Yb chalcogenides

In contrast to the pnictides, the equilibrium volumes of the Yb chalcogenides are accurately described assuming the divalent/ configuration for Yb. As pressme is applied to the Yb chalcogenides, the configuration becomes more and more favourable, and eventually a transition to an intermediate valence state occurs. Valence transitions in lanthanide systems will be discussed in the next section. 

TABLE 5 Calculated transition pressures for the electronic and structural phase transitions in the Pr pnictides and chalcogenides of Pr, Sm, Eu, Tm, and Yb. Also quoted are the volume discontinuities (relatively to the zero pressure equilibrium volume) at the transition. The notation (f") refers to SIC-LSD calculations with n localized f-electrons. Experimentally, the transitions of SmS are discontinuous, while those of SmSe, SmTe, EuO, EuS, and the Tm and Yb chalcogenides (at room temperature) are continuous. The volume changes for SmSe and SmTe as well as TmTe are obtained by extrapolation over the transition range. For Yb compounds, we quote the relative volumes at which the delocalization starts... 

In Table 5, the calculated and available experimental data are collected for the isostructural pressure transitions in Yb chalcogenides. The general trends are reproduced by the calculations. Only for the case of YbO, the calculated transition pressure seems to be significantly too high. In the intermetallic YbAls compound, pressure induces a continuous increase of valence (Kumar et al., 2008), which is in good agreement with the calculated rate of depletion of the 14th f-band in the SIC-LSD calculations. 

Wannier exciton in EuSe, as shown in fig. 19.13 by the intersection of the Urbach lines. It therefore appears possible that the sharp line inelastic light scattering is due to a resonance with this exciton. The temperature dependence and the polarization mode of the sharp line scattering is quite different from the one of the broad line scattering and no conclusive explanation is available as yet. There are, however, indications that sharp line spectra are also present in Yb chalcogenides (Vitins and Wachter, 1976b). 

The Yb chalcogenides exhibit anomalous compressibilities in the 150 to 200 kbar region (see p. 401), but the structure remains NaCl type in the entire pressure range. The anomaly is attributed to the Yb Yb valence change, Jayaraman et al. [1, p. 2515]. This is in agreement with optical studies, which indicate the semiconductor-metal transition in the region of 175 kbar, Narayanamurti etal. [2]. 

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 jumps in the lattice constants in Figure 1, seen for the elemental Eu and Yb, as well as at the chalcogenides of Sm, Eu, Tm, and Yb, are due to the change in valence from trivalent to divalent. If a transition to the trivalent state were to occur, the lattice constant would also follow the monotonous behaviour of the other lanthanides, as seen in Figure 2, where the ionic radii of trivalent lanthanide ions are displayed. For the pnictides, only CeN shows an anomaly, indicating a tetravalent state, whereas all the other compounds show a smooth, decreasing behaviour as a function of the lanthanide atomic number. 

For the lanthanide chalcogenides, it was demonstrated experimentally that Sm, Eu, and Yb are divalent in their sulphide, selenide, and telluride compounds, while Tm becomes divalent for the telluride phase only (Jayaraman, 1978, 1979). The SIC-LSD calculations find all the Eu chalcogenides, including EuO, to be insulators in the ferromagnetic state, and to have a divalent configuration... 

Figure 12 gives the lattice constants for the sulfides, selenides and tellurides of the lanthanides (Campagna et al. 1976). These constants show a regular behaviour except for some deviations for Sm, Eu, Tm and Yb. The regular curves are for the normal trivalent chalcogenides. The anomalous lattice constants occur for the divalent lanthanides, which due to the additional 4f electron have a larger ionic radius. Thus one sees that under normal conditions SmS, SmSe and SmTe are divalent (with respect to the Sm ion) but they can be driven into the mixed valent state by external pressure (Jayaraman et al. 1970). 

It is noteworthy that the two structures of the scandium derivatives, SC2X3, are both related to close packed structures of the general MX types, in which vacancies appear on the metal sites. It is a consequence of the affinity of scandium (also Yb(III) and Lu) for regular octahedral sites. This behavior is observed in the lower chalcogenides as well. 

Solid solutions formed from the CaX compounds by addition of R2X3 chalcogenides of the heavy lanthanides (fig. 11) have especially large composition ranges. They are least in the case of the sulfides, with a maximum composition in R of about 48 at%. They are face-centered cubic for Sm, Gd, Dy, Er and Y and show a transition to a rhombohedral distortion in the cases of Yb and Lu, at a R concentration of about 30 at% (Patrie et al., 1965, 1967). 

O Brien P., McAleese J. Developing an understanding ofthe processes controlling the chemical bath deposition of ZnS and CdS. J. Mater. Chem. 1998 8 2309 Ohishi Y., Mori A., Kanamori T., Fujiura K., Sudo S. Fabrication of praseodymium-doped arsenic sulfide chalcogenide fiber for 1.3-/rm fiber amplifiers. Appl. Phys. Lett. 1994 65 13 Orignac X., Barbier D., Du X.M., Almeida R.M., McCarthy O., Yeatman E. Sol-gel sUica/titania-on-silicon Er/Yb-doped waveguides for optical amplification at 1.5 /xm. Opt. Mater. 1999 12 1... 

Eu[ESi(SiMe3)3]2G5MPE)2)2(li-DMPE) result from the bis-silylamide, the Yb complexes eliminating E[Si(SiMe3)3]2 and giving the divalent chalcogenide as a dark, microcrystalline... 

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