By use of x-ray-diffraction and electrochemical techniques we have measured the phase diagram of the pseudoternary solid, ${\mathrm{Li}}_x{\mathrm{Cu}}_y{\mathrm{Mo}}_6{\mathrm{S}}_8$. We compare our measurements of the Li chemical potential, $\mu$, and the response function $-{\left(\frac{\partial x}{\partial \mu }\right)}_{y,T}$ near phase boundaries to predictions of theory.

• Received 22 October 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.1432