The value of the enthalpy of ordering is a point frequent point of confusion in formatting
THERMOCALC models for Perple_X. In papers by Holland & Powell the enthalpy of ordering for
a reaction of the form
1/2 (A + B) = C [1]
is
DH = H_C - 1/2(H_A + H_B) [2]
where H_i is the enthalpy of endmember i. Because Holland & Powell invariably choose
the endmembers A, B, and C in such a way that the configurational entropy change of
the reaction is zero, the Gibbs energy of ordering for [1] is identical to the enthalpy
of ordering. Thus [2] can be written equivalently
DH = DG = G_C - 1/2(G_A + G_B) [3]
or
G_C = 1/2(G_A + G_B) + DH [4]
where G_i is the effective Gibbs energy of endmember i. The need for distinguishing the
"effective" Gibbs energy is that Holland & Powell often introduce "DQF corrections" to
Gibbs energies indicated in their thermodynamic data bases such that
G_i = G0_i + DQF_i [5]
This is all fine and dandy, but the "make definitions" in THERMOCALC data files used to
define the Gibbs energy of the ordered species C are written in terms of the endmember
Gibbs energies without DQF corrections, i.e.,
G_C = 1/2(G0_A + G0_B) + DQF_C [6]
Because equations [4] and [6] look similar, Perple_X users frequently mistake the entry
for DQF_C in THERMOCALC solution model files with the enthalpy of ordering term DH
required in Perple_X for the definition of an ordered species. Substituting [5] into [4]
gives
G_C = 1/2(G0_A + G0_B) + 1/2(DQF_A + DQF_B) + DH [7]
and comparison of [6] and [7] gives
DH = DQF_C - 1/2(DQF_A + DQF_B) [8]
the value to be entered in Perple_X.