Experimental Determination of the P–T Melting Curve of Argon

Techniques for the precise measurement of the $\mathrm{P–T}$ melting curves of gases at pressures up to about 12 kbar have been developed. In the method used, a pressure vessel is maintained at a temperature constant to within ± 0.002°K. The pressure in the system is adjusted by manipulation of the pressure generating intensifier until solid–fluid coexistence is obtained within the vessel. The melting temperature is then measured with a platinum resistance thermometer mounted on the vessel, while the corresponding melting pressure is measured with a high‐precision manganin resistance pressure gauge, which is connected to the pressure system. This method has been employed in making extensive $\mathrm{P–T}$ melting curve measurements in argon at pressures ranging from 0.58–11.41 kbar and temperatures from 97.8–272.9°K. The estimated precision in the results is ± 0.4 bar in pressure and ± 0.002°K in temperature. The absolute accuracy of the measurements, especially pressure, may be somewhat less due to uncertainty in the defining scales. The data show systematic deviations of as much as 10 bar from a Simon melting equation of the form $P_m{\mathrm{\hspace{0.17em}=\hspace{0.17em}AT}}_m^c\mathrm{\hspace{0.17em}+\hspace{0.17em}B}$ . These deviations can be substantially reduced by use of a melting equation of the form $P_m{\mathrm{\hspace{0.17em}=\hspace{0.17em}A(T}}_m{\mathrm{\hspace{0.17em}+\hspace{0.17em}D)}}^c\mathrm{\hspace{0.17em}+\hspace{0.17em}B}$ .