Thermodynamic properties of4He. II. The bcc phase and theP-T and V-T phase diagrams below 2 K

The constant-volume heat capacity of ⁴He has been measured at molar volumes from 20.45 to 29.71 cm³/mole in the temperature range from 0.3 to 4 K. The entropy has been obtained as a function of volume and temperature by extrapolation of the data to 0 K. The P-T equilibrium curves below 2 K were obtained from the volume dependence of the entropy in the two-phase regions. The V-T curves were obtained above 1.25 K by observation of heat capacity discontinuities at the phase boundaries and below 1.25 K from the equilibrium pressure-temperature data and the properties of the pure phases. The minimum in the melting pressure occurs at 0.774 K and is 8.04 × 10^–3 atm below the 0 K value. The corresponding maxima in the molar volumes of the solid and liquid were also determined. In the bcc phase (S/V) _T and (C _v /V) _T are everywhere positive. Both the temperature and volume dependence of the heat capacity are similar to those of bcc ³He, in the very limited ranges of volume and temperature in which the phase could be studied. An unexpected rise in heat capacity in the 20 mK interval below the melting temperature was observed.Work supported by the U.S. Energy Research and Development Administration.