Pressure-volume monochalcogenide

Fig. 9.12. Pressure-volume relationship for Eu monochalcogenides (left fig.) and a plot of the log of bulk modulus against log of specific volume for R.E. monochalcogenides showing the straight line relationship. In the P-V relationship of Eu monochalcogenides the discontinuities are due to NaCI to CsCl transition. In EuO the first discontinuity is due to a valence transition in Eu (from Jayaraman et al., 1974).

Fig. 20.4. Pressure-volume relationship for rare earth monochalcogenides. The anomalous relationships are due to valence changes of the rare earth ion towards the trivalent state. The discontinuity in SmTe and EuO near 400 k bar are due to NaCl-CsCl transition (from Jayaraman et al., 1974).

Pressure-volume data for the Yb monochalcogenides and TmTe also show anomalous compression regions which have been attributed to a change in the valence state (see fig. 20.4) presumed to take place continuously with pressure (Jayaraman et al., 1975a). 

Fig. 221 shows the pressure-volume relationship up to 250 kbar. The anomaly in the 150 to 200 kbar region is due to the valence change from Yb " to Yb ". The bulk modulus Kq = 610 50 kbar is derived from an empirical relationship between the bulk modulus at atmospheric pressure and the molar volume for divalent rare earth monochalcogenides, Jayaraman et al. [3, pp. 2514/5], [4, pp. 2, 9], for the p-V diagram, also see Yayaraman [7, 8]. 

Among the chalcogenides by far the most extensively investigated compounds are the Eu monochalcogenides. Most recent pressure studies, not included by Bloch and Pavlovic (1969) are of Lara and Xavier (1974) on EuO and EuS, Hidaka (1970-71) and Schwob (1969). The pressure data of interest are summarized in table 9.9 which also shows the exchange interaction coefficients and the logarithmic volume derivatives. 

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