Phenomenology of scaling regularities at T = 0 in solid halogens under pressure

To describe phenomenologically the approach to the molecular dissociation first-order transition in cold solid halogens under high pressure, it is first pointed out that a suitable dimensionless order parameter Delta is that which describes the decrease of bonding asymmetry within the b-c plane of the orthorhombic lattice. Delta would tend to zero as the monatomic metal is approached from the lower pressure phase where diatomic halogen molecules coexist with metallic behaviour. It is known from experiment that Delta at the above transition has a value Delta(t) approximate to 1/10. It is further pointed out that to describe the scaling of the asymmetry parameter Delta with reduced volume upsilon = V/R-b(3) in the lower pressure regime, Taylor expansion around Delta(t) can be carried out, the essential additional knowledge being then the numerical value of the slope partial derivative Delta/partial derivative upsilon at the transition which is around 1/40. Finally, a property of the phase boundary at low temperature is inferred from elementary excitations plus the Clausius-Clapeyron equation. (C) 1999 Elsevier Science B.V. All rights reserved.