Fluctuating valence in SmS

Abstract

In the ground state of normal SmS, each rare-earth ion is in a (4f)^{6 7}F₀ state, with all the other electrons filling valence bands. The symmetry of the ground state is therefore known. From an examination of the observed properties it is reasoned that when the lattice collapses under hydrostatic pressure (preserving the cubic symmetry) the new ground state has the same symmetry as the ground state of the uncollapsed phase. Since the new ground state shows fluctuating valence, the problem is to incorporate this feature into its detailed description. It is shown that this can be done in a variety of ways. It is further shown that a band of low-lying states, each with J=0 at each site but with k not=0, can also be constructed, as well as higher bands with some sites having J=1. The picture which emerges is thought to be helpful in providing a theoretical framework within which to discuss the phenomena associated with fluctuating valence. Experimental tests of the model are suggested, from a theory which can be developed further if substantiated.