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The oxidation of MoSi, in air at atmospheric pressure was studied by electron diffraction, Xray diffraction, and thermogravimetric analyses. The oxidation process occurs in two parts: (1) formation of MooI and SiOz at temperatures below the boiling point of Moo3, and (2) formation of Mo6Si3 and SiOz at higher temperatures. Evidence is presented which indicates that oxygen permeation through a silica layer, which may be of a mixed crystalline-glassy nature, controls reaction rate at high temperatures and that Mo& is present directly beneath the protective oxide. The activation energy for oxidation of MoSiz above 1200°C was calculated as 81.3 kcal mole-'.

The electron spin resonance of the Gd3+ ion has been studied in two series of glasses having the molar compositions 6SiO2:5Na2O: x (Gd2O3 + Y2O3) with 0 ≤ x ≤ 0.5 and 500SiO2: x Na2O:1Gd2O3 with 2.5 ≤ x ≤ 494. The spectra were nearly independent of temperature and sample composition, but varied strongly with microwave frequency in the range 5.4–38.4 GHz. The data were interpreted in terms of the spin Hamiltonian for a strong cubic crystal field and also in terms of the usual rhombic spin Hamiltonian H = g 0β H · S + D[Sz2 − (1/3)S  (S + 1)] + E(Sx2 − Sy2) with S = (7/2). Computer calculations were made yielding all possible effective g values for this latter Hamiltonian for all physically distinct combinations of the crystal field parameters when the Zeeman field H was restricted to the x, y, or z direction. The resonance features were related to three distinct sites for the Gd3+ ion. The spectra are indicative of a chemically heterogeneous glass matrix composed of soda-rich, silica-rich, and diffuse transition regions. The heterogeneities were found to exist over the entire range of compositions studied. Variations in the resonance intensity associated with the soda-rich region suggest the existence of a very well-defined substructure within the region. The data indicate that the substructure is related to the fundamental structural features of the possible crystalline compounds which can exist within the composition range.