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.     

Systems R-Fe-O (R=Ho, Er): Thermodynamic properties of ternary oxides using differential scanning calorimetry and solid-state electrochemical cells

The thermodynamic properties of three different types of ternary oxides RFeO₃(s), R₃Fe₅O₁₂(s) and RFe₂O₄(s) (where R=Ho and Er) have been determined by calorimetric and solid-state galvanic cell methods. Heat capacities of RFeO₃(s) and R₃Fe₅O₁₂(s) have been determined by differential scanning calorimetry from 130 to 860 K. Heat capacity measurements from 130 to 860 K revealed λ-type anomalies for RFeO₃(s) and R₃Fe₅O₁₂(s) compounds which are assigned due to magnetic order-disorder transitions. The oxygen chemical potentials corresponding to the three-phase equilibria involving these ternary oxides have been determined by using solid-state electrochemical cells. The standard molar Gibbs energies of formation of RFeO₃(s), R₃Fe₅O₁₂(s) and RFe₂O₄(s) have been computed from the oxygen potential data. Based on the thermodynamic information, oxygen potential diagrams have been computed for the systems R-Fe-O (R=Ho and Er) at two different temperatures: T=1250 and 1450 K. Thermodynamic functions like , , H^o, G^o, , , , , and have been generated for the compounds RFeO₃(s) and R₃Fe₅O₁₂(s) based on the experimental data obtained in this study and the available data in the literature.     

Synthesis, characterization and standard molar enthalpy of formation of La(C7H5O3)2·(C9H6NO)

The product from reaction of lanthanum chloride seven-hydrate with salicylic acid and 8-hydroxyquinoline, La(C₇H₅O₃)₂·(C₉H₆NO), was characterized by IR, elemental analysis, molar conductance, and thermogravimetric analysis. The standard molar enthalpies of solution of [LaCl₃·7H₂O (s)], [2C₇H₆O₃ (s)], [C₉H₇NO (s)] and [La(C₇H₅O₃)₂·(C₉H₆NO) (s)] in a mixed solvent of absolute ethyl alcohol, dimethyl formamide (DMF) and perchloric acid were determined by calorimetry to be [LaCl₃·7H₂O (s), 298.15 K] = −96.45 ± 0.18 kJ mol⁻¹, [2C₇H₆O₃ (s), 298.15 K] = 14.99 ± 0.17 kJ mol⁻¹, [C₉H₇NO (s), 298.15 K] = −3.86 ± 0.06 kJ mol⁻¹ and [La(C₇H₅O₃)₂·(C₉H₆NO) (s), 298.15 K] = −117.78 ± 0.11 kJ mol⁻¹. The enthalpy change of the reaction

(1)     

Thermoanalytical study of linkage isomerism in coordination compounds: Part I. Reinvestigation of thermodynamic and thermokinetic of solid state interconversion of nitrito (ONO) and nitro (NO2) isomers of pentaaminecobalt(III) chloride by means of DSC

Solid state thermal isomerization of [Co(NH₃)₅(ONO)]Cl₂ (nitrito isomer) to [Co(NH₃)₅(NO₂)]Cl₂ (nitro isomer) and reverse reaction were investigated by non-isothermal differential scanning calorimetry (DSC) and found to be essentially an equilibrium process. The interconversions are accelerated at above 65 °C and reach to equilibrium state at about 155 °C. After establishment of the equilibrium the relative amounts of two isomers at any temperature are governed by Gibbs free energy relationship. The experimental enthalpy changes of isomerization of pure nitrito and nitro solid samples to the equilibrium state are −4.67 (±0.19) and 0.99 (±0.05) kJ mol⁻¹, respectively. From these values, total enthalpy change was calculated as: ΔH°=−5.66(±0.20) kJ mol⁻¹. Using Gibbs free energy relationship, equilibrium constant, total free energy and entropy changes were estimated at 60 °C as: K=7.72(±0.8),  kJ mol⁻¹ and  J K⁻¹ mol⁻¹.An initial rate method has been developed to determine the kinetic parameters of these reactions from non-isothermal DSC data. Both nitro to nitrito and reverse reactions obey first order kinetic law in solid state. Estimated activation parameters of forward and reverse paths at 60 °C are , , and , respectively. The negative activation entropy of both directions support the intramolecular mechanism of isomerization, including formation of a seven coordinate transition state, which formerly suggested based on spectral and X-ray methods.     

CaB3—A new calcium boride stabilized by thin film epitaxy

Codeposition of Ca and B on various single crystal substrates was carried out by MBE technique. A new calcium boride CaB₃ was epitaxially grown on Al₂O₃(0001) with substrate temperature . Structural characterization by RHEED and X-ray diffraction indicated that CaB₃ has a hexagonal cell with lattice parameters and . No evidence for superconductivity was found down to 2 K.     

X-ray powder diffraction studies and thermal behaviour of NaK2B9O15, Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15

The crystal structures of NaK₂B₉O₁₅ (, , , β=94.080(1)°, R_p=0.047, R_wp=0.059, R_B=0.026), Na(Na_.17K_.83)₂B₉O₁₅ (, , , β=94.228(2)°, R_p=0.053, R_wp=0.068, R_B=0.026), and (Na_.80K_.20)K₂B₉O₁₅ (, , , β=94.071(1)°, Z=4, R_p=0.041, R_wp=0.052, R_B=0.023) were refined in the monoclinic space groups P2₁/c(Z=4) using X-ray powder diffraction data and the Rietveld method. These nonaborates are isostructural to K₃B₉O₁₅. Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The alkali metal cations are located on three different sites in the voids of the framework. High-temperature X-ray diffraction studies show that NaK₂B₉O₁₅ decomposes at about 700 °C in accordance with the peritectic reaction NaK₂B₉O₁₅↔K₅B₁₉O₃₁+liquid. The thermal expansion of NaK₂B₉O₁₅ and Na(Na_.17K_.83)₂B₉O₁₅ is highly anisotropic. A similarity of the thermal and compositional (Na-K substitution) deformations of NaK₂B₉O₁₅ is revealed: heating of NaK₂B₉O₁₅ by 1 °C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na_1−xK_x)₃B₉O₁₅ by 0.04 at% K.     

Polycationic disorder in [Bi6O4(OH)4](NO3)6: Structure determination using synchrotron radiation and microcrystal X-ray diffraction

The bismuth basic nitrate [Bi₆O₄(OH)₄](NO₃)₆ crystallizes in a rhombohedral hexagonal unit cell with parameters , , , Z=6, space group R-3. The synthesis, formula determination, thermogravimetric analysis and nitrate assay, and finally, its crystal structure refinement determined at 150(2) K by synchrotron X-ray microcrystal diffraction are reported. Its structure is built from [Bi₆O₄(OH)₄]⁶⁺ polycations, six per unit cell, disordered over two positions. Two oxygen atoms are common to the two antagonist polycations (full occupancy) while the remaining six are partially occupied. The [Bi₆O₄(OH)₄]⁶⁺ hexanuclear clusters form columns along the c-axis. The cohesion between polycationic entities is effected by nitrate anions through either OH-ONO₂ hydrogen bonds or Bi-ONO₂ bonds. One of the two independent [NO₃]⁻ groups is also disordered over two positions. Only a local order in the columns is obtained by formation of pairs of ordered [Bi₆O₄(OH)₄]⁶⁺ polycations.     

Thermodynamic evidence for phase transition in MoO2−δ

The standard Gibbs free energy of formation of MoO_2−δ, Δ_fG^°(MoO_2−δ), has been measured over a wide temperature range (925 to 1925) K using an advanced version of bi-electrolyte solid-state electrochemical cell incorporating a buffer electrode:

Pt∣Mo + MoO_2−δ∥(Y₂O₃)ThO₂∥(CaO)ZrO₂∥O₂(0.1 MPa)∣Pt     

The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3-MO-Nb2O5 (M=Mg, Ni) systems

The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-Nb₂O₅ (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B₂O₆ octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A₂ tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (, , ) to 96 h (, , ) while the O′ oxygen was shifted from position 8b (, , ) to Wyckoff position 32e (, , ). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively.     

An open-framework three-dimensional indium oxalate: [In(OH)(C2O4)(H2O)]3·H2O

By hydrothermal reaction of In₂O₃ with H₂C₂O₄·2H₂O in the presence of H₃BO₃ at 155 °C, an open-framework three-dimensional indium oxalate of formula [In(OH)(C₂O₄)(H₂O)]₃·H₂O (1) has been obtained. The compound crystallizes in the trigonal system, space group R3c with , , , Z=6, R₁=0.0352 at 298 K. The small pores in 1 are filled with water molecules. It loses its filled water at about 180 °C without the change of structure, then the bounded water at 260 °C, and completely decompounds at 324 °C. The residue is confirmed to be In₂O₃.     

A Raman spectroscopic study of the phase transition of BaZr(PO4)2: Evidence for a trigonal structure of the high-temperature polymorph

We have studied the structural evolution of monoclinic BaZr(PO₄)₂ during heating up to 835 K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730 K during heating while upon cooling the reverse transition occurs at 705 K. However, some disagreement about the crystal structure of the high-temperature polymorph occurs in the literature. While the space group has not yet been determined, the X-ray diffraction pattern of the high-temperature phase has been indexed on either an orthorhombic or a hexagonal unit cell. We found that the number of Raman active internal PO₄ vibrational modes decrease from nine to six during the transition. A group theoretical survey through all orthorhombic, trigonal, and hexagonal factor groups revealed that the observed number of vibrations would only be consistent with the Ba and Zr atoms located at a site, the P and two O atoms at a C_3v(3m), and six O atoms at a C_s(m) site in the D_3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either , , or .     

Thermodynamic studies on lithium ferrites

Thermodynamic studies on ternary oxides of Li-Fe-O systems were carried out using differential scanning calorimetry, Knudsen effusion mass spectrometry, and solid-state electrochemical technique based on fluoride electrolyte. Heat capacities of LiFe₅O₈(s) and LiFeO₂(s) were determined in the temperature range 127-861 K using differential scanning calorimetry. Gibbs energies of formation of LiFe₅O₈(s) and LiFeO₂(s) were determined using Knudsen effusion mass spectrometry and solid-state galvanic cell technique. The combined least squares fits can be represented as

Δ_fG_m^o(LiFe₅O₈,s,T)/kJ mol⁻¹ (±6)=−2341+0.6764(T/K) (588≤T/K≤971)     

Rh-complexes of ditopic bis(pyrazol-1-yl)borate ligands: Assessment of their catalytic activity towards phenylacetylene polymerization

Two dinuclear Rh^I-cyclooctadiene complexes [1,4-(cod)Rh(B(R’)pz₂)-C₆H₄-(B(R’)pz₂)Rh(cod)], linked by a ditopic scorpionate ligand, have been prepared and fully characterized (R′ = Ph (2), C₆F₅ (2^F); pz = pyrazolide). Both compounds were tested as catalysts for phenylacetylene polymerization but showed no catalytic activity. Attempts at the synthesis of corresponding complexes of the sterically more demanding ligands (R′ = Ph (4), C₆F₅ (4^F); pz^Ph = 3-phenylpyrazolide) resulted in B-N bond cleavage and formation of the dinuclear complex [(cod)Rh(μ-pz^Ph)₂Rh(cod)] (5). Complex 5 proved to be an efficient catalyst for the preparation of highly stereoregular head-to-tail cis-transoidal poly(phenylacetylene).     

2,2′-Bis((di-tert-butylphosphino)methyl)-1,1′-biphenyl (ditbi): a bulky analogue of bisbi. The crystal structure of [Rh2Cl2(1,5-cod)2(μ-ditbi)]

Diphosphine 2,2′-bis(di-tert-butylphosphino)methyl)-1,1′-biphenyl (ditbi) is synthesised by the addition of to 2,2′-bis(bromomethyl)-1,1′-biphenyl, followed by deprotection with diethylamine. Treatment of [Rh₂Cl₂(1,5-cod)₂], with ditbi gives [Rh₂Cl₂(1,5-cod)₂(μ-ditbi)] (2) as confirmed by its X-ray crystal structure determination. Hydroformylation of 1-hexene using [Rh(acac)(CO)₂]/ditbi as catalyst gave n- and iso-heptanal in a ratio of 1:1.     

Three-dimensional framework of uranium-centered polyhedra with non-intersecting channels in the uranyl oxy-vanadates A2(UO2)3(VO4)2O (A=Li, Na)

The uranyl vanadates A₂(UO₂)₃(VO₄)₂O (A=Li, Na) have been synthesized by solid-state reaction and the structure of the Li compound was solved from single-crystal X-ray diffraction. The crystal structure is built from chains of edge-shared U(2)O₇ pentagonal bipyramids alternatively parallel to - and -axis and further connected together to form a three-dimensional (3-D) arrangement. The perpendicular chains are hung on both sides of a sheet parallel to (001), formed by U(1)O₆ square bipyramids connected by VO₄ tetrahedra, and derived from the autunite-type sheet. The resulting 3-D framework creates non-intersecting channels running down the - and -axis formed by empty face-shared oxygen octahedra, the Li⁺ ions are displaced from the center of the channels and occupy the middle of one edge of the common face. The peculiar position of the Li⁺ ion together with the full occupancy explain the low conductivity of Li₂(UO₂)₃(VO₄)₂O compared with that of Na(UO₂)₄(VO₄)₃ containing the same type of channels half occupied by Na⁺ ions in the octahedral sites.Crystallographic data for Li₂(UO₂)₃(VO₄)₂O: tetragonal, space group I41/amd, , , , Z=4, ρ_mes=5.32(2) g/cm³, ρ_cal=5.36(3) g/cm³, full-matrix least-squares refinement basis on F² yielded, R₁=0.032, wR₂=0.085 for 37 refined parameters with 364 independent reflections with I?2σ(I).     

Syntheses and crystal structures of two new pentaborates

Two new pentaborates, [Zn(DIEN)₂][B₅O₆(OH)₄]₂ (DIEN=diethylenetriamine) (I) and [B₅O₇(OH)₃Zn(TREN)] (TREN=tris(2-aminoethyl)amine) (II), have been hydrothermally synthesized and characterized by single crystal X-ray diffraction, FTIR, elemental analysis and thermogravimetric analysis. Compound I crystallizes in the monoclinic system, space group P2₁/c (No. 14), , , , β=91.259(2)°, , Z=2. The structure consists of isolated borate polyanion [B₅O₆(OH)₄]⁻ and zinc complex cation [Zn(DIEN)₂]²⁺. The anionic units, [B₅O₆(OH)₄]⁻, are linked by hydrogen bonds to form a 3D supramolecular network containing large channels, in which the templating [Zn(DIEN)₂]²⁺ cation are located. II is monoclinic, P2₁/c (No. 14), , , , β=99.635(2)°, , Z=4. The structure of II is constructed from two distinct motifs, a usual [B₅O₇(OH)₃]²⁻ cluster and a supporting zinc complex [Zn(TREN)]²⁺, which are integrated through Zn-O-B linkage. This compound represents the first example of the combination of B-O cluster with transition-metal complex.     

Synthesis and crystal structures of the layered uranyl tellurites A2[(UO2)3(TeO3)2O2] (A=K, Rb, Cs)

The reactions of UO₃ and TeO₃ with KCl, RbCl, or CsCl at 800 °C for 5 d yield single crystals of A₂[(UO₂)₃(TeO₃)₂O₂] (A=K (1), Rb (2), and Cs (3)). These compounds are isostructural with one another, and their structures consist of two-dimensional sheets arranged in a stair-like topology separated by alkali metal cations. These sheets are comprised of zigzagging uranium(VI) oxide chains bridged by corner-sharing trigonal pyramidal TeO₃²⁻ anions. The chains are composed of dimeric, edge-sharing, pentagonal bipyramidal UO₇ moieties joined by edge-sharing tetragonal bipyramidal UO₆ units. The lone-pair of electrons from the TeO₃ groups are oriented in opposite directions with respect to one another on each side of the sheets rendering each individual sheet non-polar. The alkali metal cations form contacts with nearby tellurite oxygen atoms as well as with oxygen atoms from the uranyl moieties. Crystallographic data (193 K, MoKα, ): 1, triclinic, space group , , , , α=101.852(1)°, β=102.974(1)°, γ=100.081(1)°, , Z=2, R(F)=2.70% for 98 parameters and 1697 reflections with I>2σ(I); 2, triclinic, space group , , , , α=105.590(2)°, β=101.760(2)°, γ=99.456(2)°, , Z=2, R(F)=2.36% for 98 parameters and 1817 reflections with I>2σ(I); 3, triclinic, space group , , , , α=109.301(1)°, β=100.573(1)°, γ=99.504(1)°, , Z=2, R(F)=2.61% for 98 parameters and 1965 reflections with I>2σ(I).     

Vapour pressure and excess Gibbs free energy of the ternary system {x1CH3F + x2HCl + x3N2O} at temperature of 182.33 K

The equilibrium pressure of ternary mixtures of {x₁CH₃F + x₂HCl + x₃N₂O} covering the entire composition range has been measured at temperature of 182.33 K by the static method. The system exhibits a minimum pressure for the binary {x₁CH₃F + x₂HCl}. The molar excess Gibbs free energy has been calculated from the experimental equilibrium pressure. For the equimolar mixture . The (p, x, y) surface for the ternary system and the corresponding curves for the three constituent binary mixtures obtained from the Peng-Robinson equation of state are in agreement with the experimental data.