E. Baiocchi

Optical and magnetic properties of first-row transition metal ions in lead-silicate glass

Abstract First-row transition metal ions in lead-silicate glasses of composition: 37.9% mol PbO, 61.8% mol SiO2 have been studied with regard to the oxidation state and the coordination by means of optical and magnetic measurements. Optical spectra have been recorded at 300 and 77 K. All the observed bands are assigned as being due to d-d transitions in Td and/or Oh fields. Magnetic moments are of the order of the spin-only values. By ESR measurements we reveal and quantify titanium, vanadium and manganese oxidation states not detectable by the other techniques. It turns out that the various metal ions have the following oxidation states: Ti3+ and Ti4+, V4+ and V5+, Cr3+ and Cr6+, Mn2+ and Mn3+, Fe3+, Co2+, Ni2+, Cu2+. The coordination is 4-fold for Ti4+ and V5+ and 6-fold for Mn3+ and Cu2+. Moreover it has been found that Fe3+, Co2+ and Ni2+ are both 4-fold and 6-fold coordinated and that Cr3+ and Cr6+ oxidation states coexist in the same sample.

ESR study of the equimolar PbO-V2O5 system

The semiconducting system 50 mol% PbO-V2O5 has been studied by electron spin resonance (ESR) in the temperature range 150 to 675 K. Depending on the cooling rate, this system can be obtained from the melt as an amorphous phase, as polycrystalline leadmetavanadate (PbV2O6), or as two different mixtures containing polycrystalline and amorphous phases. In all these materials the ratio V4+/Vtot has been measured by wet analysis showing that the V4+ concentration is nearly constant. It has been possible to show that the appearance of very different ESR line-shapes for the various materials is due to the difference in the hopping rate of the charge carriers. In particular the presence of a well-defined hyperfine structure is characteristic of the amorphous phase, in which the mobility of the charge carriers is lowered by the presence of a termWD in the expression of the hopping rate. The lead-metavanadate ESR spectrum shows the presence of two lines of different widths; this fact has been explained assuming the presence of two distinct jumping rates. No temperature variation of the spectra has been observed at low temperatures. The high temperature spectra have permitted us to follow the thermal evolution of the system.

Non-isothermal analysis of the crystallization of the amorphous germanium dioxide

Abstract The amorphous germanium dioxide devitrification has been investigated, with particular regard to the kinetic aspects and to the crystallization mechanism. The amorphous material transforms into the hexagonal dioxide modification, which is metastable at the considered temperatures. This fact is in accordance with the presence of tetrahedrally coordinated Ge in both the glass and the devitri fied material. The crystallization kinetics has been investigated by means of the DTA technique; a rigorous method of analysis has allowed the determination of the activation energy Eact, the Arrhenius preexponential factor A and the morphological index n characteristic of the devitrification reaction. We have so obtained the following values: E = 233 kJmol−1, A = 6.57 · 107s−1 and n = 2.44. These results are not in total accordance with previous studies carried on with similar non-isothermal techniques. Our interpretation is supported also by the morphological study of the GeO2 crystal growth carried out by scanning electron microscopy.

ESR study at various microwave frequencies of amorphous and polycrystalline ZnOxV2O5 system

The study of the system ZnO·xV2O5 has shown that the various phases obtained by different cooling rates have ESR line-shapes which are very dissimilar. It has been shown that the amorphous phase is characterized by a well resolved hyperfine structure and that the ZnV2O6 phase has a very asymmetrical line-shape which closely resembles those studied in the magnetic resonances in metals. To verify the role played by the skin depth in causing this asymmetry, the measurements have been carried out at various microwave frequencies particularly in the low range of frequencies where the lines become symmetrical. This behaviour can be explained by assuming that the low frequency skin depth of the samples containing a high amount of ZnV2O6 is of the same order of the sample size. The use of this assumption permits an evaluation of the sample conductivity (about 5×10−2 Ω−1 cm−1) which is close to the measured values (about 10−3 Ω−1 cm−1).