Phase diagram of the LaS-SmS system

Abstract In the lanthanide series the energy of the divalent and trivalent states of some elements are so close together that valence instabilities occur. SmS can have different valencies in the same crystal structure owing to changes in pressure in addition to substitution. The semiconductor-metal transition in SmS is investigated by the replacement of samarium by La 3+ in the LaS-SmS system. The phase diagram shows complete miscibility in agreement with the continuous change in the lattice parameter from LaS to SmS. It is probable that samarium is in a state intermediate between 2 + and 3 + because of the electronic configuration rather than the pressure effect induced by the LaS lattice. The lattice parameter changes continuously with the substitution of samarium by lanthanum, and discontinuously when samarium is substituted by other rare earth metals.