Calorimetric investigation of the phase transition exhibited by the hydride of Mg2Ni

545 hydrogen interaction, in these systems, and the concomitant feature is that these systems fit closely to the predictions of mean field theory, e.g. as in the values of their critical point exponents. A spinodal is the locus of points satisfying a stability limit criterion and, as such, invites consideration concerning its relevance to the description of instabilities given by catastrophe theory. A discussion is given of the care required, not always evident in the past, in identifying a spinodal with a specific member of the family of elementary catastrophes introduced by Thorn. The point is also made that in comparing catastrophe theory with the thermodynamics of phase transitions, the comparison should be limited to those systems which clearly satisfy mean field theories and display a delineable spinodal; hydrogen-in-metal systems provide the best experimental examples, found so far, of such systems.

The temperature variation of hydrogen solubility in metals* W. A. OATESa, R. LASSERb and H. WENZLb aDepartment of Chemical and Materials Engineering, Shortland, N.S. W. 2308 (Australia) bKFA,

Institut

fiir Festkorperforschung,

The University

of Newcastle,

D-51 70 Jiilich (F.R.G.)

Plots of the logarithm of the hydrogen solubility in metals versus the reciprocal temperature usually show distinct deviations from linearity. Previous suggestions which have been put forward to explain this curvature are examined and shown to be unsatisfactory. From a more detailed analysis of the experimental results for several systems and, particularly, by taking into account the solubilities of the different isotopes. some new suggestions are made. It is suggested that “band” vibrational modes may be responsible for an isotope-dependent contribution towards the observed curvature.

Calorimetric investigation of the phase transition exhibited by the hydride of Mg, Ni* MICHAEL L. POST and JOHN J. MURRAY Division

of Chemistry,

National

Research

Council of Canada, Ottawa

Ontario

(Canada)

The hydride of MgzNi exhibits low and high temperature modifications with a transition temperature near 503 K. The enthalpy characteristics of the phase transition have been investigated by heat-conduction (Calvet-Tian) calorimetry, using hydrides prepared in situ below and above 503 K and then subjected to slow temperature scans (0.2-5 K h-l) with hydrogen pressure being monitored. The data are consistent with the previously reported existence of two low temperature phases. However, there is also evidence of additional intermediate modifications which exist over narrow temperature regions within the relatively wide temperature range for the overall conversion between low and high temperature forms. Hydride preparation conditions and the thermal and pressure history of the sample are factors which determine the phase change characteristics. *Abstract of a paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 30, 1986.