Re-investigations of thermal decomposition of gadolinium sulfate octahydrate

The crystals of Gd₂(SO₄)₃·8H₂O were obtained by slow crystallization from a saturated solution previously received by dissolving Gd₂O₃ in an aqueous solution of sulfuric acid. The thermal behavior of this salt was studied using simultaneous DTA–TG technique under a nitrogen and air, in the temperature range of 298–1773 K. It was found that the dehydration of gadolinium sulfate octahydrate as well as thermal decomposition of anhydrous Gd₂(SO₄)₃ undergo in two steps. The existence of two polymorphic modification of anhydrous gadolinium sulfate has been confirmed. The new XRD data for high-temperature polymorph of Gd₂(SO₄)₃ were given. All intermediate products of dehydration and thermal decomposition were characterized by EPR method.     

Spectroscopic Investigation of the Europium(3+) Ion in a New ZnY4W3O16 Matrix

A new Zn and Eu tungstate was characterized by spectroscopic techniques. This tungstate, of the formula ZnEu₄W₃O₁₆, crystallized in the orthorhombic system and was synthesized by a solid‐state reaction. It melts incongruently at 1330°. The luminescent properties, including excitation and emission processes, luminescent dynamics, and local environments of the Eu³⁺ ions in ZnEu₄W₃O₁₆ and ZnY₄W₃O₁₆ : Eu³⁺ diluted phases (1, 5, and 10 mol‐% of Eu³⁺ ion) were studied basing on the f⁶‐intraconfigurational transitions in the 250–720 nm spectral range. The excitation spectra of this system (λ_em 615 and 470 nm) show broad bands with maxima at 265 and 315 nm related to the ligand‐to‐metal charge‐transfer (LMCT) states. The emission spectra under excitation at the O→W (265 nm) and O→Eu³⁺ (315 nm) LMCT states present the blue‐green emission bands. The emission of tungstate groups mainly originate from the charge‐transfer state of excited 2p orbitals of O^2? to the empty orbitals of the central W⁶⁺ ions. On the other hand, in the emission of the Eu³⁺ ions, both the charge transfer from O^2? to Eu³⁺ and the energy transfer from W⁶⁺ ions to Eu³⁺ are involved. The emission spectra under excitation at the ⁷F₀→⁵L₆ transition of the Eu³⁺ ion (394 nm) of ZnY₄W₃O₁₆ : Eu³⁺ diluted samples show narrow emission lines from the ⁵D₃, ⁵D₂, and ⁵D₁ emitting states. The effect of the active‐ion (Eu³⁺) concentration on the colorimetric characteristic of the emissions of the compound under investigation are presented.     

Atomic structure of defects in GaN:Mg grown with Ga polarity

The atomic structure of characteristic defects (Mg-rich hexagonal pyramids and truncated pyramids) in GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities within the defects have inside walls covered by GaN of reverse polarity. We propose that lateral overgrowth of the cavities restores matrix polarity on the defect base. From matrix to defect, exchange of Ga and N sublattices leads to a 0.6+/-0.2 A displacement of Ga sublattices. We observe a [1100]/3 shift from matrix AB stacking to BC stacking for the entire pyramid. Electron energy loss spectroscopy detected changes in N edge and presence of oxygen on the defect walls. Our results explain commonly observed decrease of acceptor concentration in heavily doped GaN:Mg.     

Diffuse reflectance spectra of iron(III) vanadates

The electronic spectra of solid iron(III) vanadates FeVO₄ and Fe₂V₄O₁₃ were investigated by the diffuse reflectance technique in the spectral range 12 500–50 000 cm⁻¹. The spectra of investigated vanadates contain 2–3 intensive CT bands in the UV region and two lowest energy d–d bands in the 12 000–22 000 cm⁻¹ range. The presence of the weak bands for FeVO₄ and Fe₂V₄O₁₃ at 16 500 cm⁻¹ and 20 500 cm⁻¹ points to the lattice deffects (oxygen deficiency and the presence of the V⁴⁺ ions) in the structure of investigated vanadates.     

New cadmium and rare-earth metal molybdato–tungstates with scheelite-type structure

A new group of cadmium and rare-earth metal molybdato–tungstates with the formula Cd_0.25 RE _0.50□_0.25(MoO₄)_0.25(WO₄)_0.75 (RE = Pr, Nd, Sm–Dy, □—vacancies in cation sublattice) were synthesized by a high-temperature solid-state reaction between RE ₂MoO₆ and CdWO₄ mixed at the molar ratio of 1:3. Powder X-ray diffraction measurements showed that obtained phases adopt the scheelite-type structure. The phases melt congruently in the temperature range of 1382–1458 K.     

The kinetics of CO2 gasification of coal chars

The gasification reactivities of three char samples derived from coals of varying ranks (“Turów” lignite, “Piast”, and “Wieczorek” sub-bituminous coals) toward CO₂ were investigated isothermally using thermogravimetric analysis. Kinetic behavior was studied at temperatures of 900, 950, and 1,000 °C under atmospheric pressure. Conditions for the chemical-controlled regime were established at these temperatures and pressure. In this paper, four kinetic models were applied to describe the varying conversion rate: volumetric model, grain model, modified volumetric model, and random pore model. From these models, only the random pore and the volumetric models positively corresponded to nearly the entire range of experimental results. Calculated values of activation energy for study samples were in the range of 180–250 kJ mol^?1, which is in accordance with other reported data. Moreover, the obtained results confirmed the significant impact of parent coal rank on its char reactivity, offering possibilities in the approximation of coal char kinetic behavior after further more detailed studies with a larger number of samples.     

Mechanism and kinetics of thermal decomposition of nickel(II) sulfate(VI) hexahydrate

This work presents results of research on thermal decomposition of nickel(II) sulfate(VI) hexahydrate in air and in helium atmosphere. On the base of TG and XRD results a mechanism of thermal decomposition of NiSO₄ hydrate was established. For calculations of kinetic parameters of the Arrhenius equation, the Coats-Redfern approximation was applied. Choice of g(a) function and thus of a mechanism best describing given stage of decomposition was performed by testing 12 kinetic models. This revised version was published online in July 2006 with corrections to the Cover Date.     

New praseodymium(III) and <Emphasis Type="Italic">d</Emphasis>-electron metals tungstates of the formula MPr<Subscript>2</Subscript>W<Subscript>2</Subscript>O<Subscript>10</Subscript> (<Emphasis Type="Italic">M</Emphasis>=Mn,Co, Cd)

Three new compounds MPr₂W₂O₁₀ (M=Mn, Co, Cd) were prepared by the solid-state reaction. They are isostructural and crystallize in the orthorhombic system. MPr₂W₂O₁₀ (M=Mn or Co) melt incongruently above 1150°C and the solid product of melting is Pr₂W₂O₉. The CdPr₂W₂O₉ compound starts decomposing in the solid-state at 1156°C to Pr₂W₂O₉ and CdO.