Melting, Sublimation, and Triple Point

The melting point of a crystalline material such as graphite is the temperature at which the solid state is in equilibrium with the liquid at a given pressure. Normal melting point occurs at a pressure of one atmosphere. Graphite does not have a normal melting point since, at one atmosphere, it does not melt but sublimes when the temperature reaches approximately 4000 K. To observe melting, a pressure of 100 atm and a temperature of 4200 K are necessary. 

The triple point (where all three phases, solid, liquid, and gas, are in equilibrium) is achieved, by recent estimates, at a temperature of4200 K and a pressure of 100 atm, as shown in the vapor-pressure curve of Fig. 3.7.I M A great deal of uncertainty still remains regarding these values of pressure and temperature, reflecting in part the difficulty of conducting experiments under such extreme conditions. 

The onset of sublimation and the melting temperatures are apparently close. At temperatures above that of the triple point and at pressures of argon greater than 100 atm, a mixture of solid and liquid carbon is detected. 

Graphite can thus be considered the most refractory of cill the elements, tungsten being second-best with a melting point of 3680 K. However hafnium carbide (HfC) and tantalum carbide (TaC) are reported to have higher melting points (approximately 4220 K and 4270 K respectively) and are the most refractory of all materials.f  

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