Oxygen, air, or chlorine gas present in the vacuum system during evaporation extend the Ge-rich boundary of the A15 phase towards the ideal 3:1 ratio. X-ray evidence indicates the presence of an amorphous component below 775°C which is detrimental to the superconductivity and which is accompanied by an abrupt increase in the A15 lattice constant. Preliminary Auger results indicate that the Nb:O ratio first decreases as the partial pressure of O2increases and then remains constant.
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Hallak et al. (1977) studied this question.