Thermal vacancies and phase separation in bcc mixtures of helium-3 and helium-4

Thermal vacancy concentrations in crystals of 3He-4He mixtures have been determined from accurate measurements of the temperature dependence of the x-ray lattice parameter. A new x-ray diffractometer-position sensitive detector system is used to make measurements of the absolute lattice parameter of the helium crystals with an accuracy of 300 ppm, and measurements of changes in lattice parameter to better than 60 ppm. The phase separation of the concentrated 3He-4He mixtures has been studied in detail with the x-ray measurements. Vacancy concentrations in crystals with 99%, 51%, 28%, 12%, and 0% 3He have been determined. The vacancy data in the nearly pure 3He crystals confirm previous x-ray results from pure 3He. The first direct measurements of vacancies in pure hcp 4He' are reported, along with study of vacancies in bcc 4He . The vacancy concentrations in the 51% and 28% 3He mixed crystals are found to be as large as 5%. Progress is made toward characterization of the detailed temperature dependence of the vacancy concentration in all the crystals. The low temperature x-ray molar volumes of the crystals are determined to better than 0.1%. Vacancy volumes of formation are inferred by several methods. Phase separation has been studied in mixed crystals with concentrations of 51%, 28%, and 12% 3He and melting pressures between 3.0 and 6.1 MPa. The phase separation temperatures determined in this work are in general agreement with previous work. The pressure dependence of Tc, the phase separation temperature for a 50% mixture, is found to be linear: dTc /dP = -34 mdeg/MPa. The x-ray measurements are used to make several comments on the low temperature phase diagram of the helium mixtures.U of I OnlyThesi