Chemical-potential measurements and phase diagram of a pseudoternary solid: LixCuyMo6S8.

Intercalation cotnpounds with one type of guest atom are pseudobinary systems as long as the host retains its structure, and the behavior of such compounds can often be understood with two-component models. '2 One component is then the guest and the other is the interstitial site where the guest resides. Similarly, intercalation compounds with two types of guests are pseudoternary systems. Adding the second guest should complicate and enrich the behavior of intercalation systems just as adding a third component to a binary alloy3 or to a binary fluid4 does. The presence of two mobile guests can lead to surprises, because even if the average concentration of one of the guests is fixed, that guest can segregate in the host so that its local concentration varies. A complete understanding of these systems requires a pseudoternary phase diagram, a phase diagram presented as a function of the concentration of both guests. Here we present such a phase diagram for Li„Cu«Mo6SB [Fig. 1(a)], the first for any intercalation compound. Although phase diagratns of ternary and pseudoternary systems (A„B~Ct „«) have been measureds and calculated, 6 7 the chemical potentials p, and v of the components A and 8 have not been measured. In addition, there is no theoretical treatment of the behavior of response functions like (Bx/t)iL)«T near critical points of ternary systems. We have used electrochemical techniquess to measure the chemical potential, iM, , of Li in Li„Cu«Mo6S8, as well as (t)x/ Bp)«T. W, e show how knowing p, simplifies the measurement of the phase diagram. We also present the first measurement of the chemical potentials of all the coexisting phases in any ternary or pseudoternary solid. These are shown in Fig. 1(b) for Li„Cu«Mo6SS. The phase diagram [Fig. 1(a)] was determined by isothermal cuts at constant y. We intercalate Li into Cu«Mo6SS using Li/Li„Cu«Mo6S8 electrochemical cells while monitoring the cell's voltage, V(x). We correlate —(t)x/Bp, )» T, calculated from V(x), with the structure of Li„Cu«Mo6SS measured in situ by use of cells with x-ray windows. For samples with two phases, we determined the fraction of the sample in phase 1 (f) and 2 (1 —f) from the intensities of the Bragg peaks. Thus for a single cut across the phase diagram we can measure x, f, and V(x) at a fixed y. Figure 2 shows an example of our results for y = 2. Here, as Li intercalates, the sample changes from a single phase to a two-phase mixture and back again to