The heat liberated by isothermal compression permits the determination of the thermodynamic properties of liquid water−heat capacity, compressibility, and expansivity. The results, as a function of pressure and temperature, have been extrapolated as far down as −40 °C. They confirm the existing data in the region of the supercooled liquid and also the existence of a divergence of the measured properties. This could be explained as a lambda transition occuring at −45 °C on isobars p = 0. The abnormal heat capacity which characterizes this transition is well described as a function of the temperature by a formula with critical coefficients as already suggested by others.
The expansivity of liquid CO2 and of n-butane is determined as a function of pressure up to 4 kbar, from the melting temperatures up to the critical temperatures. The experimental method is piezothermal. The set of data is processed through a fit with a multipoint Padé approximant using an algorithm due to Werner. This particular technique determines, in its reduced form, the unique rational function relevant to the problem. The expansivity of the whole liquid phase is described in terms of a simple equation with four coefficients which enables calculation of the other thermodynamic properties. The general aspect of the phenomenological equations is discussed in this instance, underlining the particular behavior of the heat capacity as a function of pressure.
An analysis of the potential energy has shown that a cubic lattice cannot be mechanically stable unless the lattice parameter a remains inside relatively narrow limits. The stability field so defined depends on the kind of lattice, face-centred, body-centred or simple cubic, the interatomic potential being the same. In the case of the face-centred cubic lattice, which is the real lattice of krypton, a computation of the free energy F(T,a), made at the quasiharmonic approximation shows that at T=0°K F(O,a) reaches a minimum at a value a(0) of the lattice parameter, and a(0) is inside the field of mechanical stability. But for body-centred and simple cubic lattices, no field of mechanical stability has been found. As the temperature increases, the equilibrium value a(T) of the face-centred lattice increases too and the shape of the curve F(T,a) changes as function of a. The thermodynamical equilibrium which is stable at low temperatures becomes metastable and disappears from the field of mechanical stability.
Limite de stabilit6 m6canique
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