Pressure-Induced Electronic Collapse and Structural Changes in Rare-Earth Monochalcogenides

The compressibilities of the monotellurides of Pr, Sm, Eu, Tm, and Yb and the monoselenide and sulfide of Sm have been investigated to ∼ 300 kbar using high-pressure $x$-ray-diffraction techniques. SmTe, SmSe, TmTe, and YbTe show abnormal volume changes in the 20-50-, 15-40-, 15-30-, and 150-200-kbar regions of pressure, respectively. SmS shows an abrupt decrease in volume at 6.5 kbar. Since there is no change in structure, the anomalously large volume changes have been explained on the basis of a pressure-induced $4f-5d$ electronic collapse which involves a change in the valence state of the rare-earth ion from $2^{+}$ towards the $3^{+}$ state. The results of high-pressure x-ray studies on Sm chalcogenides are consistent with the conclusions drawn in the earlier work from high-pressure resistivity measurements. PrTe, SmTe, and EuTe exhibit a phase transition from NaCl-type to CsCl-type structure at pressures of about 90 ± 10, 110 ± 10, and 110 ± 10 kbar, respectively. It appears that a pressure-induced NaCl-to-CsCl transition may be commonly encountered in rare-earth monochalcogenides.