and
at high pressure. Verhoogen [1980] pointed out the
danger of assuming that 
of liquid iron would be the same
as that of 
-iron at ambient conditions. The basic formula
for 
is 
which is the change of pressure
with energy density at constant V. This formula applies to both
solids and liquids. A careful experimental analysis by
W.W. Anderson and Ahrens [1994] shows that the ambient
of liquid iron is 1.735. The reported experimental
value of 
is 1.401 [ Brown and McQueen, 1986].
These results can be fitted by
where the exponent is known as q,
).
The low value of the exponent is typical of
the liquid state. In solids, 
would be closer to 1
(though preceded by a negative sign). These experimental results agree
qualitatively with the new calculations of
Stacey [1995] (see his Figure 2),
which show that 
starts out with a high
value (probably 
) and tends to zero at
: in particular, 
is 1.63 and 
is 1.29.
For pure solid iron, a number of calculations indicate
that the lattice contribution to 
for inner core conditions
is close to 1.5 (see Anderson [1995] p. 268). There is, however, a contribution to
from the electronic density of states
that effectively adds a small amount to the lattice 
.
The total 
for the inner core used to calculate thermal
pressure should be greater than 1.5, but not
by much [ Anderson, 1995]. Stacey [1995] points out
that we should expect 
of solid
iron to be less than 
of liquid iron.
The value of 
for liquid iron with
impurities might be slightly different
from that for pure liquid iron. From
the above, 
is close to 1.3;
this should be compared with the values used by
Braginsky and Roberts [1995]: 
;
.