Resistivity and lattice-constant measurements under high pressure on SmS show that a $4f\to 5d$ electronic transition in SmS occurs discontinuously at 6.5 kbar at room temperature, whereas such a transition takes place continuously over a broad pressure range in SmTe and SmSe. The pressure-induced semiconductor-to-metal transition in the Sm chalcogenides and their pressure-volume relationship are consistent with the conversion of ${\mathrm{Sm}}^{2+}$ to ${\mathrm{Sm}}^{3+}$. Optical-absorption measurements in these materials correlate well with the resistivity data under pressure. The semiconductor-to-metal transition in Sm chalcogenides appears to fit the model recently proposed by Falicov and Kimball for a system with a localized state and a conduction band.

• Received 18 September 1970

DOI:https://doi.org/10.1103/PhysRevLett.25.1430