Yb-pnictides

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). 

The Eu and Yb pnictides in comparison with the alkaline-earth pnictides. Phases marked with contain partly or exclusively (the NaCl-type phases)... 

Earlier measurements of the magnetic susceptibility of Yb pnictides yielded values for the paramagnetic moment and the paramagnetic Curie temperatures. These measurements are reported by Didchenko and Gortsema (1963), Busch (1967), Bodnar... 

The susceptibility measurements of Ott et al. (1982) are plotted in fig. 80. Ott et al. (1982) were the first authors to realize that at room temperature Yb behaves like a trivalent ion, but at low temperatures order did not set in. They concluded that the Yb pnictides are heavy-fermion materials, a break-through was the detection of order (at very low temperatures) by specific-heat measurements (Ott et al. 1985). Mossbauer experiments by Bonville et al. (1987) confirmed the heavy-fermion behaviour of YbP below 0.41 K. The ordered moment was estimated to be 0.8/tb, even lower than the moment of the crystalline electric field Tg ground state, which yields a moment of 1.33/xb- The moment depression is attributed to hybridization. Order was confirmed by NMR measurements (Takagi et al. 1988). 

Ternary compounds have been observed for the R-Si, Ge, Sn, Ga, In, Se, Te-Sb systems. The systems with arsenic and bismuth are characterized by formation of substitutional solid solutions between isotypic binary pnictides. No compounds have been found in the partly investigated ternary La-Al-Sb system at 773 K (Muravjova, 1971). For the Yb-Al-Sb, the formation and crystal structure of the Yb AlSbn have been reported (Fisher et al., 2000). 

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. 

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... 

Phases and structure types observed in binary rare earth pnictides. For the Eu and Yb compounds see table 33.2. Phases marked with contain (also)... 

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