Isotope separation during thin film compound growth

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be a metastable two-phase region associated with this PE phase line. We have indicated one possible metastable extension of the CE phase diagram in Figure 4. It is difficult to envisage circumstances where this metastable equilibrium could be observed since the atoms of the alloy must have some mobility in order for it to occur (composition changes are involved) and, with such atomic mobility it is likely that the CE decomposition reaction would occur in preference. Perhaps, however, the metastable equilibrium could be of significance in understanding local equilibrium3 during the /3 ~ / 3 ' transformation. The usefulness of these PE phase diagrams in understanding some features of alloy-hydrogen systems will be discussed elsewhere.

/:/+7 Both authors would like to express their thanks to the NSF for financial support, and WAO would also like to thank the Australian Research Grants Scheme.

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Fig. 4 - - C E ( - - ) and PE ( m ) phase diagrams for a system in which A and B form an intermediate phase AB which decomposes under CE but which also exhibits pseudo-binary behavior (PE). A possible metastable ( . . . . . . . ) extension of the CE phase diagram is also indicated. The phase fields are labeled with respect to PE.

the CE and PE phase diagrams for a similar but more general case than the ones in Figures 1. AX is now taken as being structurally different from BX and their AGe's are assumed unequal. A new two-phase line corresponding to the PE between A(B)X-B(A)X is now present in the A/Xx-~ diagram, Figure 2(a), and a new PE two-phase region appears in the r/-~ diagram shown in Figure 2(b). It can be seen that the PE phase diagram in the r/-~ cooordinates has the appearance of a binary temperature-composition diagram if it is rotated through 90 deg. In Figures 1 through 4 the primed and unprimed symbols represent the compound and metalrich phases, respectively, of the phase fields of PE. Consider now the reverse situation of that discussed above, i.e., A X and BX form a continuous series of solid solutions while A and B are substantially immiscible at CE. In this case the PE phase diagram bears a different relation to the CE one from the case considered previously. This difference arises from the fact that the a- and/3-phases in the alloy cannot change their composition in the PE case. The resulting phase diagrams are shown in Figures 3(a) and 3(b). The idea of three- and four-phase regions (actually five if the gas phase is included) on the PE phase diagram in Figure 3(b) may seem surprising, but it arises from the fact that we are really dealing with two separate binary systems, i.e., the a and/3 phases of fixed sc. With a little practice it is not difficult to draw PE phase diagrams for more complex systems. As an example, in Figure 4 we show the/Xx-~: diagram for the case where a single intermediate phase is indicated on the solvent phase diagram. The phase is seen to undergo the decomposition reaction, /3 ~ a ' + y, unde