Thermal decomposition of ammonium fluoroferrates (NH4)xFeF2x (2≤x≤3)

Different ammonium fluoroferrates (NH₄)_xFeF_2x (2≤x≤3) have been investigated. The thermal decomposition of the compounds obtained can be interpreted by their identical crystal structures (cryolite type). The decomposition products of all ammonium fluoroferrates formed in initial stage are isostructural of NH₄FeF₄. The decomposition is accompanied by the partial reduction of Fe(III) to Fe(II) by ammonium isolated. The end product of the thermal decomposition is FeF₂ and FeF₃ mixture.     

Stabilization of ammonium dinitramide in the liquid phase

The kinetics of accumulation of the main products of thermal decomposition of ammonium dinitramide in the melt was investigated. The isotope composition of nitrogen-containing gases evolved by the decomposition of ¹⁵NH₄N(NO₂)₂ and NH₄ ¹⁵N(NO₂)₂ was found. Easily oxidized salts, amines, amides, iodides, and other compounds soluble in the melt interfere with the liquid-phase decomposition of ammonium dinitramide.     

Thermal behavior of glutamic acid and its sodium,lithium and ammonium salts

Glutamic acid (H2_glu) and its lithium, sodium and ammonium monosalts were submitted to thermal analysis using thermogravimetry (TG) and differential thermal analysis (DTA). The main goal of these studies was to compare the relative thermal stability and to evaluate the effect of the counter ion in the thermal decomposition pathways. Salts were obtained by direct neutralization of the purified acid with LiOH, NaOH or NH₄OH and were characterized by elemental analysis (C, H and N) and IR spectroscopy. Decomposition occurred after conversion to the pyroglutamic acid or the respective pyroglutamates and ammonium salt loosing NH₃ being converted to H₂glu before decomposition.     

Tricritical point in ferroelastic ammonium titanyl fluoride: NMR study

Ionic mobility and phase transitions in ammonium titanyl pentafluoride (NH₄)₃TiOF₅ were studied using the ¹⁹F and ¹H NMR data. The high-temperature phase (I) is characterized by spherically symmetric (isotropic) reorientation of [TiOF₅]³⁻ anions and by uniaxial reorientation of these anions in the ferroelastic phase II. A previously unknown second-order phase transition to the low-temperature modification (NH₄)₃TiOF₅(III) was found at 205 K. The transition is accompanied by hindering of uniaxial rotations of [TiOF₅]^3- anions and by noticeable change of ¹⁹F magnetic shielding tensor associated with the influence of pseudo-Jahn-Teller effect. A pressure-induced tricritical point with coordinates p_TCR≈2 kbar and T_TCR≈170 K is estimated on the base of ¹⁹F NMR chemical shift data, and previously studied p-T diagram of (NH₄)₃TiOF₅.     

Zur thermischen Zersetzung von Ammoniumfluorochromaten

On the Thermal Decomposition of Ammonium Fluoro Chromates The thermal decomposition of the ammonium fluoro chromates (NH₄)₂[CrF₅(H₂O)], (NH₄)₃CrF₆, and (NH₄)₂[Cr(H₂O)₆]F₅ has been investigated by thermoanalytical and X-ray investigations. Steps of decomposition are the formation of (NH₄)₃CrF₆ und NH₄CrF₄. Further the formation of a chromium fluoride ammoniate CrF₃ · xNH₃ (x ≥ 1) has been detected. The end product of decomposition is the rhomboedrical CrF₃. The hydrolysis reactions have been suppressed by using quasi-isobaric conditions. As by-reaction a partial reduction of CrF₃ has been detected leading to the formation of Cr₂F₅.     

Synthesis of NH<Subscript>4</Subscript>TiOF<Subscript>3</Subscript> Crystals in the Presence of Polyoxyethylene Ethers

The effect of polyoxyethylene ethers as surfactants on the morphology of NH₄TiOF₃ crystals obtained by hydrolysis of (NH₄)₂TiF₆ in the presence of boric acid is analyzed. Depending on the molecular weight of the polyoxyethylene ether, the geometric dimensions (thickness and lateral size) of the crystals change. The obtained samples were characterized by X-ray and electron diffraction, Raman spectroscopy, and scanning electron microscopy. The heat treatment of NH₄TiOF₃ powders gives anatase mesocrystals retaining the initial particle morphology.     

Phase transitions in perovskite-like oxyfluorides (NH4)3WO3F3 and (NH4)3TiOF5

Calorimetric and X-ray measurements have been performed on ammonium oxyfluorides (NH₄)₃WO₃F₃ and (NH₄)₃TiOF₅ from 120 up to 300 K. Two and one structural phase transitions were found for the former and latter compounds, respectively. In accordance with the entropy parameters both compounds undergo phase transitions of order–disorder type.     

Thermal decomposition of ammonium fluoromanganates(III)

The thermal decomposition of ammonium fluoromanganates (III) has been investigated in air and argon by simultaneous thermogravimetry and differential thermal analysis. Chemical analysis, X-ray powder data, and infrared spectra have been employed to characterise the intermediate and final products. The thermal decomposition can be described by the sequence (NH₄₃MnF₆ → (NH₄)₂MnF₅ → NH₄MnF₄ → MnF₂. Although penta- and tetra-fluoromanganates are well-defined compounds, the intermediate states could not be separated. In addition, a high temperature form of ammonium hexafluoromanganate has been observed.     

THERMAL REACTIONS IN SYNTHETIC APATITE–AMMONIUM SULFATE MIXTURE

The thermal reactions in the mixtures of hydroxylapatite or fluorapatite and (NH₄)₂SO₄up to 500°C were studied with the purpose of elaborating the conditions of obtaining calcium–ammonium cyclophosphate that could be used as fertilizer. Thermal analysis with a simultaneous FTIR analysis of the evolved gases as well as the analyses of chemical and phase composition of solid products were performed. The thermal changes in the mixtures could be divided into three steps: (1) decomposition of (NH₄)₂SO₄and reactions of apatite with these products at 250–420°C, (2) calcium ammonium polyphosphate formation at 290–450°C, and (3) reaction of CaSO₄with CaNH₄P₃O₉at 320–500°C. Higher concentrations of NH₃in the gas phase promote the formation of CaNH₄P₃O₉and increase its stability. Calcination at temperatures above 350°C causes decomposition of CaNH₄P₃O₉with a decrease in the content of water-soluble phosphorus and evolvement of SO₂.     

Investigation of oxofluorotitanates (NH4)3TiOF5 and Rb2KTiOF5 by vibrational spectroscopy and quantum-chemical methods

The phase transition effects in (NH₄)₃TiOF₅ and Rb₂KTiOF₅ were studied by vibrational spectroscopy over a wide range of temperatures. Changes in the spectra of these crystals were found for a PT in the region of the internal vibrations of the [TiOF₅]^3? ion. Quantum-chemical simulation of the vibrational spectra of (NH₄)₃TiOF₅ was performed, and band assignments were carried out. The structure of the compound and the mechanism of the structural phase transition were examined. It is shown that the vibrational spectra of the compounds at room temperature depend on the dynamic disordering of the complex anion.     

Thermal stability of ammonium chlorocobaltates(II)

The thermal destruction of ammonium penta-, tetra-, and trichlorocobaltates(II) was studied. Thermogravimetric analysis (TGA) data was used to calculate the thermal destruction activation energy, which was 8.6 kJ/mol for (NH₄)₃CoCl₅, 14.8 kJ/mol for (NH₄)₂CoCl₄, and 117.4 kJ/mol for NH₄CoCl₃. The decomposition enthalpy of 35.9 kJ/mol for (NH₄)₃CoCl₅, 12.1 kJ/mol for (NH₄)₂CoCl₄, and 166.3 kJ/mol for NH₄CoCl₃ was determined from differential scanning calorimetry (DSC) data.     

The thermal decomposition of ammonium heptamolybdate

The thermal decomposition of ammonium heptamolybdate has been investigated by TG. The decomposition is discussed, making use of additional information obtained from isothermal studies, X-ray and IR measurements. The formation of two new compounds, namely (NH₄)₂O · 14 MoO₃ and (NH₄)₂O · 22 MoO₃, prior to the formation of MoO₃ is detected, as well as the main intermediate compounds (NH₄)₂O · 2.5 MoO₃ or (NH₄)₂O · 3 MoO₃ (according to the water content of the starting material) and (NH₄)₂O · 4 MoO₃.     

Thermal decomposition study of selected isopolymolybdates

The thermal decomposition of ammonium trimolybdate (NH₄)₂Mo₃O₁₀·H₂O, anilinium trimolybdate (C₆NH₈)₂Mo₃O₁₀·4H₂O and anilinium pentamolybdate (C₆NH₈)₂Mo₅O₁₆ in air and nitrogen has been investigated. The decomposition of molybdates was studied in situ by powder X-ray diffraction. Moreover, results of TG, as well as scanning microscopy studies, are presented. It was found that during thermal treatment in air phases of MoO_x type are obtained, while thermal treatment in nitrogen leads to obtaining a mixture of Mo_yC_z and Mo_pN_q. It is worth noting that even though chemical decomposition and formation of new compounds took place, in some cases needle-like or plate-like shapes of crystallites were preserved during thermal treatment.     

Studies on the thermal decomposition kinetics and mechanism of ammonium niobium oxalate

Ammonium niobium oxalate was prepared and characterized by elemental analysis, XRD and FTIR spectroscopy analysis, which confirmed that the molecular formula of the complex is NH₄(NbO(C₂O₄)₂(H₂O)₂)(H₂O)₃. Dynamic TG analysis under air was used to investigate the thermal decomposition process of synthetic ammonium niobium oxalate. It shows that the thermal decomposition occurs in three stages and the corresponding apparent activation energies were calculated with the Ozawa–Flynn–Wall and the Friedman methods. The most probable kinetic models of the first two steps decomposition of the complex have been estimated by Coats–Redfern integral and the Achar–Bridly–Sharp differential methods.     

Evidence of quasi-intramolecular redox reactions during thermal decomposition of ammonium hydroxodisulfitoferriate(III), (NH4)2[Fe(OH)(SO3)2]·H2O

Synthesis of ammonium hydroxodisulfitoferriate(III), (diammonium catena-{bis(μ ²-sulfito-κO,κO)-μ ²-hydroxo-κ²O}ferrate(III) monohydrate) (NH₄)₂[Fe(OH)(SO₃)₂]·H₂O (compound 1) and its thermal behavior is reported. The compound is stable in air. Its thermal decomposition proceeds without the expected quasi-intramolecular oxidation of sulfite ion with ferric ions. The disproportionation reaction of the ammonium sulfite, formed from the evolved NH₃, SO₂ and H₂O in the main decomposition stage of 1, results in the formation of ammonium sulfate and ammonium sulfide. The ammonium sulfide is unstable at the decomposition temperature of 1 (150 °C) and transforms into NH₃ and H₂S which immediately forms elementary sulfur by reaction with SO₂. The formation and decomposition of other intermediate compounds like (NH₄)₂S_nO_x (n = 2, x = 3 and n = 3, x = 6) results in the same decomposition products (S, SO₂ and NH₃). Two basic iron sulfates, formed in different ratios during synthesizing experiments performed under N₂ or in the presence of air, have been detected as solid intermediates which contain ammonium ions. The final decomposition product was proved to be α-Fe₂O₃ (mineral name hematite).

     

Investigations on thermal stability of adduct aluminium nitrate(V)-urea (1/6)

The adduct of Al(NO₃)₃·6CO(NH₂)₂ has been prepared and characterized by means of chemical analysis, IR spectroscopy, X-ray patterns and microscopy. A thermoanalytical study of Al(NO₃)₃·6CO(NH₂)₂ as well as urea, for comparison purposes, under conventional dynamic and quasi-isothermal—quasi-isobaric conditions in air has been carried out. It has been found that the adduct is thermally stable up to about 200°C, i.e. up to higher temperature than the decomposition temperature of the constituent compounds. The thermal decomposition mechanism of the adduct is complex, thus infrared spectroscopy and X-ray diffraction techniques have been used to determine the intermediate products. Aluminium oxide(III) is the final decomposition product.     

Untersuchungen zur thermischen Zersetzung der Ammoniumyttriumhalogenide III. Ammoniumyttriumbromid, (NH4)3YBr6

Investigation on the Thermal Decomposition of Ammonium Yttrium Halides. III. Ammonium Yttrium Bromide, (NH₄)₃YBr₆ The decomposition equilibria of NH₄Br and (NH₄)₃YBr₆ were determined by total pressure measurements. It was shown by high temperature X-ray patterns and DTA that (NH₄)₃YBr₆ and YBr₃ show phase transitions in the measured temperature region. An endothermic transition of YBr₃ starts near 300°C and is very slow. By interpretation of the thermal decomposition of (NH₄)₃YBr₆, the enthalpy of formation, and the standard entropy was derived. In the system NH₄Br/YBr₃ only the described phase (NH₄)₃YBr₆ exists.     

Structure and thermal decomposition of ammonium metatungstate

The structure and morphology of ammonium metatungstate (AMT), (NH₄)₆[H₂W₁₂O₄₀]?4H₂O, and its thermal decomposition in air and nitrogen atmospheres were investigated by SEM, FTIR, XRD, and TG/DTA-MS. The cell parameters of the AMT sample were determined and refined with a full profile fit. The thermal decomposition of AMT involved several steps in inert atmosphere: (i) release of crystal water between 25 and 200 °C resulting in dehydrated AMT, (ii) formation of an amorphous phase between 200 and 380 °C, (iii) from which hexagonal WO₃ formed between 380 and 500 °C, and (iv) which then transformed into the more stable m-WO₃ between 500 and 600 °C. As a difference in air, the as-formed NH₃ ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of AMT was similar to ammonium paratungstate (APT), (NH₄)₁₀[H₂W₁₂O₄₂]?4H₂O, the only main difference being the lack of dry NH₃ evolution between 170 and 240 °C in the case of AMT.     

Tungsten Sulfides obtained by Decomposition of Ammonium Tetrathiotungstate

The decomposition of ammonium tetrathiotungstate, (NH₄)₂WS₄, and the nature of the resulting tungsten sulfides have been studied, mainly by X-ray diffraction, differential thermal analysis, optical microscopy and electron spin resonance. Non-stoichiometric tungsten trisulfide may be obtained by decomposition of (NH₄)₂WS₄ at about 200°C. At 330°C crystallization of tungsten disulfide starts. Characteristic electron spin resonance spectra have been obtained for both sulfides.     

Synthesis,open-framework structure and thermal behaviour of ammonium tin oxalate,Sn2(NH4)2(C2O4)3·3H2O

A new mixed ammonium tin oxalate trihydrate, Sn₂(NH₄)₂(C₂O₄)₃·3H₂O, has been prepared from evaporation of a solution of tin and ammonium oxalates. Its crystal structure has been solved from single-crystal diffraction data. The symmetry is orthorhombic, space group Pnma (No. 62), cell dimensions a=15.1821(5) Å, b=11.7506(2) Å, c=10.8342(3) Å, and Z=4. The structure consists of macroanionic layers built from [Sn(C₂O₄)₃]^2– groups. The SnO₆ polyhedron can be described as a pseudo pentagonal bipyramid, with the lone pair of electrons presumably occupying one apex. The resulting framework displays holes in which the water molecules and ammonium groups are located. The thermal behaviour of the mixed ammonium tin oxalate has been investigated with temperature-dependent X-ray powder diffraction and conventional thermal analysis. The degradation process has been completely explained, as well as that of oxammite, a phase always obtained in the preparations. The thermal decomposition of oxammite leads to (NH₄)₂C₂O₄ and a new acid salt, NH₄HC₂O₄. The mixed ammonium tin oxalate decomposes successively into the amorphous compounds, Sn₂(NH₄)₂(C₂O₄)₃·H₂O and Sn₂(NH₄)₂(C₂O₄)₃, SnC₂O₄ and, finally, cassiterite SnO₂.