M. Salis

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.

Synthesis and characterization of Fe and P substituted 3:2 mullite

Abstract The incorporation in mullite structure of foreign elements has been studied since 1924. In this paper the isovalent 1 2 P plus 1 2 Al for Si substitution has been investigated employing wet chemistry preparative route with a final firing temperature of 1300°C. The extent of solid solution and the crystallographic features of the end term with composition Al6.22Si1.56P0.22O13 are described on the basis of XRPD profile fitting. This substitution has also been examined in the presence of the contemporary Fe for Al substitution; the end term of this coupled isovalent compositional series Al5.90Fe0.20Si1.80P0.10O13 has been characterized too. In this latter case, the crystallographic results are confirmed by Mossbauer spectroscopy. The simple Fe for Al substitution at 1300°C has also been re-examined in the same preparative conditions. The composition and the structure of the end term (Al5.60Fe0.40Si2O13 ) are in agreement with previous studies. The incorporation degree of Fe and P in the coupled substitution results in being approximately halved in respect to simple substitutions of the two elements.

A Debye model for point defects in aluminosilicate crystals

SummaryIn aluminosilicate lattices, exchanges between silicon and aluminum ions are expected to originate charged point defects of opposite signs. Thermodynamic arguments, based on a model analogous to the Debye electrolyte model, allow evaluation of the equilibrium density of defects as a function of temperature. The electric field of point defects is found to be screened by charge distribution even at distances small enough to affect the ground-state energy of electrons trapped by the defect positive charges. Calculations show that large fluctuations of trapped electron energy are expected. These results agree with data from thermoluminescence experiments on different aluminosilicates, which showed broad distributions of electron traps.

Mössbauer spectroscopy of tektites

SummaryDifferent tektite samples have been investigated by57Fe room temperature Mössbauer spectroscopy. The data indicate iron degrees of oxidation larger than those previously determined by wet chemical analysis.

Thermodynamics of ion exchange defects in aluminosilicates

SummaryThe Debye-Huckel model for electrolytes is utilized to evaluate the density of defects originated by the exchange of Al and Si ions in aluminosilicate lattices.

A study of lattice defects in solids belonging to Al2O3−B2O3 and Al2O3−SiO2 systems by thermoluminescence experiments

SummarySpectrally resolved thermoluminescent emission has been investigated in solids belonging to Al2O3−B2O3 and Al2O3−SiO2 systems. These solids, which are characterized by a structure like that of alumino-silicate mullite, contain a high density of oxygen and aluminum or boron lacunae. This fact favours the occurrence of both-sign interstitial ions acting as carrier traps. The results show a close similarity with those obtained when studying, with the same experimental method, the thermoluminescent emission of alumina. The overall analysis of data allows for reliable conclusions on the nature of crystal defects in these materials.

On the radioluminescent emission of BaF2

SummaryThe luminescence of barium fluoride excited by X-rays has been recordedvs. wavelength. The spectrum shows two components due to recombination of excitons bound to Frenkel defects and to luminescent centres also active in the thermoluminescent process. The results are interpreted by means of a model based on the Prener and Williams recombination scheme.

A re-examination of fermi's hyperfine contact interaction

SummaryThe hyperfine coupling between spin of electrons ins states and nuclear spin is generally represented by a contact Hamiltonian in which a δ(r) factor appears. Utilizing relativistic equations and considering pointlike nuclei, we show that the δ(r) factor must be replaced by a steeply decreasing radial function of half-maximum width δr=5.8·10−14Z cm. For hydrogen, the correction with respect to the contact Hamiltonian turns out to be small, but for high-Z nuclei this correction acquires substantial importance. For iron 1s states, it rises up to 9.6%.

A Mössbauer study of the silica-like structures of FePO4 and (Al0.67Fe0.33)PO4

SummarySamples of FePO4 and (Al0.67Fe0.33)PO4 have been investigated by Mössbauer spectroscopy. The data obtained on quadruple splittings are in agreement with neutron scattering and XRD data on (Al, Fe)O4 tetrahedra structures.

Investigation of cordierite lattice defects by thermoluminescence experiments

SummaryThe thermoluminescence of α- and μ-cordierite samples has been recordedvs. temperature and wavelength. Arguments ascribing the observed emission to the easy ion exchanges characterizing cordierite lattices are advanced.