Eu-pnictides

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. 

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)... 

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

The extensive NMR work on the rare-earth pnictides (RN, RP, etc.) was reviewed in section 2.2.3 and 2.2.3.2. Various of the rare-earth chal-cogenides (oxides, sulfides, etc.) also form in the cubic rock-salt structure, in particular those of Eu and Sm. The europium monochalcogenides are semiconducting magnetic materials whose properties have been reviewed by Wachter (1972) also see ch. 19. Both NGR and NMR methods afford opportunity to study these compounds, and the work on oxides has been reviewed in the fore-going section (section 2.3.3). Table 18.24 provides a brief summary listing of experimental work. 

It is quite natural that not all authors and not all papers could be included in this review due to the limited amount of space and capability of the author. But since this article is not the only review and probably not the last one on this topic the problems and materials will always be illuminated from different points of view. With respect to magnetic semiconductors we want to recall that non-rare earth compounds like the Cr spinels (Haas, 1970) offer similar effects as the Eu chalcogenides. Also the group of the RE-pnictides may become ferromagnetic semiconductors with increasing purity and perfection of the samples, but at the moment their conduction mechanism is still in discussion (see also F. Hulliger, vol. 4, ch. 33 of this Handbook). 

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