Liquid-Solid Phase Transition inHe3-He4Mixtures

Measurements on the liquid-solid phase transition of ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures have been performed by means of a strain-gauge technique. It was found that the freezing curves of mixtures with initial concentrations of greater than 5% ${\mathrm{He}}^3$ merged into a common curve with negative slope in the $P-T$ plane. This curve is interpreted to be a three-phase equilibrium line or univariant involving the coexistence of bcc solid helium and the two phase-separated liquids, one rich in ${\mathrm{He}}^3$ and the other rich in ${\mathrm{He}}^4$. A number of other univariant lines involving these and other phases of helium have also been observed. Univariants intersect at quadruple points which are defined as points where four phases coexist. A quadruple point has been observed in these experiments involving coexistence of bcc solid helium, hcp solid helium, and the two phase-separated liquids at a temperature $T=0.37\mathrm{}°$K and a pressure $P=26\mathrm{}$ atm. The data is discussed in terms of a three-dimensional phase diagram which is consistent with the Gibbs phase rule and with the results obtained by other investigators.