Divalent monochalcogenides

Monochalcogenides, LnZ (Z = S, Se, Te), have been prepared for all the lanthanides except Pm, mostly by direct combination.They are almost black and, like the monoxides, have the NaCl structure. However, with the exceptions of SmZ, EuZ, YbZ, TmSe and TmTe, they have metallic conductivity and evidently consist of Ln -t- Z ions with 1 electron from each cation delocalized in a conduction band. EuZ and YbZ, by contrast, are semiconductors or insulators with genuinely divalent cations, but SmZ seem to be intermediate and may involve the equilibrium ... 

Most monochalcogenides of the Group 3 metals adopt the rock salt (NaCl) structure. Note that the crystal chemistry of divalent europium is very similar to that of the alkaline earths, particularly strontium, as the radius of Eu is almost the same as that of Sr ". For the Yb compounds, the cell dimensions are practically identical with those of the Ca compounds. 

The monochalcogenides of Sm, Eu and Yb, and also YbTe, have lattice parameters which are larger than those of the neighboring compounds formed by typically trivalent rare earths. They are semiconductors or insulators and have no metallic luster. The rare earth is in a divalent state. The monochalcogenides of Eu are described in ch. 19. Moreover, the Sm, Eu and Yb monochalcogenides show a pressure induced semi-conductor to metal transition due to electron delocalization. This phenomenon is presented in ch. 20. Consequently, we shall only describe the monochalcogenides of trivalent lanthanides. 

The samples were characterized through lattice parameter, optical absorption, magnetic susceptibility and resistivity measurements. In table 20.2 some relevant data are given. Figure 20.1 shows a plot of the lattice parameter of the rare earth monochalcogenides, wherein the divalent cases are evident. If a transition to the trivalent state were to occur, the lattice parameter should be consistent with that of the other trivalent members shown in the fig. 20.1. 

The divalent rare earth monochalcogenides are semiconductors. The general features of the electronic band structure have been deduced from optical... 

Some properties of divalent rare earth monochalcogenides from Jayaraman et al., 1975 and Bucher et al., 1971. 

Fig. 2. Lattice parameters of Bl-type lanthanide and actinide compounds at ambient conditions, (a) Actinide monocarbides. Monocarbides are not known for the heavier actinides and for the lanthanides, (b) Monopnictides. Note the regular decrease with Z for lanthanide compounds (exception CeN). The lattice parameters of actinide compounds, in contrast, go through a minimum in the region Pa to Pu. See also sect. 4. (c) Monochalcogenides. Note the regular decrease with Z for lanthanide compounds (exception divalent lanthanides). The lattice parameters of actinide compounds, in contrast, %o through a minimum in the region Pa to Pu. See also sect. 4. ( ) monoxides of actinides only observed as surface layers or stabilized by impurities.

The Yb monochalcogenides are all diamagnetic, since Yb is divalent and thus nonmagnetic. 

Reason (b), which was the driving scientific reason for studying these systems is now recognized to be much more complex. Divalent uranium or other actinide species are almost impossible to stabilize, and so are not expected. Numerous studies, see the reviews byS choenes (1984), Brooks et al. (1984), and Baer (1984), have shown that all the monopnictides and monochalcogenides (and thus all the solid solutions as well) are essentially semimetals. It is generally accepted that the monopnictides are... 

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]. 

The divalent state of Sm, Eu, Tm and Yb should become unstable under high pressure. This can be foreseen from the fact that they have smaller ionic radii in their trivalent state and hence high pressure could favor the higher valence state. Recent high pressure experiments on a series of lanthanoid compounds, in particular the monochalcogenides Sm, Eu, Yb, Tm, have demonstrated the occurrence of such transformations. 

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