P-T-x phase diagrams of the light lanthanide-hydrogen systems (Ln = La - Nd)

The P-T-x phase diagram of the La-H2 system has been measured with vapor pressure measurements as a function of composition in the range 868 <= T <= 11110C, using a quartz-glass reactor capable of T <= 11500C and P <= 6 bar. It is shown that the T-x phase diagram does not have a critical point in the two-phase region γ-La(s) + LaHsub(x)(s) as mentioned in the literature. Extrapolation of the phase boundaries indicates the existence of a peritectic at T approx. 12000C. From the Van't Hoff isochore a value of P > 4 bar has been estimated. Just above the peritectic temperature it should be possible to grow crystals of the light lanthanide hydrides. Since these conditions cannot be reached in a quartz-glass reactor with the necessary purity, a steel autoclave is under construction. Measurements of the Pr-H2 system also indicate the presence of a peritectic. In addition, inflections in the phase boundaries of the β-Pr(s) + PrHsub(x)(s) region at approx. 10600C indicate the presence of a transition which has not been reported previously. The existence of peritectic diagrams for the La-H2 and Pr-H2 systems is in agreement with that of the Ce-H2 system; a peritectic diagram is also forecast for the Nd-H2 system. (Auth.)