Heat capacity,entropy of Ln<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> (Ln = La,Sm, and Gd), and the high-temperature enthalpy of Ln<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> (Ln = Eu,Dy, and Ho)

The low-temperature heat capacity of Ln₂(MoO₄)₃ (Ln = La, Sm, and Gd) is investigated by means of adiabatic calorimetry within the range of 60–300 K. The temperature dependences of the heat capacity are found and the values of the standard entropy are calculated, based on extrapolations to 0 K. Characteristic temperatures for molybdates are determined from the results of IR spectroscopic studies. The high-temperature enthalpy of Ln₂(MoO₄)₃ (Ln = Eu, Dy, and Ho) is measured via high-temperature microcalorimetry, and the temperature dependence of heat capacity is calculated in the range of 298–1000 K. Since samarium and gadolinium molybdates are of the same structural type as terbium molybdate, we can estimate the anomaly of the heat capacity in the low-temperature region using the data for terbium molybdate and find the entropy of samarium and gadolinium molybdates.     

Silver magnesium molybdate and silver cobalt molybdate: Synthesis and ionic conductivity

Impedance spectroscopy, X-ray powder diffraction, and electron microscopy are used to study silver magnesium and silver cobalt molybdates of composition Ag₂A ₂ ^II (MoO₄)₃ (A = Mg, Co) and the products of their aliovalent doping by scandium(III) and vanadium(V). The double molybdates have high ionic conductivities at temperatures above 600 K. A partial aliovalent substitution of scandium for magnesium or vanadium for molybdenum increases the ionic conductivity of the molybdates below 473 K. The defect mobility and the enthalpy of defect formation in the Ag₂Mg₂(MoO₄)₃ structure are estimated proceeding from the experimental data.     

The standard molar enthalpy of formation of Ce2(MoO4)3(s) and Sm2(MoO4)3(s) using solution calorimetry

The enthalpies of formations of Ce₂(MoO₄)₃(s) and Sm₂(MoO₄)₃(s) have been measured at 298.15 K using semi adiabatic solution calorimetry. The precipitation reaction between RE(NO₃)₃·6H₂O(s) (R= Ce, Sm) and ammonical solution of Na₂MoO₄(s) was studied. From the enthalpy of precipitation and other required auxiliary data, $ \Updelta_{\text{f}} H_{\text{m}}^{ \circ } \left( { 2 9 8. 1 5 {\text{ K}}} \right) $ Δ f H m ° ( 2 9 8.1 5 K ) of Ce₂(MoO₄)₃(s) and Sm₂(MoO₄)₃(s) have been calculated for the first time as ?4388.7 ± 3.6 and ?4363.4 ± 4.1 kJ mol^?1, respectively. The enthalpy of hydration of anhydrous Ce(NO₃)₃(s) to Ce(NO₃)₃·6H₂O(s) has been calculated. $ \Updelta_{\text{f}} H_{\text{m}}^{ \circ } \left( {{\text{MoO4}}^{ 2- } ,\,{\text{aq}},\, 2 9 8. 1 5 \,{\text{K}}} \right) $ Δ f H m ° ( MoO4 2 ? , aq , 2 9 8.1 5 K ) has also been measured and calculated as ?995.1 kJ mol^?1 from required literature data.     

Synthesis and Selected Physicochemical Properties of Complex Na<Subscript>2</Subscript>[Zr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The interaction of Zr(NO₃)₄ and Na₂MoO₄ in an aqueous medium has been studied by the method of residual concentrations at 20°C. The compound Nа₂[Zr(MoO₄)₃] is formed starting at the molar ratio Zr(NO₃)₄/Na₂MoO₄ ≥ 0.66. The compound has been characterized by X-ray diffraction, IR spectroscopy, and thermal analysis.     

Thermochemical properties of the Fe-analogue of cryolite, Na3FeF6

Relative enthalpies for low-and high-temperature modifications of Na₃FeF₆ and for the Na₃FeF₆ melt have been measured by drop calorimetry in the temperature range 723–1318 K. Enthalpy of modification transition at 920 K, δ_trans H(Na₃FeF₆, 920 K) = (19 ± 3) kJ mol⁻¹ and enthalpy of fusion at the temperature of fusion 1255 K, δ_fusH(Na₃FeF₆, 1255 K) = (89 ± 3) kJ mol⁻¹ have been determined from the experimental data. Following heat capacities were obtained for the crystalline phases and for the melt, respectively: C _p(Na₃FeF₆, cr, α) = (294 ± 14) J (mol K)⁻¹, for 723 = T/K ≤ 920, C _p(Na₃FeF₆, cr, β) = (300 ± 11) J (mol K)⁻¹ for 920 ≤ T/K = 1233 and C _p(Na₃FeF₆, melt) = (275 ± 22) J (mol K)⁻¹ for 1258 ≤ T/K ≤ 1318. The obtained enthalpies indicate that melting of Na3FeF6 proceeds through a continuous series of temperature dependent equilibrium states, likely associated with the production of a solid solution.      

Solution and solvation thermochemistry of copper(II) and nickel(II) nitrates in water and acetonitrile mixtures at 298 K

The calorimetric enthalpies of mixing of aqueous solutions of Cu(NO₃)₂ and Ni(NO₃)₂ in mixtures of water with acetonitrile (AN) at 298 K are measured over the entire range of the compositions. The ionic enthalpies of transfer from pure water to water mixed with AN are calculated. The behavioral features of d cations, as distinct from simple ions, are recognized. The contributions from the universal, chemical, and electrostatic interactions between the ion and solvent to the enthalpy of ion transfer are calculated. The structural term of the enthalpy of ion transfer that reflects the energy (enthalpy) changes in the solution induced by the chemical interaction of an ion with the solvent is obtained; the regions of d-cation resolvation in the solution are recognized as a result.     

Synthesis and crystal structure of new complex sodium lanthanide phosphate molybdates Na2MIII(MoO4)(PO4)(MIII = Tb, Dy, Ho, Er, Tm, Lu)

New complex sodium lanthanide phosphate molybdates Na₂M^III(MoO₄)(PO₄)(M^III=Tb, Dy, Ho, Er, Tm, Lu) have been synthesized by the ceramic method (T = 600°C, τ = 48 h), and their unit cell parameters have been determined. The structures of Na₂M^III(MoO₄)(PO₄)(M^III = Dy, Ho, Er, Lu) were refined by the Rietveld method. The compounds are isostructural: they are orthorhombic (space group Ibca, Z = 8) and have layered structures. In the structures of phosphate molybdates, chains of M^IIIO₈ polyhedra and MoO₄ tetrahedra are linked by PO₄ tetrahedra to form layers. The MoO ₄ ^2? anions are involved in dipole-dipole interaction. The sodium ions are arranged in the interlayer space. The compounds melt incongruently at 850–870°C.     

Heat capacity and thermodynamic properties of the alkali metal compounds in the temperature range 300–800 K. I. Cesium and rubidium molybdates

The heat capacities of cesium and rubidium molybdates, Cs₂MoO₄ and Rb₂MoO₄, have been measured by differential scanning calorimetry (DSC) in the temperature range 300–800 K. These values have been combined with published low-temperature heat capacity data for Cs₂MoO₄ to obtain thermodynamic functions to 800 K. For Rb₂MoO₄, however, these functions could not be calculated because low-temperature heat capacities are unavailable. Instead, only heat capacity data are reported.     

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.     

Enthalpic analysis of the CaTiSiO5 system

The relative enthalpy of titanite and enthalpy of CaTiSiO₅ melt have been measured using drop calorimetry between 823 K and 1843 K. Enthalpies of solution of titanite and CaTiSiO₅ glass have been measured by the use of hydrofluoric acid solution calorimetry at 298 K. Enthalpy of vitrification at 298 K, δ_vitr H(298 K) = (80.7 ± 3.4) kJ mol⁻¹, and enthalpy of fusion at the temperature of fusion 1656 K, δ_fus H(1656 K) = (139 ± 3) kJ mol⁻¹, of titanite have been determined from experimental data. The obtained enthalpy of fusion is considerably higher than up to the present published values of this quantity.     

Phase formation in Li<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-K<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-MMoO<Subscript>4</Subscript> (M = Ca,Pb, Ba) systems and the crystal structure of α-KLiMoO<Subscript>4</Subscript>

Solid-phase interactions in Li₂MoO₄-K₂MoO₄-MMoO₄ (M = Ca, Pb, Ba) systems were studied, and the subsolidus regions of these systems were triangulated. The lead and barium systems were studied in a more detailed way to discover that, along KLiMoO₄-K₂M(MoO₄)₂ (M = Pb, Ba), KLiMoO₄-PbMoO₄, and Li₂MoO₄-K₂Ba(MoO₄)₂ quasi-binary sections, there are homogeneity regions reaching 6–11 mol % based on K₂M(MoO₄)₂ and lead molybdate. Triple molybdates are formed in none of the systems, which is verified by experiments on spontaneous crystallization from solution in melt. Crystallization experiments yielded crystals of potassium dimolybdate and simple and double molybdates from the boundary systems. The crystal structure was solved for a hexagonal KLiMoO₄ phase: (Na,K){ZnPO4}, a = 18.8838(7) Å, c = 8.9911(6)Å, Z = 24, space group P6₃, R = 0.065. The structure comprises a three-dimensional tridymite framework built by an alternation of corner-sharing LiO₄- and MoO₄ tetrahedra wherein voids are occupied by potassium cations.     

Phase formation in the Li2MoO4-A2MoO4-NiMoO4 (A = K, Rb, Cs) systems, the crystal structure of Cs2Ni2(MoO4)3, and color characteristics of alkali-metal nickel molybdates

The subsolidus regions of the Li₂MoO₄-A₂⁺MoO₄-NiMoO₄ (A⁺ = K, Rb, Cs) systems at 510°C have been triangulated by the intersecting-joins method. The A₂MoO₄-Li₂Ni₂(MoO₄)₃, Li₂MoO₄-A₂Ni₂(MoO₄)₃, A₂Ni₂(MoO₄)₃-Li₂Ni₂(MoO₄)₃ (A = K, Rb, Cs), and ALiMoO₄-A₂Ni₂(MoO₄)₃ (A = K, Rb) joins have been investigated. The subsolidus phase formation study has also been completed by spontaneous flux crystallization. No triple salts have been identified, but only compounds belonging to the boundary binary systems. The crystal structure of Cs₂Ni₂(MoO₄)₃ (a = 10.7538 ?, Z = 4, space group P2₁3, R = 0.0082) belonging to the langbeinite type has been determined. It is built of a three-dimensional framework of vertexsharing MoO₄ tetrahedra and NiO₆ octahedra and cesium ions occupying large out-of-framework cavities. All alkali-metal nickel molybdates are yellow. These compounds are usable as pigments, as judged from their reflection spectra and calculated color characteristics, namely, colorfulness (C), lightness (L), and hue (H).     

Standard enthalpy of formation of lanthanum oxybritholites

Lanthanum-bearing silicate-oxyapatites or britholites, Ca_10–xLax(PO₄)_6–x(SiO₄)_xO with 1≤x≤6, have been synthesized by solid state reaction at high temperature. They were characterized by X-ray diffraction and IR spectroscopy. Using two microcalorimeters, the heat of solution of these compounds have been measured at 298 K in a solution of nitric and hydrofluoric acid. A strained least squares method was applied to the experimental results to obtain the solution enthalpies at infinite dilution, and the mixing enthalpy in two steps. In the first step the mixing enthalpy obtained is referenced to the britholite monosubstituted and to the oxysilicate. The mixing enthalpy referenced to the oxyapatite and to the oxysilicate is then extrapolated. In order to determine the enthalpies of formation of all the terms of the solution, thermochemical cycles were proposed and complementary experiments were performed. The results obtained show a decrease of the enthalpy of formation with the amount of Si and La introduced in the lattice. This was explained by the difference in the bond energies of (Ca–O, P–O) and (La–O, Si–O).     

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.     

Isotope exchange of strontium and molybdate ions in strontium polymolybdates

The heterogeneous isotopic exchange reactions in strontium polymolybdates of Sr²⁺ and MoO₄ ^2- ions in the strontium nitrate and sodium molybdate solutions have been studied using ⁹⁰Sr and ⁹⁹Mo as tracers. Electrometric methods have been used to study the compositions of strontium molybdates obtained by adding strontium chloride to a progressively acidified solution of sodium molybdate. It has been found that the exchange fraction increases with increasing chain length of strontium polymolybdate. The exchange equilibrium constant (K _ex) has been calculated between 298 and 348 K as well as DG°, DH° and DS°. The results indicate that Sr²⁺ cations have a much higher affinity for exchangers than MoO₄ ^2- anions. By fitting the data to the Dubinin-Radushkevich (D-R) isotherm it has been shown that the exchange capacity (X _m ) for both ions is affected by the ion adsorption process at low temperatures and by the ion exchange process at high temperatures. At high concentrations, the recrystallization process contributes to on the cation exchange but is ineffective on the anion exchange mechanism.     

Thermochemical study of gaseous salts of oxygen-containing acids: XXII. Tin molybdates

Gas-phase reactions involving tin molybdates were studied. Standard enthalpies of formation and atomization of gaseous salts SnMoO₄ and Sn₂MoO₅ were determined.     

The luminescence of the scheelite NaBi(MoO4)2

The luminescent properties of the scheelite NaBi(MoO₄)₂ are reported. Below 50 K an efficient, red emission occurs. The results agree with those for other bismuth molybdates. The decay measurements indicate an energy level structure of the emitting state which is very similar to that in PbMoO₄. The distribution of the Na⁺ and Bi³⁺ ions is probably short-range ordered.     

Phase Relations and Electrical Properties of Phases in Systems Na2MoO4-AMoO4-R2(MoO4)3 (A = Mg, Mn, Co, Ni; R = Cr, Fe)

The subsolidus region of ternary salt systems Na₂MoO₄-AMoO₄-R₂(MoO₄)₃ was studied. It was established that phases Na_1-x A_1-x R_1+x (MoO₄)₃, 0 x 0.2-0.3 of variable composition with rhombohedral lattice (R-3c) and ternary molybdates NaA₃R(MoO₄)₅ of triclinic crystal system (P-1) are formed in the system. The electrical characteristics of ternary molybdates and the effect of the cation nature on the homogeneity region of the phases and type of conduction were studied.     

Low-temperature heat capacity and high-temperature thermal behavior of sodium lutetium molybdate phosphate Na<Subscript>2</Subscript>Lu(MoO<Subscript>4</Subscript>)(PO<Subscript>4</Subscript>)

The low-temperature heat capacity of Na₂Lu (MoO₄)(PO₄) was measured by adiabatic calorimetry in the range of 7.47–345.74 K. The experimental data were used to calculate the thermodynamic functions of Na₂Lu (MoO₄)(PO₄). At 298.15 K, the following values were obtained: C _p ⁰ (298.15 K) = 237.7 ± 0.1 J/(K mol), S ⁰(298.15 K) = 278.1 ± 0.8 J/(K mol), H ⁰(298.15 K) ? H ⁰ (0 K) = 42330 ± 20 J/mol, and Φ⁰(298.15 K) = 136.1 ± 0. 3 J/(K mol). A heat capacity anomaly was found in the range of 10-67 K with a maximum at T _tr = 39.18 K. The entropy and enthalpy of transition are ΔS = 12.39 ± 0.75 J/(K mol) and ΔH = 403 ± 16 J/mol. The thermal investigation of sodium lutetium molybdate phosphate in the high-temperature range (623–1223 K) was performed using differential scanning calorimetry. It was found that during melting in the range of 1030–1200 K, Na₂Lu(MoO₄)(PO₄) degrades to simpler compounds; the degradation scenario is verified by X-ray powder diffraction.     

Electrical Properties of Molybdates in the Systems M2MoO4-AMoO4-Zr(MoO4)2

The temperature dependence of conductivity of molybdates in the systems M₂MoO₄-AMoO₄-Zr(MoO₄)₂ was studied. The starting molybdates and molybdates of 5 : 1 : 3 and 1 : 1 : 1 compositions, formed in the systems, exhibit mixed conduction, turning ionic at high temperatures. The sharp bends in linear portions of the temperature dependence of conductivity coincide with the temperatures of polymorphic transitions in the corresponding molybdates.