Tm chalcogenides

In the Tm chalcogenides, the calculated energy differences between divalent... 

The Tm chalcogenides are standard model substances for the study of mixed-valence effects. 

Table 4.5. A selection of Si chemical shifts for chalcogenides and related compounds, in ppm with respect to TMS.

Substitution reactions involving silylated chalcogen reagents represent a powerful approach in metal-group-XVI cluster synthesis.Compounds such as E(TMS)2 and RE-TMS react readily with metal salts to form metal chalcogenide or chalcogenolate bonds (Equations (13)-(15)). The driving force for the reaction is the thermodynamically favorable formation of an X—Si bond and elimination of X-TMS (where X = halide, OAc, etc.). 

A peculiar class is formed by ternary metal chalcogenides M, tMQ2 (M is a 3d TM, M=Group 4-6 metal Q=S, Se, Te, x < 1). They formally can be regarded as intercalates of 3d metal between the chalcogen layers of parent MQ2. The resulting structure features linear chains of... 

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... 

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... 

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). 

Similar observations can be obtained for the thulium chalcogenides. TmTe is obviously divalent. With increasing internal pressure (decreasing lattice constant), the tendency towards trivalency increases. This means that TmS is purely trivalent and TmSe is mixed valent as is evident from the magnitude of their lattice constants. One of the first intermediate valent system studied by photoemission was the... 

Here / and are the relative fractions of the normalized absorption jumps in the 4f" and 4f" reference spectra and m is the number of outer valence electrons. This procedure was applied e.g. by Launois et al. (1980), Martin et al. (1980), Ravot et al. (1981) to extract the valence from the L, spectra of mixed valent lanthanide chalcogenides. Figure 14 reproduces the spectra of mixed valent TmSe together with its (isostructural) references TmS (nominally 3" ) and TmTe (nominally 2" ). Divalent Tm in TmTe exhibits a smaller and narrower line than trivalent Tm in... 

Wilson and coworkers assessed the performance of 13 hybrid-GGA, hybrid-meta-GGA, and double-hybrid DFT functionals on the cc-CA-TM/11 dataset [83]. The hybrid B97-1 [42, 84] functional and the double-hybrid functional mPW2-PLYP [85] were found to give the best performance over experimental results with mean absolute deviations of 7.2 and 7.3 kcal mol . The dataset (cc-CA-TM/11) is rather diverse and consists of AyH of 193 3d transition metal molecules (Sc(C5H5)3 and (Cr03)3, halides, oxides, and heavy chalcogenides, hydrides, nitrides, small clusters, metal carbonyls, and other coordination complexes) [86]. 

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