Valence transition ofSm1−xLaxSand related compounds

The pressure-induced semiconductor-metal transition was studied experimentally in ${\mathrm{Sm}}_{1-x}{\mathrm{La}}_x\mathrm{S}$ and ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$. Transition pressure, reflectivity, and lattice compression were measured as a function of composition and hydrostatic pressure. While being strongly first order in Y alloys, the transition in La alloys softens and becomes continuous with increasing concentration. The observed behavior as well as various properties reported in the literature on other alloys can be interpreted in terms of a Falicov-Kimball model. In the theoretical part, we consider the extensions to this model which are to be made to include the effects of alloying. Calculations on the basis of this model agree semiquantitatively with experimental results. In particular, a "universal" critical concentration of $x_{\mathrm{cr}}\simeq 27\%$ is predicted for the changeover from a first-order to a continuous transition independent of the particular type of alloyed cation.