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 diffusion of oxygen in monocrystalline ScaO3, YzO3, Dy,O2, HosO3, Er2O3, Tm2O2, and Lu2O3 was measured by a thermobalance technique. The reoxidation of partially reduced crystals of these materials was treated as a diffusion‐controlled process with a moving boundary. Large diffusion coefficients and fairly low activation energies were found for this process, in agreement with a proposed migration mechanism which involves the inherently defective anion sublattice of these materials.

WEIGHT-GAIN technique can be used to study oxygen diffusion in a semiconducting oxide if certain conditions exist. The conditions required, method of material preparation, treatment of data, assumptions made, and evidence to support the assumptions were presented in detail. ' Briefly, the oxide is slightly reduced, to GdOl.,, in this case, and then reoxidized in air at various constant temperatures. Migration of oxygen interstitials associated with holes is assumed to occur during reoxidation. In contrast to the cubic type C structures of the oxides of the earlier report, the gadolinia in this study was in the monoclinic type B form. This structure has oxygen ions in a less than close-packed arrangement,' but does not contain the discrete anion voids found in the cubic structure common to the heavy rare earth oxides. The movement of a sharp reduced-oxidized material interface during oxidation nevertheless indicates interstitial oxygen diffusion. Progress of the moving boundary between oxidized and reduced material was monitored with an electrobalance, and diffusion information, was developed with techniques described by Crank.3 This treatment yields values for the diffusion product, DC,, where D is the diffusion coefficient and Co the concentration of the diffusing species dissolved in the oxidized layer at the oxygen-oxide interface. Figure 1 is a plot of diffusion products for Gd,O, fitted to an Arrhenius equation: DCo = 1.83X10-4 exp (TT-) -28,890Data for C,, are not available for Gd,Ol. While the monoclinic structure probably has a smaller interstitial oxygen solubility than the cubic structure, the ability of the latter structure to accommodate an additional 25 mol% oxygen is used here as an upper limit of the solubility in the monoclinic structure. This maximum assumed solubility gives the diffusion coefficients plotted in Fig. 2. In this case:It should be stressed that the D values in Fig. 2 are lower limits reached by assuming maximum solubility.In the range 750' to 1050°C oxygen self-diffusion in Gd'O, is more rapid than for other rare earth oxides examined by this method.' Data for LuzO, having the cubic type C structure are included in Fig. 2 for comparison. In a recent study, Stone et al., reported diffusion data for SmzO, having the same type B structure as the Gd,O, used in this study.

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