First, the free energy of Ta
2D
1+x is represented as a function of the long-range order parameter
z.
z=1 corresponds to the β type deuteride based on the Ta
2D superstructure;
z=0 represents the δ type deuteride which is a nonstoichiometric form of the TaD superstructure. At the composition of Ta
2D
1+x,
z and temperature are shown to have the following relationship:
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Tc0 represents the β→δ phase transition temperature at
x=0. Analysis of the above equation shows that
Tcx decreases according to
Tc0(1−
x2). At
x=0.6,
Tcx calculated from the observed
Cp data by Asano et al. is very close to room temperature, which is consistent with the observed β⁄δ phase boundary of the Ta-D phase diagram.
Secondly, Khachaturyan’s method of static concentration, waves and the theory of stress-induced interaction are discussed in relation to our treatment for the β−δ tantalum deuteride system. The following points are presented: (1) In the completely disordered state where
n(
p,
R)=\bar
n, the partial molar configurational entropy in the Khachaturyan’s theory has the form, −
K[ln\bar
n⁄(1−\bar
n)]. This, however, contradicts the generally supported blocking models, and is not consistent with the observed entropy in the α phase. (2) In Khachaturyan’s method, the relation,
n(3,
R)=
n(\bar3,
R), always holds, whereas
n(3,
R)\leavevmode\hbox
to0
pt=\llap\
n(\bar3,
R) in the actual Ta
2D
1+x superstructure. (3) The internal energy of β type Ta
2D calculated by the Khachaturyan’s theory is in good agreement with that derived from the observed
Cp data and the β⁄δ phase boundary.
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