The sodium chloride primary pressure gauge

Abstract

The failure of a central force model for sodium chloride is discussed. It is noted that it does not closely satisfy the Cauchy conditions at low temperatures, and that it fails the central force requirement of the Love condition. It is also noted that the model as used in the usual way, with B0 as the zero-pressure value of the bulk modulus and the lattice parameter as input, fails to predict the pressure derivative of the bulk modulus. It is therefore concluded that there is no scientific justification for basing a pressure scale on a central force equation of state. The available shock data for sodium chloride and its analysis is examined, and two reasons why the Hugoniot transformation pressure is likely to be less than 231 kbar are discussed. These reasons are (1) the presence of overpressure (the additional pressure beyond the equilibrium transformation pressure needed to cause the transformation under dynamic conditions) and (2) the lack of data points between 212 and 231 kbar. The important (but unjustified) theoretical assumptions made in converting Hugoniot to isothermal data is discussed; it is noted that serious error can enter for very large pressures (P ? B0) for a given material and that at such high pressures the isothermal data should thus be considered only semiquantitative even if the Hugoniot data itself is accurate. An alternate method of estimating the isothermal transformation pressure from the Hugoniot transformation pressure is used. This method is based on the temperature derivative of the transformation pressure. On this basis it is concluded that an upper bound for the isothermal transformation of NaCl (to a CsCl-type structure) at room temperature is 257 kbar; it is noted that the actual value may be considerably less than this.