Structural and thermal studies on the solid products in the system MnSeO<Subscript>3</Subscript>-SeO<Subscript>2</Subscript>-H<Subscript>2</Subscript>O

The solubility of MnSeO₃-SeO₂-H₂O system was studied in the temperature region 25–300°C. The compounds of the three-component system were identified by the Schreinemaker’s method. The phase diagram of manganese(II) selenites was drawn and the crystallization fields for the different phases were determined. Depending on the conditions for hydrothermal synthesis, MnSeO₃·H₂O, MnSeO₃·3/4H₂O, MnSeO₃·l/3H₂O and MnSe₂O₅ were obtained. The different phases were proven and characterized by chemical, powder X-ray diffraction and thermal analyses, as well as IR spectroscopy. The kinetics of dehydration and decomposition of MnSeO₃·H₂O was studied under non-isothermal heating. Based on 4 calculation procedures and 27 kinetic equations, the values of activation energy and pre-exponential factor in Arrhenius equation were calculated for both processes.     

Phase equilibria in the GeSe2–SnSe system

The phase diagram of the system GeSe₂–SnSe is studied by means of X-ray diffraction, differential thermal analysis and measurements of the density and the microhardness of the material. There are no intermediate compounds in it, as well as regions of range of solid solutions at room temperature on the base of GeSe₂ and SnSe. There are two non-variant equilibria in the system: eutectic (where T _e=530±5°C and x _e= 40 mol% SnSe) and metaeutectic (where T _m=550±5°C and x _m=98 mol% SnSe).     

Revised Phase Diagram of the System NaF–NaBF4

Summary. The phase diagram of the binary system NaF–NaBF₄ was determined using the thermal analysis method. Subsequent coupled analysis of the thermodynamic and phase diagram data was carried out to calculate the thermodynamically consistent phase diagram. The system NaF–NaBF₄ forms a simple eutectic phase diagram with the calculated coordinates of the eutectic point: 8.1 mol% NaF, 91.9 mol% NaBF₄, and 385.7°C. The probable inaccuracy in the calculated binary phase diagram is 9°C.     

Phase diagrams of the KF-K<Subscript>2</Subscript>TaF<Subscript>7</Subscript> and KF-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> systems

The phase diagrams of the systems KF-K₂TaF₇ and KF-Ta₂O₅ were determined using the thermal analysis method. The phase diagrams were described by suitable thermodynamic model. In the system KF-K₂TaF₇ eutectic points at x _KF=0.716 and t=725.4°C and at x _KF=0.214 and t=712.2°C has been calculated. It was suggested that K₂TaF₇ melts incongruently at around 743°C forming two immiscible liquids. The system KF-Ta₂O₅ have been measured up to 8 mol% of Ta₂O₅. The eutectic point was estimated to be at x _KF∼0.9 and t∼816°C. The formation of KTaO₃ and K₃TaO₂F₄ compounds has been observed in the solidified samples.     

Phase Diagram of the System NaF-KF-AlF3

The phase diagrams of the binary system KF-AlF₃ as well as the ternary system NaF-KF-AlF₃ in the range up to 50 mol% AlF₃, were measured using the thermal analysis method. In the system KF-AlF₃ the coordinates of the eutectic points are: E ₁: 8.0 mol% AlF₃, 821.2°C, and E ₂: 45.5 mol% AlF₃, 565.0°C. In the investigated concentration range of the ternary system 2 eutectic points have been found with the calculated coordinates: E ₁: 36.3 mol% NaF, 62.7 mol% KF, 1.0 mol% AlF₃; t=711.2°C; and E ₂: 51.9 mol% NaF, 27.4 mol% KF, 20.7 mol% AlF₃; t=734.5°C. Other eutectic points lie most probably beyond the investigated part of the system. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase Equilibria in the K,Cа∥SO<Subscript>4</Subscript>,CO<Subscript>3</Subscript>,HCO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O System at 25°С

Phase equilibria in the system K,Cа∥SO₄,CO₃,HCO₃–H₂O have been studied at 25°С. This system at 25°С involves 7 invariant points, 21 monovariant curves, and 22 divariant fields. The data gained served to plot the first phase diagram (phase complex) of the studied system at 25°С.     

Solid-state synthesis of undoped and Sr-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

The solid-state synthesis of undoped K_0.5Na_0.5NbO₃ (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 °C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 °C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 °C and finishes at 540 °C, and the second step has an onset at 540 °C and finishes with the end of the reaction between 630 and 675 °C, depending on the particle size distribution of the Nb₂O₅ precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 °C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 °C.     

Phase equilibria in the system CaO-CoO-Co<Subscript>2</Subscript>O<Subscript>3</Subscript>-MnO-MnO<Subscript>2</Subscript>

Phase equilibria in the system CaO-CoO-Co₂O₃-MnO-MnO₂ at 700–1200°C in air were studied on the basis of published data and proper experiments. No new compounds were found. The results are presented in the form of 13 isothermal sections of the phase diagram.     

Phase diagram study of CaBr2–LiBr system using DTA

The phase diagram of binary LiBr–CaBr₂ system was investigated using differential thermal analysis (DTA) between room temperature and 800 °C. From the DTA results obtained over the entire range of composition from pure LiBr to pure CaBr₂ in steps of ~5 mol%, the phase diagram was constructed and is reported here. The results indicated the possible existence of a compound at 50 mol% LiBr, namely, LiCaBr₃. The compound undergoes peritectic decomposition at 552 °C. The system shows a eutectic reaction at 532 °C between this compound and LiBr phase, and the eutectic composition is close to 80 mol% LiBr. The compound LiCaBr₃ decomposes into CaBr₂ and LiBr phases below 272 °C. Co-existing phases in different phase fields are characterized by X-ray diffraction analysis.     

Phase equilibria in the Na,K∥SO<Subscript>4</Subscript>, CO<Subscript>3</Subscript>, F-H<Subscript>2</Subscript>O system at 25°C

Phase equilibria in the Na, K∥SO₄, CO₃, F-H₂O system at 25°C are studied using the translation technique. Twenty four divariant fields, 22 univariant curves, and seven invariant points are found in the system. The complete phase diagram (the phase complex) of the system is designed on the basis of these data.     

Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> phase diagram

Phase relations in the Y₂O₃-Ga₂O₃ system were studied by the anneal-and-quench technique in air within 1000–2300°C, and a phase diagram was plotted. Three compounds were found to form: Y₃GaO₆, Y₄Ga₂O₉, and Y₃Ga₅O₁₂; the temperature and concentration bounds of stability were determined for these compounds. Indexing results for Y₃GaO₆ are given.     

Thermal behaviour and fragility of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>-containing zinc borophosphate glasses

Thermal behaviour of the glass series (100-x)[50ZnO-10B₂O₃-40P₂O₅]·xSb₂O₃ (x=0-42 mol%) and (100-y)[60ZnO-10B₂O₃-30P₂O₅]·ySb₂O₃ (y=0-28 mol%) was investigated by DSC and TMA. The addition of Sb₂O₃ results in a decrease of the glass transition temperature and crystallization temperature in both compositional series. All glasses crystallize on heating in the temperature range of 522–632°C. Thermal expansion coefficient of the glasses monotonously increases with increasing Sb₂O₃ content in both series and varies within the range of 6.6–11.7 ppm °C⁻¹. From changes of thermal capacity within the glass transition region it was concluded that with increasing Sb₂O₃ content the ‘fragility’ of the studied glasses increases.     

Zeit- und temperaturaufgelöste Röntgenpulverdiffraktometrie: Die Reaktion von (NH4)2SnF6 mit Ammoniak

Time and Temperature Resolved in situ X-Ray Powder Diffractometry. The Reaction of (NH₄)₂SnF₆ with Ammonia The thermal decomposition of (NH₄)₂SnF₆ under an atmosphere of ammonia is reported. The complicated reaction paths were illucidated by time and temperature resolved in situ x-ray powder diffractometry. It is shown that this technique is a powerful tool to observe structural changes during reaction. It offers also a valuable access to thermodynamic and kinetic data for solid state and gas phase reactions. (NH₄)₂SnF₆ decomposes under ammonia below room temperature to NH₄F and amorphous SnF₄ · x NH₃. At a temperature of 80°C an intermediate product, (NH₄)₄SnF₈, is formed, which decomposes at 140°C into (NH₄)₂SnF₆ and NH₄F. At 250°C (NH₄)[Sn(NH₃)F₅] and Sn(NH₃)₂F₄ are formed. The latter crystallises C-centered monoclinic with lattice constants a = 844.1(5) pm, b = 630.5(3) pm, c = 520.2(3) pm and b? = 114.02(7)°. At 330°C a further decomposition yields SnF₂(NH₂)₂ with a C-centered monoclinic cell and lattice constants a = 1 069(7), b = 325.3(2), c = 504.8(3) pm and b? = 105.83(7)°. Finally above 500°C tin metal is formed.     

Anion influence on the solution-solid phase equilibria in the MX<Subscript>2</Subscript>-NEt<Subscript>4</Subscript>X-H<Subscript>2</Subscript>O systems (M = Cd,Cu, Co; X = Cl,Br) at 25°C

Solubility of the ternary systems M-NEt₄Cl-H₂O (M = Cd, Cu, Co) at 25°C was determined by the method of isothermal saturation. Compositions and fields of crystallization of solid compounds in equilibrium with a liquid phase were determined. Changes in the processes of hydration, association of tetraethyl ammonium salts, and acido-complex formation of d-element ions on the replacement of a halide anion affect the solution-solid phase equilibria in the studied systems.     

The possibility of separating hydrogen fluoride from its gaseous mixture with silicon tetrafluoride

We report in this paper investigations of the conditions necessary to effect the selective absorption of hydrogen fluoride from its gaseous mixture with silicon tetrafluoride when that mixture is brought into contact with solid sodium fluoride. Thus, when an anhydrous HF-SiF₄ gas mixture is brought into contact with granular NaF at room temperature only hydrogen fluoride is absorbed (giving NaF·HF), while silicon tetrafluoride remains in the gaseous state. In this way it is possible to separate HF from SiF₄. The hydrogen fluoride may subsequently be regenerated by heating the sodium hydrogen fluoride at 350–400°¹. If, however, the gaseous mixture contains only small traces of water vapor then SiF₄ also reacts with NaF to give Na₂SiF₆ in addition to NaF·HF. Under these circumstances it is not possible to effect a separation.     

Electrical conductivity studies in the system Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript>-Li<Subscript>1.33</Subscript>Ti<Subscript>1.67</Subscript>O<Subscript>4</Subscript>

Electrical conductivity in the monoclinic Li₂TiO₃, cubic Li_1.33Ti_1.67O₄, and in their mixture has been studied by impedance spectroscopy in the temperature range 20–730 °C. Li₂TiO₃ shows low lithium ion conductivity, σ₃₀₀≈10^–6 S/cm at 300 °C, whereas Li_1.33Ti_1.67O₄ has 3×10^–8 at 20 °C and 3×10^–4 S/cm at 300 °C. Structural properties are used to discuss the observed conductivity features. The conductivity dependences on temperature in the coordinates of 1000/T versus log_e(σT) are not linear, as the conductivity mechanism changes. Extrinsic and intrinsic conductivity regions are observed. The change in the conductivity mechanism in Li₂TiO₃ at around 500–600 °C is observed and considered as an effect of the first-order phase transition, not reported before. Formation of solid solutions of Li_{2– x} Ti_{1+ x} O₃ above 900 °C significantly increases the conductivity. Irradiation by high-energy (5 MeV) electrons causes defects and the conductivity in Li₂TiO₃ increases exponentially. A dose of 144 MGy yields an increase in conductivity of about 100 times at room temperature. Electronic Publication     

Subsolidus phase diagram of binary system in the perovskite type layer compounds (<Emphasis Type="Italic">n</Emphasis>-C<Subscript>n</Subscript>H<Subscript>2n+1</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>MnCl<Subscript>4</Subscript>

The thermotropic phase transitions in the perovskite type layer compound (n-C₁₀H₂₁NH₃)₂MnCl₄ and (n-C₁₄H₂₉NH₃)₂MnCl₄ were synthesized and, at the same time, a series of their mixtures C₁₀Mn-C₁₄Mn were prepared. The experimental binary phase diagram of C₁₀Mn-C₁₄Mn was established by differential thermal analysis (DTA), IR and X-ray diffraction. In the phase diagram new material (n-C₁₀H₂₁NH₃)(n-C₁₄H₂₉NH₃)MnCl₄ and two eutectoid invariants were observed, two eutectic points temperatures are about 29.8 and 27.9°C. Contrasting other similar system, there are three noticeable solid solution ranges (α, β, γ) at the left and right boundary and middle of the phase diagram.     

Diagram of the PbF<Subscript>2</Subscript>–SnF<Subscript>2</Subscript> system

A phase diagram of the PbF₂–SnF₂ system has been studied by differential thermal analysis and X-ray powder diffraction. The system forms Pb_1–хSn_хF₂ (х ≤ 0.33) solid solution and three compounds. Pb₂SnF₆ decomposes in solid state by a peritectoid reaction at 350°С; Pb₃Sn₂F₁₀ and PbSnF₄ melt by peritectic reactions at 565 and 380°С, respectively. The eutectic coordinates are 180°С, 90 mol % SnF₂.     

Soft chemical synthesis of NaYF<Subscript>4</Subscript> nanopowders

Hydrated NaYF₄ powders dominated by the metastable high-temperature cubic phase were precipitated with a fivefold NaF excess from acid solutions of yttrium nitrate. Transformation into the stable hexagonal phase (a = 5.969(2), c = 3.503(1) Å) occurs under heating with an exotherm at ～400°C.     

Influence of heat treatment on the phase transition of ZrMo<Subscript>2</Subscript>O<Subscript>8</Subscript> and photocatalytic activity

ZrMo₂O₇(OH)₂·2H₂O was obtained from ZrOCl₂·2H₂O and Na₂MoO₄·2H₂O by a coprecipitation method. The phase and structural changes occurred during the heat-treatment of ZrMo₂O₇(OH)₂·2H₂O were investigated by XRD, IR and XPS analysis. The sequence of phase transformation can be divided into three stages: (1) transformation of ZrMo₂O₇(OH)₂·2H₂O to orthorhombic LT-ZrMo₂O₈ up to 300°C; (2) obtaining of mixture of both polymorphs of ZrMo₂O₈: cubic and trigonal at 400°C; (3) conversion to single trigonal (α) ZrMo₂O₈ above 450°C. The microstructure of the obtained trigonal (α) ZrMo₂O₈ was observed by scanning electron microscopy (SEM). The particle sizes were below 0.5 μm. The specific surface area was measured by modified BET method. The photocatalytic activity of the obtained trigonal (α) ZrMo₂O₈ powders was investigated by degradation of a model aqueous solution of Malachite Green (MG) upon UV-light irradiation.