Study of the valence transition in Eu-, Yb-, and Ca-substituted SmS under high pressure and some comments on other substitutions

The effect of Eu, Yb, and Ca substitution for Sm in SmS (Sm_1-xMₓ²⁺S), as well as the effect of pressure on these compounds have been investigated. The mixed compounds, which are semiconducting, show no anomaly in the lattice parameter indicative of a valence transition at ambient pressure, and high pressure is required to affect the valence change. This shows that a favorable size of the substituent alone is insufficient for promoting the valence change, and that the electronic structure of the substituent has to be right. Resistivity and volume studies under high pressure show that the pressure-induced transition is first order in Sm_1-xEuₓS and Sm_1-xCaₓS for $x<0.3$, in Sm_1-xYbₓS for $x<0.60$ and continuous at higher concentrations. In Eu- and Yb-substituted SmS there are two $4f$ levels; one corresponding to the Sm 4f⁶ which lies closer to the $5d$ conduction band and the Eu 4f⁷, or the Yb 4f¹⁴ levels which are located deep in the energy gap. In the valence transition, only the Sm 4f⁶ level is delocalized. From the present as well as earlier results on substituted SmS, three generalizations are proposed, which seem to explain the behavior of rare-earth- and Th-substituted SmS, as well as SmS_1-xAsₓ compounds. To account for the first-order and continuous transitions, the presence or absence of S character in the conduction band is invoked.