Melting Pressure andα−γSolid-Phase Transformation ofHe4near the Upper Lambda Point

Measurements are reported of the slope ${\left(\frac{\mathrm{dP}}{\mathrm{dT}}\right)}_m$ of the melting curve of ${\mathrm{He}}^4$ over the temperature range $-3\mathrm{}\times {10}^{-3\mathrm{}}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}<T-{\mathrm{T}}_{{\lambda }^{\prime }}<15\mathrm{}\times {10}^{-3\mathrm{}}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}$ for the $\gamma$ solid phase and $9\times {10}^{-3\mathrm{}}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}<T-T_{{\lambda }^{\prime }}<19\mathrm{}\times {10}^{-3\mathrm{}}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}$ for the $\alpha$ solid phase, where $T_{{\lambda }^{\prime }}$ is the temperature of the upper lambda point. The upper triple point, where the two solid phases and the liquid are in equilibrium, is (10.00±0.10)×${10}^{-3\mathrm{}}$°K and 0.305±0.003 atm above the upper lambda point. The slope of the $\alpha -\gamma$ transition line at the triple point is 14.3±0.7 atm/°K. The temperature and pressure of the upper lambda point are 1.7632±0.0001°K and 29.83±0.02 atm, respectively, in satisfactory agreement with earlier measurements.