Observation of a continuous phase transformation in LaNi5-D

We have used high-resolution neutron powder diffraction to study the phase diagram of LaNi5Dx at temperatures up to 100 degrees C. Diffraction pattern s were recorded during consecutive absorption-desorption cycles at 100, 65 and 30 degrees C. In a sequence of four patterns taken during desorption at 100 degrees C, we observed the beta phase transforming continuously to the gamma phase: the first pattern was 100% beta phase; the fourth one was 100 % gamma phase; the second and third patterns were also single phase, but ha d lattice parameters intermediate between those of the beta and gamma phase s. The succeeding gamma-->alpha transformation remained discontinuous, that is, co-existing gamma and alpha phases were observed via two diffraction p atterns with distinct lattice parameters. The following absorption-desorpti on cycle was conducted at 65 degrees C. Again, the beta-->gamma transformat ion was continuous. In the final cycle, conducted at 30 degrees C, the beta -->gamma transformation was discontinuous. Hence, there appears to be a the rmodynamic critical point of the beta+gamma phase 'equilibrium' in this sam ple, between 65 and 30 degrees C. This is the first such observation in the LaNi5-H/D system. The pressure-composition isotherms resemble those of a M m(NiCoMnAl)(5) battery alloy in that a considerable fraction of the D capac ity appears to lie within the concentration range of the 'hydride phase'. ( C) 1999 Elsevier Science S.A. All rights reserved.