EFFECT OF SMALL OXYGEN ADDITIONS ON ELASTIC CONSTANTS AND LOW TEMPERATURE ULTRASONIC ATTENUATION OF NB SINGLE CRYSTALS

Elastic constants and ultrasonic attenuation of pure Nb single crystals and single crystals doped with oxygen were measured by the pulse superposition technique at temperatures from 4.2° to 313°K. The principal effects of the added oxygen were to decrease slightly all constants but C′ and C44, the two 〈110〉 shear constants, and also to bring about the appearance of a secondary dip in the attenuation at temperatures below the superconducting transition. No discontinuities in the elastic constants were observed, so it was concluded that spontaneous ordering of oxygen in Nb did not occur in our case. The substructure responsible for the extra dip in the attenuation below Tc is therefore due to other causes, the most likely of which is oxygen clustering above room temperature. This hypothesis is supported by a variety of other data. © 1969.