Yb monochalcogenides

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. 

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). 

Similar qualitative observations with diamond anvil apparatus have shown that Yb monochalcogenides undergo the electronic transition near about 200 kbar pressure (Jayaraman et al., 1974). At high pressures YbS acquires a golden yellow color, YbSe a copperish luster and YbTe a purple color, reminiscent of the changes observed in SmS, SmSe and SmTe at high pressure. In table 20.3 data obtained from pressure experiments are presented. 

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

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. 

Through optical work on the monochalcogenides of Eu 9d), Yb (96) and Sm(9c,excitation energies which increase in the order -0, -S, -Se, -Te. This very unusual ordering customarily has been ascribed to an upward shift in the t g state, originating from a reduction in TO Dq with the increase in Uq. Such a description is however surely inappropriate for f -> d excitations in the present chalcogenides. 

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). 

At ambient conditions, the Yb monopnictides and monochalcogenides crystallize in the B1 structure. As outlined, the Yb pnictides are all foimd to be well described by the nominally trivalent scenario, where the effective valence varies from 2.88 in YbN to 2.69, 2.63, and 2.53 in YbP, YbAs, and YbSb, respectively. Experimentally, the position of the/ band is found 0.2 eV above the Fermi level in YbN, YbP, and YbAs (Degiorgi et al., 1990,1993). Other experiments have revealed heavy-electron behaviour in Yb pnictides (Ott et al., 1985 Sakon et al., 1992 Takeda et al., 1993), but this can be a reflection of sample non-stoichiometry (Degiorgi et al., 1990,1993). The discrepancy between the present electronic structure and the pictme provided by Degiorgi et al. (1990,1993) can be due to the LSD approximation, since the position of the narrow band in the theory is solely determined by the LSD potential (no correlation correction). LDA - - U calculations on YbN (Larson et al., 2007) include a positive correlation shift of the unoccupied f-states that leads to an ideal trivalent Yb ion in accordance with Degiorgi et al. (1990,1993). 

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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