Rare-earth monochalcogenides

Rare-earth monochalcogenides are trivalent in the ground state and have metal-type conduction. These compounds, particularly the monosulfides, are highly stable in the thermal sense [8j. They melt without decomposing. For example, the vapor of lanthanum monosulfide consists mainly of LaS molecules [9]. Europium, ytterbium, and samarium monochalcogenides are semiconductors. All the monochalcogenides have the NaCl-type structure. 

RARE-EARTH MONOCHALCOGENIDES HAVING SEMICONDUCTOR CONDUCTIVITY 43... 

The experimental and theoretical aspects of the valence changes mainly in rare earth monochalcogenides have been reviewed by Jayaraman et al. (1975b), Varma (1976), by Campagna et al. (1976) and by Suryanarayanan (1978) (see also Parks, 1977). In discussing valence changes in rare earth compounds much material has been drawn from the first two reviews. 

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. 20.2. Schematic energy band diagram for rare earth monochalcogenides (Methfessel et al., 1966). On the left the atomic 5d states associated with the rare earth ion is shown. In the crystal these broaden to form the conduction band, the lowest having the Tj, symmetry. On the right the multiple splitting of the 4f states and its mixing with the crystal field split 5d states are shown. On the extreme right the absorption line shapes that result from 4f to 5d transitions are shown (from Holtzberg and Torrance, 1972).

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).

The valence change in the rare earth monochalcogenides shows itself in a rather spectacular manner in the optical reflectivity. Samarium sulfide which appears black in the semiconducting state turns brilliant gold in color after the 6.5 kbar transition (Kirk et al., 1972), SmSe a bright copper-like color on... 

Jayaraman, A., 1974, Electronic Phase Transitions in Rare Earth Monochalcogenides Under Pressure and Alloying, in Haschke, J.M. and H.A. Eick, eds.. Proceedings of the Eleventh Rare Earth Research Conference, Traverse City, Michigan, 1974, Vol. II (U.S. Department of Commerce, Springfield, Virginia) pp. 830-839. 

In and T1 monohalides are isoelectronic with the Sn and Pb monochalcogenides. The character of their structures, however, is different, although geometrically, the two structures are closely related. Common to both groups is, furthermore, the occurrence of NaCl-type phases and NaCl-type derivatives (see Table 51). CsCl-type modifications, however, are known only for the In and T1 halides, similar to the alkali halides and rare-earth monochalcogenides, but no such modifications are reported for the isoelectronic Sn and Pb chalcogenides. 

For data on the electronic structure of metallic GdSe and TmSe and of semiconducting SmSe, EuSe, and YbSe, see the individual sections. Reviews on semiconducting rare earth monochalcogenides have been given by Zhuze et al. [4, pp. 238/71], Jayaraman et al. [9], Guntherodt [10], and Zhuze [11] and include, for example, the following data based on optical studies ... 

A. Jayaraman, Electronic Transitions in Rare Earth Monochalcogenides, Comments Solid State Phys. 7 [1977] 135/48. 

A. Jayaraman, P. D. Dernier, L. D. Longinotti, Valence Electron Transition in Rare Earth Monochalcogenides Induced by Pressure, Alloying and Temperature, High Temp.-High Pressures 7 [1975] 1/28. 

A completely calculated energy band structure scheme of TmSe did not exist up to now. The models proposed for other rare earth monochalcogenides with NaCl structure, for example those of EuS, or SmS (see Rare Earth Elements C7, 1983, pp. 252 and 259, respectively) have been used to describe the properties of TmSe. Accordingly, the valence band is derived predominantly from the 4p states of Se and the conduction band from the 5d and 6s states of Tm. The cubic crystal field splits the 5d states into lower energy tsg (triplet) and... 

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]. 

---