Let us consider the coherency strain associated with composition fluctuations:
the change in lattice paramter with composition is given by
;
thus, for a total composition change of
, molar volume
, Young's modulus
and Poisson's
ratio
, the coherency strain energy is given by
 |
(10) |
where
 |
(11) |
and
 |
(12) |
Adding the above energy
to Eq. 7,
one obtains the equation
from which the condition for spinodal is derived as
 |
(14) |