Positron annihilation in vanadium hydrides

Doppler broadening of the positron annihilation radiation line shape has been studied for VH/sub x/(x = 0.0, 0.018, 0.105, 0.29, 0.38, 0.54, 0.61, and 0.63), VD/sub 0.49/, and cold-deformed pure V. Measurements of the line-shape parameter were performed as a function of the hydrogen concentration (x) and the sample temperature in the range 20--500 K. The results indicate that the lattice defects, mainly the dislocations formed during the phase changes accompanying the temperature cycling, play an inportant role in determining the interaction of positrons with the hydrogen in the host metal. The observed dependence of the line-shape parameter on x and temperature is attributed to the trapping of positrons at lattice defects. A comparison of the results from VH/sub 0.54/ and VD/sub 0.49/ shows the absence of any isotope effect. Because of the positron affinity for lattice defects the positron annihilation techniques are found to be useful for determining the electronic structure of vanadium hydride, only, if the measurements are performed at high temperatures in the α phase to prevent phase transformations and hence the production of lattice defects in the sample