Studies of low temperature phase transformations in high field superconductors and the phonon spectrum and mechanical properties of vanadium. Final report

Extensive studies of the structure and phase transformations of high field superconductors of the BETA -tungsten type were made. A phase transformation was discovered at 43 deg K in Nb/sub 3/Sn and the structure of the low temperature phase was measured. The details of the transformation in terms of the formation of the low temperature tetragonal phase from the high temperature cubic phase was investigated both by x-ray observations and optical microscopy below 40 deg K. The complete phonon dispersion of vanadium was measured by x-ray diffuse scattering, thus complementing the neutron measurements on other body-centered cubic elements. Incidental to the work on phonons in vanadium a strong effect was discovered which was related to hydrogen precipitations in vanadium. In separate electron microscopy studies the structure of the vanadium hydride phase was determined directly and the relative positions of the hydrogen atoms were determined. In a neutron diffraction study complementing the phonon work with x-rays the existence of anharmonic vibrations in single crystals of silicon was shown directly by determining, for the first time, a nuclear forbidden reflection. This reflection is due solely to anharmonic vibration of the nucleus of the atom and it was shown that its intensity increases with increasing temperature roughly as the 4th power of temperature. (auth