Monochalcogenides, lanthanide

The experimental lattice parameters as a function of lanthanide atomic number show the famous lanthanide contraction, the decrease of the lattice parameter across the lanthanide series, with the exception of the two anomalies for Eu and Yb, as seen in Figure 1 (top panel). What is plotted there b actually the atomic sphere radius S (in atomic units) as a function of the lanthanide element A similar behaviour is abo observed, for example, for lanthanide monochalcogenides and monopnictides, whose lattice parameters are abo shovm in Figure 1 (middle and bottom panels). 

Some other lanthanide monochalcogenides such as SmS, or TmSe exhibit other interesting properties associated with valence fluctuations (Varma, 1976) and are the subject of much current interest (see ch. 20). 

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 ... 

At ambient conditions, the lanthanide monopnictides RX (X = N, P, As, Sb, Bi) and monochalcogenides RX (X = 0, S, Se, Te, Po) crystallize in the NaCl structure. Given the combination of an electropositive R-ion with an electronegative pnic-tide or chalcogenide, one might assume that an ionic picture can be applied here. But based on the observed properties, Rhyne and McGuire (1972) proposed a classification distinguishing between the so-called valence balanced compoimds... 

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. 

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.

Fig. 18. Isothermal bulk moduli of Bl-type compounds of the lanthanides and actinides, (a) Monopnictides with P, As, Sb, and Bi. (b) Monochalcogenides with S, Se, and Te.

Fig. 20. Pressure of upstroke transition versus Z for Bl-type phases of lanthanide and actinide compounds, (a) Monopnictides. (b) Monochalcogenides. Mononitrides and monoxides are not included because only isolated data are available. Indications of crystal structures in the graphs mark transitions to structures other than B2. Downward arrows for ThBi and ThTe indicate theoretical transition to B2 at negative...

Figures 20 and 21 compare the transition pressures of AnX compounds and of some LnX compounds. In particular, fig. 21 gives the dependence of the transition pressures on the cation-anion radius ratio at ambient pressure. This ratio is in general between 0.41 and 0.73 for Bl-type compounds. In the actinide monopnictides and monochalcogenides, as well as in the lanthanide monotellurides, the transition pressure is seen to increase with decreasing rdv/. This even includes a few of the...

A. A. Eliseev, Monochalcogenides of Lanthanides with Variable Valence, Zh. Neorgan. Khim. 29 [1984] 1650/4 Russ. J. Inorg. Chem. 29 [1984] 945/8. 

---