Calorimetric study of the transition between the close-packed phases ofHe4from the triple point to 3.9 kbar

Abstract

The hcp-fcc ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ transition was studied using a calorimetric method from the triple point at 14.992 K and 1127 bar to a pressure of 3882 bar. The transition shows temperature hysteresis and a finite thermal width, both of which increase with increasing pressure. Erratic progress of the transition, including burstlike behavior leading to self-cooling during the heating transition and self-heating during the cooling transition was observed. The transition is most likely of martensitic type. Above about 15.8 K, the equilibrium phase line is linear with a slope of 550 bar/K. Between 15.8 K and the triple point, the phase line increases in slope, becoming very large and possibly infinite at the triple point. Transition entropy measurements give ${\left(\Delta S\right)}_{\mathrm{tr}}=18\mathrm{}$ mJ/mole K ± 10% with no temperature dependence discernible within experimental scatter. The molar volume change is 0.33 ${\mathrm{mm}}^3$/mole above 15.8 K, this decreases to zero or near zero as the triple point is approached. The transition entropy and the molar volume change at 15.8 K and above are in good agreement with theoretical predictions. The data do not show, however, the predicted increase in slope of the phase line with increasing pressure.