Simon–Glatzel equation explained

The Simon–Glatzel equation^[1] is an empirical correlation describing the pressure dependence of the melting temperature of a solid. The pressure dependence of the melting temperature is small for small pressure changes because the volume change during fusion or melting is rather small. However, at very high pressures higher melting temperatures are generally observed as the liquid usually occupies a larger volume than the solid making melting more thermodynamically unfavorable at elevated pressure. If the liquid has a smaller volume than the solid (as for ice and liquid water) a higher pressure leads to a lower melting point.

The equation and its variations

T_m=T_ref\left(

	P-P_ref
	a

+1\right)^b

T_ref

and

P_ref

are normally the temperature and the pressure of the triple point, but the normal melting temperature at atmospheric pressure are also commonly used as reference point because the normal melting point is much more easily accessible. Typically

P_ref

is then set to 0.

and

are component-specific parameters.

The Simon–Glatzel equation can be viewed as a combination of the Murnaghan equation of state and the Lindemann law,^[2] and an alternative form was proposed by J. J. Gilvarry (1956):^[3]

T_m=T_ref

	'
\left(K
	0

	P-P_ref
	K₀

	2(\gamma-1)
	3+f

\right)

where

K₀

is general

P=0

	'
K
	0

is pressure derivative

P=0

\gamma

is Grüneisen ratio, and

is the coefficient in Morse potential.

Example parameters

For methanol the following parameters^[4] can be obtained:

a	188158	kPa
a	188.158	MPa
b⁻¹	5.15905
T_min	174.61	K
T_max	228.45	K
P_max	575000	kPa
P_max	575.000	MPa

The reference temperature has been T_ref = 174.61 K and the reference pressure P_ref has been set to 0 kPa.

Methanol is a component where the Simon–Glatzel works well in the given validity range.

Extensions and generalizations

The Simon–Glatzel equation is a monotonically increasing function. It can only describe the melting curves that rise indefinitely with increasing pressure. It may fail to describe the melting curves with a negative pressure dependence or local maximums. A damping term that asymptotically slopes down under pressure,

D(P)=\exp[-c(P-P_ref)]

(c is another component-specific parameter), is introduced by Vladimir V. Kechin to extend the Simon–Glatzel equation^[5] so that all melting curves, rising, falling, and flattening, as well as curves with a maximum, can be described by a unified equation:

T_m=F(P) ⋅ D(P)

where

F(P)

is the Simon–Glatzel equation (rising) and

D(P)

is the damping term (falling or flattening).

The unified equation may be rewritten as:

T_m=T_ref\left(

	P-P_ref
	a

+1\right)^b ⋅ \exp[-c(P-P_ref)]

This form predicts that all solids have a maximum melting temperature at a positive or (fictitious) negative pressure.

Notes and References

Simon F. E., Glatzel G., Z. Anorg. (Allg.) Chem., 1929, 178, 309–312
Book: Anderson, Orson L.. Equations of State of Solids for Geophysics and Ceramic Science. Oxford University Press. 1995. 0-19-505606-X. 281.
Gilvarry. John James. 1956. Equation of the Fusion Curve. Physical Review. 102. 2. 325–331. 10.1103/PhysRev.102.325. 1956PhRv..102..325G.
[Dortmund Data Bank]
Kechin V. V., J. Phys. Condens. Matter, 1995, 7, 531–535