Pressure-volume relationship and pressure-induced electronic and structural transformations in Eu and Yb monochalcogenides

The pressure-volume relationships for EuSe, EuS, YbSe, and YbS have been obtained from high-pressure x-ray-diffraction studies to nearly 300 kbar. Like EuTe, EuSe and EuS exhibit normal compression and undergo the NaCl-to-CsCl-type transition at about 145 and 215 kbar. The compression curves of YbSe and YbS are anomalous in the 150-200-kbar region, which we believe is because of a change in the valence state of Yb from $2^{+}$ towards the $3^{+}$ state. Among the Eu monochalcogenides EuO is the only substance that shows a valence transformation (∼300 kbar) followed by a NaCl-to-CsCl-type transition (∼400 kbar). However, all the Yb monochalcogenides undergo the valence transformation continuously with pressure. The bulk moduli evaluated from the experimental $P-V$ data and the measured pressure coefficients of the energy gap between the $4f$ level and the conduction-band edge from optical-absorption studies of divalent rare-earth monochalcogenides are compared. These data are used to rationalize the occurrence or nonoccurrence of the valence transitions in this series of compounds.