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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.
Chatterjee et al. (Fri,) studied this question.