Phase separation in solid mixtures of helium-3 and helium-4

Phase separation temperatures have been determined in bee 3He-4He mixtures as a function of 3He concentration and melting pressure from measurements of changes in the x-ray lattice parameter and peak shape. A new rigid tail dilution refrigerator cryostat is used to study 3 He-4He crystals with 3He concentrations of 0.10,0.20,0.30,0.45,0.60 and 0.70, and melting pressures between 3.0 and 4.3 MPa. The phase separation temperatures determined are in good agreement with regular solution theory and give little support for an asymmetry in the coexistence curve expected from a Nosanow-type model and reported from previous experiments using other signatures of phase separation. Differences in phase separation temperatures determined from slow cooling and warming data are as much as 25 mdeg, but this is less than half the differences reported from previous experiments. The pressure dependence of Tc, the highest temperature on the coexistence curve, is found to be linear with dTc/dP = -26 mdeg/MPa. dTc/dP is related to the excess volume. A bcc-hcp transformation is seen for the 10% mixture at about T = 0.3 K for a melting pressure of 3.7 MPa.U of I OnlyThesi