Continuous and Discontinuous Semiconductor-Metal Transition in Samarium Monochalcogenides Under Pressure

Resistivity and lattice-constant measurements under high pressure on SmS show that a 4f→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 Sm²⁺ to Sm³⁺. 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.