloric acid and of CsCl(s) in aqueous (hydrochloric acid + boric acid + calcium oxide) were determined. From these results and the enthalpies of solution of H₃BO₃(s) in approximately 1 mol dm⁻³ HCl(aq) and of CaO(s) in aqueous (hydrochloric acid + boric acid), the standard molar enthalpy of formation of −(10328 ± 6) kJ mol⁻¹ for Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) was obtained from the standard molar enthalpy of formation of CaO(s), CsCl(s), H₃BO₃(s) and H₂O(l). The standard molar entropy of formation of Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) computed from a group contribution method.     

Excess enthalpies of alkane-1-amines {CnH2n+1NH2, n = 3-8} + methyl methylthiomethyl sulfoxide and +dimethyl sulfoxide at 298.15 K

Excess enthalpies of binary mixtures between each of alkane-1-amines {C_nH_2n+1NH₂, n=3-8} and methyl methylthiomethyl sulfoxide (MMTSO) or dimethyl sulfoxide (DMSO) have been determined at 298.15 K. All mixtures showed positive enthalpy changes over the whole range of mole fractions.The limiting excess partial molar enthalpies of the aliphatic amines, H₁^E,∞, of all the mixtures with MMTSO or DMSO studied were smaller than those of MMTSO or DMSO, H₂^E,∞, respectively. Linear relations are obtained between limiting excess partial molar enthalpies and number of methylene groups.     

Vaporization of DyI3(s) and thermochemistry of the homocomplexes (DyI3)2(g) and (DyI3)3(g)

The vaporization of DyI₃(s) was investigated in the temperature range between 833 and 1053 K by the use of Knudsen effusion mass spectrometry. The ions DyI₂⁺, DyI₃⁺, Dy₂I₄⁺, Dy₂I₅⁺, Dy₃I₇⁺, and Dy₃I₈⁺ were detected in the mass spectrum of the equilibrium vapor. The gaseous species DyI₃, (DyI₃)₂, and (DyI₃)₃ were identified and their partial pressures determined. Enthalpies and entropies of sublimation resulted according to the second- and third-law methods. The following sublimation enthalpies at 298 K were determined for the gaseous species given in brackets: 274.8±8.2 kJ mol⁻¹ [DyI₃], 356.0±11.3 kJ mol⁻¹ [(DyI₃)₂], and 436.6±14.6 kJ mol⁻¹ [(DyI₃)₃]. The enthalpy changes of the dissociation reactions (DyI₃)₂=2 DyI₃ and (DyI₃)₃=3 DyI₃ were obtained as Δ_dH°(298)=193.3±5.6 and 390.3±13.0 kJ mol⁻¹, respectively.     

Heat capacity, enthalpy and entropy of bismuth niobate and bismuth tantalate

The heat capacity and the heat content of bismuth niobate BiNbO₄ and bismuth tantalate BiTaO₄ were measured by the relaxation method and Calvet-type heat flux calorimetry. The temperature dependencies of the heat capacities in the form C_pm=128.628+0.03340 T−1991055/T²+136273131/T³ (J K^-1 mol^-1) and 133.594+0.02539 T−2734386/T²+235597393/T³ (J K^-1 mol^-1) were derived for BiNbO₄ and BiTaO₄, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K S_m(BiNbO₄)=147.86 J K^-1 mol^-1 and S_m(BiTaO₄)=149.11 J K^-1 mol^-1 were assessed from the low temperature heat capacity measurements. To complete a set of thermodynamic data of these mixed oxides an attempt was made to estimate the values of the heat of formation from the constituent binary oxides.     

Determination of cadmium, copper and lead in soils, sediments and sea water samples by ETAAS using a Sc + Pd + NH(4)NO(3) chemical modifier

Cadmium, copper and lead in soils, sediments and spiked sea water samples have been determined by electrothermal atomic absorption spectrometry (ETAAS) with Zeeman effect background corrector using NH₄NO₃, Sc, Pd, Sc + NH₄NO₃, Pd + NH₄NO₃, Sc + Pd and Sc + Pd + NH₄NO₃ as chemical modifiers. A comprehensive comparison was made among the modifiers and without modifier in terms of pyrolysis and atomization temperatures, atomization and background absorption profiles, characteristic masses, detection limits and accuracy of the determinations. Sc + Pd + NH₄NO₃ modifier mixture was found to be preferable for the determination of analytes in soil and sediment certified and standard reference materials, and sea water samples because it increased the pyrolysis temperature up to 900 °C for Cd, 1350 °C for Cu and 1300 °C for Pb. Optimum masses of mixed modifier components found are 20 μg Sc + 4 μg Pd + 8 μg NH₄NO₃. Characteristic masses of Cd, Cu and Pb obtained are 0.6, 5.3 and 15.8 pg, respectively. The detection limits of Cd, Cu and Pb were found to be 0.08, 0.57 and 0.83 μg l⁻¹, respectively. Depending on the solid sample type, the percent recoveries were increased up to 103% for Cd, Cu and Pb by using the proposed modifier mixture. The accuracy of the determination of analytes in the sea water samples was also increased.     

Ionothermal synthesis of β-NH4AlF4 and the determination by single crystal X-ray diffraction of its room temperature and low temperature phases

β-NH₄AlF₄ has been synthesised ionothermally using 1-ethyl-3-methylimidazolium hexafluorophosphate as solvent and template provider. β-NH₄AlF₄ crystals were produced which were suitable for single crystal X-ray diffraction analysis. A phase transition occurs between room temperature (298 K) and low temperature (93 K) data collections. At 298 K the space group=I4/mcm (no. 140), α=11.642(5), c=12.661(5) Å, Z=2 (10NH₄AlF₄), wR(F²)=0.1278, R(F)=0.0453. At 93 K the space group=P4₂/ncm (no. 138), α=11.616(3), c=12.677(3) Å, Z=2 (10NH₄AlF₄), wR(F²)=0.1387, R(F)=0.0443. The single crystal X-ray diffraction study of β-NH₄AlF₄ shows the presence of two different polymorphs at low and room temperature, indicative of a phase transition. The [AlF_4/2F₂]⁻ layers are undisturbed except for a small tilting of the AlF₆ octahedra in the c-axis direction.     

Determination of the enthalpy of fusion of K3TaF8 and K3TaOF6

The areas of the fusion and crystallization peaks of K₃TaF₈ and K₃TaOF₆ have been measured using the DSC mode of the high-temperature calorimeter (SETARAM 1800 K). On the basis of these quantities and the temperature dependence of the used calorimetric method sensitivity, the values of the enthalpy of fusion of K₃TaF₈ at temperature of fusion 1039 K: Δ_fusH_m(K₃TaF₈; 1039 K) = (52 ± 2) kJ mol⁻¹ and of K₃TaOF₆ at temperature of fusion 1055 K: Δ_fusH_m(K₃TaOF₆; 1055 K) = (62 ± 3) kJ mol⁻¹ have been determined.     

Synthesis, crystal structure and thermochemical behaviour of a barium complex [Ba(5-OH-BDC)(H2O)3] [5-OH-H2BDC = 5-hydroxyisophtalic acid]

A novel complex [Ba(5-OH-BDC)(H₂O)₃] [5-OH-H₂BDC = 5-hydroxyisophtalic acid] was synthesized and characterized by X-ray crystallography. The complex is Monoclinic P2₁/c, a = 11.1069(4), b = 14.8192(6), c = 6.5005(2) Å, β = 103.465(3)° and Z = 4, which exhibits a three-dimensional framework formed by linkage of adjacent two-dimensional (6, 3) layers via intermolecular hydrogen bonds. The title complex has been studied by IR spectrum and TG-DTG. The constant-volume combustion energy of the complex, Δ_cU, was determined as being (−3210.45 ± 1.41) kJ mol⁻¹ by a precise rotating-bomb calorimeter at 298.15 K. The standard enthalpy of combustion, , and the standard enthalpy of formation, , were calculated as being (−3207.97 ± 1.41) and (−1922.80 ± 1.76) kJ mol⁻¹, respectively. A calculation model for determining the specific heat capacity of the complex with an improved RD496-III microcalorimeter is also derived. The specific heat capacity of the complex was (6158.387 ± 0.187) J mol⁻¹ K⁻¹.     

Crystal structure and the standard molar enthalpy of formation of a coordination polymer [Cu(nip)(phen)]n

A coordination polymer [Cu(nip)(phen)]_n was hydrothermally synthesized by the reaction of Cu(NO₃)₂ with 5-nitroisophthalic acid and phen. Single-crystal structure analysis showed that the complex crystallized in the monoclinic space group P2₁/c; a = 10.6566(13); b = 12.5931(15); c = 13.0514(16) Å; β = 95.474(2)°, V = 1743.5(4) Å³; Z = 4. The standard molar enthalpy of formation of the complex was determined to be −554 ± 11 kJ mol⁻¹.