Radovan Dimitrijević

FTIR spectroscopy of framework aluminosilicate structures: carnegieite and pure sodium nepheline

Abstract In this work the spectroscopic studies of polymorph transformation of framework silicates containing six-membered rings and different Si/Al ratio were carried out. Two model systems with different stoichiometries (Na-LTA, Si/Al=1 and Na-FAU, Si/Al=1.23) were investigated. Thermally induced phase transformations of initial zeolites resulted in forming of stuffed derivatives of cristobalite (carnegieite) and tridymite (nepheline). Powder XRD method was used for the recognition of new phases. All obtained phases have framework structures built by single six-membered rings. The changes of middle range order (rings symmetry), which take place during transformations of Na-LTA and Na-FAU into low-temperature carnegieite, low-temperature carnegieite into pure sodium (ps) nepheline as well as ps-nepheline into high-temperature carnegieite, were investigated by IR spectroscopy. The rings symmetry is found to be dependent on a phase stoichiometry as well as on polymorph type.

Structure and proton conductivity of 12-tungstophosphoric acid doped silica

Abstract Sol–gel syntheses offer a wide range of possibilities for introduction of materials with specific properties (electronic, optic and ion-conductive, etc.) into optically clear matrices. Preparation of silica gel bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as its structural and conduction characteristics are reported. Characteristics of the obtained doped gels depend on the gel preparation, gelation process and the WPA content. According to the obtained results, especially for the high conductivity ( σ ∼0.1 S/cm), WPA doped silica gel is a promising material for solid electrolytes.

Thermally induced phase transformations of Ca-exchanged LTA and FAU zeolite frameworks: Rietveld refinement of the hexagonal CaAl2Si2O8 diphyllosilicate structure

Thermally induced phase transformations of Ca-exchanged LTA and FAU zeolites are followed in the range from room temperature to 1500°C. Both frameworks collapse into amorphous intermediate products after heating between 800 and 900°C. Prolonged heating of the intermediate product obtained from the Ca-LTA zeolite over 900°C induces recrystallization of CaAl 2 Si 2 O 8 phases (hexagonal CaAl 2 Si 2 O 8 phase and anorthite) into a mixture. The crystal structure of the hexagonal CaAl 2 Si 2 O 8 phase [ a 0 = 5.1154 (A), c 0 = 14.767(9) A] is refined by the Rietveld method from X-ray powder diffraction data to final agreement factors R p = 8.99%, R WP = 11.87%, and R B = 4.31%. The refined hexagonal Ca-diphyllosilicate phase is unstable in prolonged calcination, and at 1050°C it can be transformed polymorphously into an highly ordered anorthite LTA . Prolonged heating of the amorphous intermediate product obtained from Ca-FAU zeolite over 900°C results directly in formation of a less ordered anorthite FAU Ca 0.88 0.12 Al 1.77 Si 2.33 O 8.00 framework with vacant Ca 2+ sites.

The thermal transformation of Na LiA zeolites. A new polymorph in the system Li2OAl2O3SiO2

Abstract High-temperature phase transformations of A zeolite with various degrees of exchange of Na + with Li + ions were investigated. An increase in the number of Li + ions per unit cell accelerates the thermal transformation of the zeolite framework to the amorphous state. Above 730°C, four phases (carnegieite, nepheline, β-eucryptite, and a new phase—γ-eucryptite) were identified. Only γ- and β-eucryptite phases were obtained from pure LiA zeolite. γ-eucryptite is a new metastable polymorph in the system Li 2 OAl 2 O 3 SiO 2 . γ-eucryptite a 0 = 7.231(3)A, b 0 = 10.270(6) A, c 0 = 12.054(7) A) is transformed to β-eucryptite ( a 0 = 10.533(5) A, c 0 = 11.148(5) A) above 840°C.

Structural properties and proton conductivity of the 12-tungstophosphoric acid doped aluminosilicate gels

Abstract Preparation of aluminosilicate gels bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as their structural and conduction characteristics are reported. Doped gels are prepared from aluminum–silicon alkoxide AlSi(OR) x , iso-propanol, water and WPA·29H 2 O mixtures. Characteristics of the obtained doped gels depend on the WPA content. Results such as WPA-filled mesopores and infra red (IR) band shifts indicate differences with respect to material obtained by mechanical alloying and interaction between the WPA and aluminosilicate host framework. According to the obtained results, especially of the higher conductivity ( σ ∼10 −3 S/cm), WPA-doped aluminosilicate gel is a promising material for solid electrolytes.

Structural modifications of dodecatungstophosphoric acid hexahydrate induced by temperature in the 10–358 K range. In situ high-resolution neutron powder diffraction investigation

Abstract Dodecatungstophosphoric acid hexahydrate H 3 PW 12 O 40 ·6H 2 O crystal structure has been investigated by neutron powder diffraction (NPD) at different temperatures in the 10–358 K range. A nonconvergent reversible phase transition has been noticed at about 320 K. This transition is associated with a change in dynamic equilibrium of hydrate species and partial reduction/oxidation (redox) W 6+ ↔W 5+ . Expressive structure changes lie in the PO bonding inside Keggins anion and the H 5 O 2 + conformational angle.

Keggin’s ion structural modification and expansion of dodecatungstophosphoric acid hexahydrate induced by temperature treatment: In situ X-ray powder diffraction and raman investigations

Abstract The changes in lattice and molecular structure of the dodecatungstophosphoric acid hexahydrate (WPA-6) induced by the temperature treatment between 244 and 373 K, were studied in situ by X-ray powder diffraction and Raman spectroscopy. The increase in temperature was followed with the slight modifications of Keggin’s anion, expansion of the unit cell and an abrupt change in the crystallite microstructure of WPA-6 what was monitored by the Rietveld and Warren–Averbach methods. Those two discontinuities in the thermal expansion of 4 0 0 interplanar spacing, observed at about 320 and 340 K, are result of the non-convergent reversible phase transition, produced by partial reduction and oxidation of the tungsten, known as an ion with mixed valence. The extraordinary compression of P–Oa bond length [1.61(1) A at 298 K and 1.44(2) A at 373 K], broadening and splitting of ν1(P–Oa) bond with the increase in temperature, the abrupt change of crystallite dimension and the strain at 338 K, indicate a modification in the charge distribution followed with the partial reduction of W6+ into W5+ ions (i.e. ‘heteropoly blue’ formation). The partial reduction is closely related to the change in a dynamic equilibrium between different proton species and hydrated proton entities, such as: oxonium H3O+, dioxonium H5O2+, triaquaoxonium H7O3+, etc.

Structural investigations of celsian glass derived from Ba-LTA zeolite

The structure of stoichiometric celsian glass was investigated by 29Si MAS NMR and IR spectroscopies and reverse Monte Carlo (RMC) simulation of X-ray diffraction data. The glass was prepared by thermally induced phase transformation of Ba2+-exchanged LTA (Linde Type A) zeolite, under annealing conditions prior to hexacelsian crystallization. NMR and IR measurements have shown that the local silicon–aluminium ordering of the starting zeolite framework, as well as the framework fragments in the form of deformed tetrahedral rings, is retained in the glass structure. Interatomic distances and mean coordination numbers were calculated from the three-dimensional RMC derived structure model. The local Ba2+ environment in the glass was compared with the corresponding crystalline polymorphs. Ba–O distances in the glass are longer than the distances in hexacelsian but the coordination number 12.00 and the increase in the Ba–T distance, compared to zeolite, indicate the establishment of hexacelsian-like coordination. Inspection of the three-dimensional RMC model of the glass showed a layered structure, with Ba2+ cations between the layers.

High-temperature synchrotron powder diffraction investigation of thermal expansion, strain and microstructure for the co-elastic α ↔ β hexacelsian transition

Abstract High-temperature investigations of thermal expansion, strain and microstructure of the α ↔ β-hexacelsian reversible phase transition are reported. HexacelsianLTA and hexa-celsianFAU were synthesised from LTA and FAU zeolites, respectively. Only the former show discontinuities near the transition temperature, ∼ 580 K. The discontinuity in the thermal expansion near the transition temperature for the α ↔ β-hexacelsianLTA suggest a co-elastic character for the phase transition. A distortion of the co-ordination polyhedra and ordering of framework cations induce this phase transition. The absence of discontinuities for hexacelsianpFAU suggests non-existence of a phase transition. The similarities in the measured and calculated values for α-hexacelsianLTA and hexacelsianPAU suggest that hexacelsianFAU crystallise exclusively as the β-form.

Multiple hydriding/dehydriding of Zr1.02Ni0.98 alloy

Abstract The hydriding kinetics and hydrogen absorption capacity of Zr 1.02 Ni 0.98 alloy powder were investigated in a temperature range of 150-250C. An isothermal volumetric method within a pressure range of 0-1bar was used. As expected, the temperature increase causes both an increase of the hydriding rate and a shift in the equilibrium of the Zr 1.02 Ni 0.98 + x /2H 2 -Zr 1.02 N 0.98 H x reaction towards the left. Performing multiple hydriding/dehydriding cycles, an increase in hydriding rate accompanied by a decrease in maximal hydriding capacity was observed after each subsequent hydriding cycles of the same sample. Using X-ray diffractometry, this was shown to be a consequence of an irreversible destruction of the alloy.