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).     

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.     

Transition metal tetramolybdate dihydrates MMo4O13·2H2O (M=Co,Ni) having a novel pillared layer structure

Hydrothermal synthesis in the M/Mo/O (M=Co,Ni) system was investigated. Novel transition metal tetramolybdate dihydrates MMo₄O₁₃·2H₂O (M=Co,Ni), having an interesting pillared layer structure, were found. The molybdates crystallize in the triclinic system with space group P−1, Z=1 with unit cell parameters of a=5.525(3) Å, b=7.058(4) Å, c=7.551(5) Å, α=90.019(10)°, β=105.230(10)°, γ=90.286(10)° for CoMo₄O₁₃·2H₂O, and a=5.508(2) Å, b=7.017(3) Å, c=7.533(3) Å, α=90.152(6)°, β=105.216(6)°, γ=90.161(6)° for NiMo₄O₁₃·2H₂O The structure is composed of two-dimensional molybdenum-oxide (2D Mo-O) sheets pillared with CoO₆ octahedra. The 2D Mo-O sheet is made up of infinite straight ribbons built up by corner-sharing of four molybdenum octahedra (two MoO₆ and two MoO₅OH₂) sharing edges. These infinite ribbons are similar to the straight ones in triclinic-K₂Mo₄O₁₃ having 1D chain structure, but are linked one after another by corner-sharing to form a 2D sheet structure, like the twisted ribbons in BaMo₄O₁₃·2H₂O (or in orthorhombic-K₂Mo₄O₁₃) are.     

Synthesis of MnOOH nanorods by cluster growth route from [Mn12O12(RCOO)16(H2O)n] (R=CH3, C2H5). Rational conversion of MnOOH into Mn3O4 or MnO2 Nanorods

Single-crystalline nanorods of γ-MnOOH (manganite) phase with diameters of 120 nm and lengths of 1100 nm have been prepared using a new cluster growth route under low-temperature hydrothermal conditions starting from [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄]·2CH₃COOH·4H₂O or [Mn₁₂O₁₂(C₂H₅COO)₁₆(H₂O)₃]·4H₂O without any catalyst or template agents. The so-obtained nanorods were studied by X-ray diffraction (XRD), infrared (IR) spectroscopy, Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Their thermal conversion opens an access to Mn₃O₄ (hausmannite) and β-MnO₂ (pyrolusite) nanorods, respectively, under argon or air atmosphere. A coercive field of 12.4 kOe was obtained for the Mn₃O₄ nanorods.     

Subsolidus phase relations and crystal structures of the mixed-oxide phases in the In2O3-WO3 system

Subsolidus phase relationships in the In₂O₃-WO₃ system at 800-1400°C were investigated using X-ray diffraction. Two binary-oxide phases—In₆WO₁₂ and In₂(WO₄)₃—were found to be stable over the range 800-1200°C. Heating the binary-oxide phases above 1200°C resulted in the preferential volatilization of WO₃. Rietveld refinement was performed on three structures using X-ray diffraction data from nominally phase-pure In₆WO₁₂ at room temperature and from nominally phase-pure In₂(WO₄)₃ at 225°C and 310°C. The indium-rich phase, In₆WO₁₂, is rhombohedral, space group (rhombohedral), with Z=1, a=6.22390(4) Å, α=99.0338(2)° [hexagonal axes: a_H=9.48298(6) Å, c=8.94276(6) Å, a_H/c=0.9430(9)]. In₆WO₁₂ can be viewed as an anion-deficient fluorite structure in which 1/7 of the fluorite anion sites are vacant. Indium tungstate, In₂(WO₄)₃, undergoes a monoclinic-orthorhombic transition around 250°C. The high-temperature polymorph is orthorhombic, space group Pnca, with a=9.7126(5) Å, b=13.3824(7) Å, c=9.6141(5) Å, and Z=4. The low-temperature polymorph is monoclinic, space group P2₁/a, with a=16.406(2) Å, b=9.9663(1) Å, c=19.099(2) Å, β=125.411(2)°, and Z=8. The structures of the two In₂(WO₄)₃ polymorphs are similar, consisting of a network of corner sharing InO₆ octahedra and WO₄ tetrahedra.     

Magnetic properties of Mn2V2O7 single crystals

Magnetic properties of Mn₂V₂O₇ single crystals are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. A structural phase transition of the α-β forms is clearly observed at the temperature range of 200-250 K and an antiferromagnetic ordering with magnetic anisotropy is observed below 20 K. A spin-flop transition is observed with magnetic field applied along the [110] axis of β-Mn₂V₂O₇, of which corresponds to the [001] axis of α-Mn₂V₂O₇, suggesting that the spins of Mn²⁺ ions locate within honeycomb layers which point likely in the [110] direction of β-Mn₂V₂O₇ or the [001] axis of α-Mn₂V₂O₇. However, a rather small jump of magnetization at spin-flop transition suggests a possible partition of crystal to some domains through β-to-α transition on cooling or much complex spin structure in honeycomb lattice with some frustration.     

Energetics of formation of alkali and ammonium cobalt and zinc phosphate frameworks

Alkali and ammonium cobalt and zinc phosphates show extensive polymorphism. Thermal behavior, relative stabilities, and enthalpies of formation of KCoPO₄, RbCoPO₄, NH₄CoPO₄, and NH₄ZnPO₄ polymorphs are studied by differential scanning calorimetry, high-temperature oxide melt solution calorimetry, and acid solution calorimetry.α-KCoPO₄ and γ-KCoPO₄ are very similar in enthalpy. γ-KCoPO₄ slowly transforms to α-KCoPO₄ near 673 K. The high-temperature phase, β-KCoPO₄, is 5-7 kJ mol⁻¹ higher in enthalpy than α-KCoPO₄ and γ-KCoPO₄. HEX phases of NH₄CoPO₄ and NH₄ZnPO₄ are about 3 kJ mol⁻¹ lower in enthalpy than the corresponding ABW phases. There is a strong relationship between enthalpy of formation from oxides and acid-base interaction for cobalt and zinc phosphates and also for aluminosilicates with related frameworks. Cobalt and zinc phosphates exhibit similar trends in enthalpies of formation from oxides as aluminosilicates, but their enthalpies of formation from oxides are more exothermic because of their stronger acid-base interactions. Enthalpies of formation from ammonia and oxides of NH₄CoPO₄ and NH₄ZnPO₄ are similar, reflecting the similar basicity of CoO and ZnO.     

K2Mo4O13 phases prepared by hydrothermal synthesis

Hydrothermal synthesis in the K-Mo oxide system was investigated as a function of the pH of the reaction medium. Four compounds were formed, including two K₂Mo₄O₁₃ phases. One is a new low-temperature polymorph, which crystallizes in the orthorhombic, space group Pbca, with Z=8 and unit cell dimensions a=7.544(1) Å, b=15.394(2) Å, c=18.568(3) Å. The other is the known triclinic K₂Mo₄O₁₃, whose structure was re-determined from single crystal data; its cell parameters were determined as a=7.976(2) Å, b=8.345(2) Å, c=10.017(2) Å, α=107.104(3)°, β=102.885(3)°, γ=109.760(3)°, which are the standard settings of the crystal lattice. The orthorhombic phase converts endothermically into triclinic phase at ca. 730 K with a heat of transition of 8.31 kJ/mol.     

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.     

Nano size crystals of goethite, α-FeOOH: Synthesis and thermal transformation

An aqueous suspension of amorphous iron(III) hydroxide was kept at room temperature (298 K) for 23 years. During this period of time the pH of the liquid phase changed from 4.3 to 2.85, and nano size crystals of goethite, α-FeOOH crystallised from the amorphous iron(III) hydroxide. Transmission electron microscopy (TEM) investigations, Mössbauer spectra, and powder X-ray diffraction using Co Kα radiation showed that the only iron containing crystalline phase present in the recovered product was α-FeOOH. The size of these nano particles range from 10 to 100 nm measured by TEM. The thermal decomposition of α-FeOOH was investigated by time-resolved in situ synchrotron radiation powder X-ray diffraction and the data showed that the sample of α-FeOOH transformed to α-Fe₂O₃ in the temperature range 444-584 K. A quantitative phase analysis shows the increase in scattered X-ray intensity from α-Fe₂O₃ to follow the decrease of intensity from α-FeOOH in agreement with the topotactic phase transition.     

Ab Initio Structure Determination of a New Compound, β-SrGaBO4, from Powder X-Ray Diffraction Data

A new compound, β-SrGaBO₄, has been attained through solid phase transition from α-SrGaBO₄ at high temperatures. Its crystal structure has been determined from powder X-ray diffraction data by direct methods. The refinement was carried out using the Rietveld method and the final refinement converged with Rp=11.42 % and Rwp=15.16 %. It has an orthorhombic P2₁2₁2 space group with cell parameters a=15.3706(2) Å, b=8.9921(1) Å, c=5.9191(1) Å, and Z=8. The structure of β-SrGaBO₄ is built up from Ga₂BO₈ units formed by two GaO₄ tetrahedra and one BO₃ triangle, and Sr₂O₁₂ units formed by two SrO₇ groups. Tetrahedra [GaO₄] are linked by shared O(3) and O(7) atoms to form infinite chains along the c axis.     

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.     

Effects of partial anion substitution on the thermoelectric properties of silver(I) chalcogenide halides in the system Ag5Q2X with Q=Te, Se and S and X=Br and Cl

A selection of mixed conducting silver chalcogenide halides of the general formula Ag₅Q₂X with Q=sulfur, selenium and tellurium and X=chlorine and bromine has been investigated due to their thermoelectric properties. Recently, the ternary counterpart Ag₅Te₂Cl showed a defined d¹⁰-d¹⁰ interaction in the disordered cation substructure at elevated temperatures where Ag₅Te₂Cl is present in its high temperature α-phase. A significant drop of the thermal diffusivity has been observed during the β−α phase transition reducing the values from 0.12 close to 0.08 mm² s⁻¹. At the same transition the thermopower reacts on the increasing silver mobility and jumps towards less negative values.Thermal conductivities, thermopower and thermal diffusivity of selected compounds with various grades of anion substitution in Ag₅Q₂X were determined around the silver-order/disorder β−α phase transition. A formation of attractive interactions could be observed for selenium substituted phases while no effect was detected for bromide and sulfide samples. Depending on the grade and type of substitution the thermopower changes significantly at and after the β−α phase transition. Thermal conductivities are low reaching values around 0.2-0.3 W m⁻¹ K⁻¹ at 299 K. Partial anion exchange can substantially tune the thermoelectric properties in Ag₅Q₂X phases.     

Enzyme mimics of spinel-type CoxNi1−xFe2O4 magnetic nanomaterial for eletroctrocatalytic oxidation of hydrogen peroxide

A series of spinel-type Co_xNi_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) magnetic nanomaterials were solvothermally synthesized as enzyme mimics for the eletroctrocatalytic oxidation of H₂O₂. X-ray diffraction and scanning electron microscope were employed to characterize the composition, structure and morphology of the material. The electrochemical properties of spinel-type Co_xNi_1−xFe₂O₄ with different (Co/Ni) molar ratio toward H₂O₂ oxidation were investigated, and the results demonstrated that Co_0.5Ni_0.5Fe₂O₄ modified carbon paste electrode (Co_0.5Ni_0.5Fe₂O₄/CPE) possessed the best electrocatalytic activity for H₂O₂ oxidation. Under optimum conditions, the calibration curve for H₂O₂ determination on Co_0.5Ni_0.5Fe₂O₄/CPE was linear in a wide range of 1.0 × 10⁻⁸–1.0 × 10⁻³ M with low detection limit of 3.0 × 10⁻⁹ M (S/N = 3). The proposed Co_0.5Ni_0.5Fe₂O₄/CPE was also applied to the determination of H₂O₂ in commercial toothpastes with satisfactory results, indicating that Co_xNi_1−xFe₂O₄ is a promising hydrogen peroxidase mimics for the detection of H₂O₂.     

Calorimetric study and thermal analysis of [Gd4/3Y2/3(Gly)6(H2O)4](ClO4)6·5H2O and [ErY(Gly)6(H2O)4](ClO4)6·5H2O (Gly: glycin)

Two solid-state coordination compounds of rare earth metals with glycin, [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O and [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O were synthesized. The low-temperature heat capacities of the two coordination compounds were measured with an adiabatic calorimeter over the temperature range from 78 to 376 K. [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 342.90 K, while [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O melted at 328.79 K. The molar enthalpy and entropy of fusion for the two coordination compounds were determined to be 18.48 kJ mol⁻¹ and 53.9 J K⁻¹ mol⁻¹ for [Gd_4/3Y_2/3(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, 1.82 kJ mol⁻¹ and 5.5 J K⁻¹ mol⁻¹ for [ErY(Gly)₆(H₂O)₄](ClO₄)₆·5H₂O, respectively. Thermal decompositions of the two coordination compounds were studied through the thermogravimetry (TG). Possible mechanisms of the decompositions are discussed.     

Thermodynamic properties of hydrated sodium l-threonate Na(C4H7O5)·H2O(s)

Low-temperature heat capacities of the compound Na(C₄H₇O₅)·H₂O(s) have been measured with an automated adiabatic calorimeter. A solid-solid phase transition and dehydration occur at 290-318 K and 367-373 K, respectively. The enthalpy and entropy of the solid-solid transition are Δ_transH_m = (5.75 ± 0.01) kJ mol⁻¹ and Δ_transS_m = (18.47 ± 0.02) J K⁻¹ mol⁻¹. The enthalpy and entropy of the dehydration are Δ_dH_m = (15.35 ± 0.03) kJ mol⁻¹ and Δ_dS_m = (41.35 ± 0.08) J K⁻¹ mol⁻¹. Experimental values of heat capacities for the solids (I and II) and the solid-liquid mixture (III) have been fitted to polynomial equations.     

Synthesis and decarboxylation of N-acyl-α-triphenylphosphonio-α-amino acids: a new synthesis of α-(N-acylamino)alkyltriphenylphosphonium salts

4-Phosphoranylidene-5(4H)-oxazolones 1 undergo hydrolysis in THF in the presence of HBF₄ at room temperature to give N-acyl-α-triphenylphosphonioglycines 3 (R² = H) in very good yields. 4-Alkyl-4-triphenylphosphonio-5(4H)-oxazolones 2 react with water in CH₂Cl₂/THF solution without any acidic catalyst at 0-5 °C in a few days yielding N-acyl-α-triphenylphosphonio-α-amino acids 3 (R² = Me) or α-(N-acylamino)alkyltriphenylphosphonium salt 4 (R² = CH₂OMe). α-Triphenylphosphonio-α-amino acids 3, on heating up to 105-115 °C under reduced pressure (5 mmHg) or on treatment with diisopropylethylamine in CH₂Cl₂ at 20 °C undergo decarboxylation to give the corresponding α-(N-acylamino)alkyltriphenylphosphonium salts 4, usually in very good yields.     

A simple method of fabricating large-area α-MnO2 nanowires and nanorods

α-MnO₂ nanowires or nanorods have been selectively synthesized via the hydrothermal method in nitric acid condition. The α-MnO₂ nanowires hold with average diameter of 50 nm and lengths ranging between 10 and 40 μm, using MnSO₄·H₂O as manganese source; meanwhile, α-MnO₂ bifurcate nanorods with average diameter of 100 nm were obtained by adopting MnCO₃ as starting material. The morphology of α-MnO₂ bifurcate nanorods is the first one to be reported in this paper. X-ray powder diffraction (XRD), field scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the products. Experimental results indicate that the concentrated nitric acid plays a crucial role in the phase purity and morphologies of the products. The possible formation mechanism of α-MnO₂ nanowires and nanorods has been discussed.     

Crystal chemistry of M[PO2(OH)2]2·2H2O compounds (M=Mg, Mn, Fe, Co, Ni, Zn, Cd): Structural investigation of the Ni, Zn and Cd salts

The compounds M[PO₂(OH)₂]₂·2H₂O (M=Mg, Mn, Fe, Co, Ni, Zn, Cd) were prepared from super-saturated aqueous solutions at room temperature. Single-crystal X-ray structure investigations of members with M=Ni, Zn, Cd were performed at 295 and 120 K. The space-group symmetry is P2₁/n, Z=2. The unit-cell parameters are at 295/120 K for M=Ni: a=7.240(2)/7.202(2), b=9.794(2)/9.799(2), c=5.313(1)/5.285(1) Å, β=94.81(1)/94.38(1)°, V=375.4/371.9 Å³; M=Zn: a=7.263(2)/7.221(2), b=9.893(2)/9.899(3), c=5.328(1)/5.296(2) Å, β=94.79(1)/94.31(2)°, V=381.5/377.5 Å³; M=Cd: a=7.356(2)/7.319(2), b=10.416(2)/10.423(3), c=5.407(1)/5.371(2) Å, β=93.85(1)/93.30(2)°, V=413.4/409.1 Å³. Layers of corner-shared MO₆ octahedra and phosphate tetrahedra are linked by three of the four crystallographically different hydrogen bonds. The fourth hydrogen bond (located within the layer) is worth mentioning because of the short O_h?O bond distance of 2.57-2.61 Å at room temperature (2.56-2.57 Å at 120 K); only for M=Mg it is increased to 2.65 Å. Any marked temperature-dependent variation of the unit-cell dimension is observed only vertical to the layers. The analysis of the infrared (IR) spectroscopy data evidences that the internal PO₄ vibrations are insensitive to the size and the electronic configuration of the M²⁺ ions. The slight strengthening of the intra-molecular P-O bonds in the Mg salt is caused by the more ionic character of the Mg-O bonds. All IR spectra exhibit the characteristic “ABC trio” for acidic salts: 2900-3180 cm⁻¹ (A band), 2000-2450 cm⁻¹ (B band) and 1550-1750 cm⁻¹ (C band). Both the frequency and the intensity of the A band provide an evidence that the PO₂(OH)₂ groups in M[PO₂(OH)₂]₂·2H₂O compounds form weaker hydrogen bonds as compared with other acidic salts with comparable O?O bond distances of about 2.60 Å. The observed shift of the O-H stretching vibrations of the water molecule in the order M=Mg>Mn≈Fe≈Co>Ni>Zn≈Cd has been discussed with respect to the influence of both the character and the strength of M↔H₂O interactions.     

Thermal stability and crystal structure of β-Ba3YB3O9

A new compound, β-Ba₃YB₃O₉, has been attained through solid phase transition from α-Ba₃YB₃O₉ at high temperatures. Differential thermal analysis (DTA) revealed the phase transition at about 1120°C, the melting temperature at about 1253°C. Its crystal structure has been determined from powder X-ray diffraction data. The refinement was carried out using the Rietveld method and the final refinement converged with R_p=10.5% and R_wp=13.7%. This compound belongs to the hexagonal space group R-3, with lattice parameters a=13.0441(1) Å and c=9.5291(1) Å. There are 6 formulas per unit cell and 7 atoms in the asymmetric unit. The structure of β-Ba₃YB₃O₉ is built up from Ba(Y)O₈, BaO₆ and YB₆O₁₈ units formed by one YO₆ octahedron and six BO₃ triangles with shared O atoms.