Liquid-Solid Phase Transition inHe3-He4Mixtures

Abstract

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 $P\ensuremath{-}T$ 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 $T=0.37\ifmmode^\circ\else\textdegree\fi{}$K and a pressure $P=26$ 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.