Phase Separation inHe3-He4Mixtures near Absolute Zero

Phase separation in liquid mixtures of ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ at temperatures close to 0°K is discussed on the basis of the isotopic impurity models of Pomeranchuk and Zharkov and Silin. It is predicted that for ${\mathrm{He}}^3$-rich mixtures the phase separation line is asymptotic to the temperature axis as $T\to 0$, so that phase separation should occur in extremely dilute solutions of ${\mathrm{He}}^4$ in ${\mathrm{He}}^3$ at relatively high temperatures. For example, 0.003% of ${\mathrm{He}}^4$ in liquid ${\mathrm{He}}^3$ should begin to separate at about 0.1°K. The phase separation curve for ${\mathrm{He}}^4$-rich mixtures near 0°K is difficult to predict with the data available at present. One possibility is that dilute solutions of ${\mathrm{He}}^3$ in ${\mathrm{He}}^4$ may be stable at 0°K.