M. Lucco-Borlera

High-quartz solid-solution phases from xerogels with composition 2MgO.2Al2O3.5SiO2 (μ-cordierite) and Li2O.Al2O3.nSiO2 (n = 2 to 4) (β-eucryptite): Characterization by XRD, FTIR and surface measurements

Abstract The title phases are prepared by the sol-gel route employing prehydrolyzed tetraethylorthosilicate (TEOS) and Al, Li or Mg nitrates in aqueous solution. Amorphous xerogels are obtained at 400°C; they are characterized as high to very high surface oxides with peculiar pore-size distributions. High-quartz solid-solution phases originate from these xerogels by heating at 600°C (lithia-containing compositions) or at 900°C (magnesia-containing composition). They are characterized by X-ray powder diffraction and FTIR spectroscopy as phases with strong disorder in the cation distribution, both in the Al/Si sites and Li sites, with the exception of the stoichiometry LiAlSiO 4 , which shows a certain degree of Li and Si/Al ordering.

Some equilibrium relationships in the barium rich part of the BaOCuOO system

Abstract The authors have examined the barium rich region of the system BaOCuOO at temperatures between 800 and 900°C in presence of gaseous atmospheres with different partial pressures of oxygen (i.e. 1; 0.21; 1.10 −6 atm). In flowing argon (pO 2 = 10 −6 atm) two new compounds corresponding to the formulae Ba 2 CuO 3 and Ba 3 CuO 4 have been identified, for which cell parameters and structural type were defined. In presence of pure oxygen or CO 2 free air the formation was observed instead of a phase with composition Ba 2 CuO 3+δ . It can exist with two modifications characterized by a different distribution of oxygen vacancies.

Subsolidus phase relationships in the SrO-CaO-CuO-O and SrO-Y2o3-CuO-O systems

The solid-state equilibrium relationships were investigated in the SrO-CaO-CuO-O and SrO-Y2O3-CuO-O systems at 900°C in air. The first system is characterized by three wide solid solution series. One stretches from the compound Sr2CuO3 to the compound Ca2CuO3. The other ones originate from the compounds SrCuO2 and Sr14Cu24O41 respectively, by substitution of calcium for strontium up to a limit of 70 at .% in the first case and of slightly more than 50 at .% in the second. In this system a ternary phase is also present, whose composition lies between (Sr0.88Ca0.12)CuO2 and (Sr0.85Ca0.15CuO2In the system SrO-Y2O3-CuO-O no proper ternary phases originate. In the compounds Sr14Cu24O41 yttrium can sustitute for strontium up to a limit of about 30 at .%.

On the substitution of Fe and B for Al in the pollucite (CsAlSi2O6) structure

Abstract The partial or total substitution of Fe and B for Al in the pollucite (CsAlSi2O6) structure was examined or re-examined by means of XRD and FTIR. The samples, prepared through a sol-gel technique, showed a complete solid solution series along the join CsAlSi2O6-CsFeSi2O6 and along the join CsAlSi2O6-CsBSi2O6, whilst along the join CsFe-Si2O6-CsBSi2O6 the mutual substitution of boron for iron is no longer complete; indeed a miscibility gap exists, probably due to the large difference between the ionic radii of Fe and B. Along the latter join, XRD intensity measurements on selected samples (CsFeSi2O6, Cs(Fe0.5B0.5)Si2O6 and CsBSi2O6) allowed to carry out their structural analysis in the Space Group Ia3d.

Effect of the substitution of boron for aluminium in the β-eucryptite LiAlSiO4 structure

Abstract The substitution of B for Al in the β-eucryptite structure was examined along the compositional series described by the stoichiometry LiAl (1− x ) B x SiO 4 with x ranging from 0 to 1. These samples were prepared according to a sol-gel route which employs prehydrolysed tetraethylorthosilicate (TEOS), Al and Li nitrates and boric acid in aqueous solution. Two different monophasic fields were found with the β-eucryptite and the β-spodumene structure, respectively, with 0 ≤ x ≤ 0.40 and 0.60 ≤ x ≤ 0.80; their relative stability is discussed in terms of averaged ionic radius of B, Al and Si ions and of lithium content.

Thermoluminescence experiments to study lattice defects in aluminosilicates

SummaryThe thermoluminescent emissions of β-eucryptite and β-spodumene have been recordedvs. temperature and wavelength. Arguments are advanced which allow the observed emission to be ascribed to point defects originated by the exchange of Si+4 and Al+3 ions in (Si, Al)O4 tetrahedra.

New phases in the most basic zone of the system Ba−Cu−O

SummaryIn the most basic part of the binary system BaO−CuO two new compounds were identified corresponding to the Ba2CuO3 and Ba3CuO4 formulae. Another more oxidized phase of composition Ba2CuO3+δ was also obtained, which can exist in two crystalline modifications. For all these new substances the structural type and the lattice parameters are reported.

Research on the system BaO-La2O3-CuO-O 1° the join Ba2CuO3+δ — Ba1.33La0.67CuO3+δ

Abstract The phases deriving from Ba 2 CuO 3+δ by partial substitution of lanthanum for barium atoms up to the limit composition Ba 1.33 La 0.67 CuO 3+δ were investigated. The reaction environment was CO 2 free air, in order to avoid any pollution of the samples by CO −− 3 ions. At 950°C the system is characterized by the presence of two solid solution fields Ba 2−x La x CuO 3+δ : the first shows tetragonal structure K 2 NiF 4 -type with disordered anionic vacancies and stretches from x =0 to x =0.05, the second has an orthorhombic structure deriving from that of Sr 2 CuO 3 and extends from x =0.25 to x =0.67. Under 450°C the tetragonal phase disappears and we observe a unique solid solution fields, with orthorhombic symmetry

Investigation of catalytic activities by differential thermal analysis

A calorimetric method is proposed to evaluate the catalytic activity of a solid catalyst with respect to the exothermic oxidation of Volatile Organic Compounds (VOC).This method employs a differential thermal analyzer in which an inert reference and a catalytically active sample are both fluxed at a constant rate with a reactive gaseous mixture composed of an inert gas (N2 90% vol.), oxygen and VOC, the last typically 900 to 5000 ppm. While the temperature is varied according to a predefined cycle, the output signal due to the exothermic reaction on the catalyst is continuously recorded. The design of the test chamber, the amount of catalyst, the shape of the holders and finally the flow rate and composition of the gaseous mixture should be carefully selected in order to achieve reproducible results.

Study of defects inZr lacunar Nasicon crystals

SummarySamples of Nasicon compounds NaxZr2-ySi3-zPzO12, withx - 4y +z = 4, were investigated by radioluminescence and thermoluminescence experiments. It has been found that disorder in the Zr lacunar sublattice originates charged defects active as centres of electron-hole radiative recombinations in excited crystals. The analysis of radioluminescent spectra shows that the minimum displacements of Na+ ions exceed 5 Å. This result yields information on the paths of Na+ ions in the ionic conduction of these materials. PACS 61.72.Ji - Point defects (vacancies, interstitials, color centers, etc.) and defect clusters.