Rare earth compounds types

Other rare earth compounds of this type are also prepared by similar methods. 

RU2EU2O7. — Rare earth compounds of the pyrochlore type, Ru , belong to the space group FdSm (Oj) and show analogy with the structures of U02 and U4O8. The lattice parameter [333] of Ru2Eu2C>7 is a = 10.252 A. 

Th02—ternary oxides or oxide phases with tetravalent americium are stabilized. The solid-state reaction of Am02 with most group V elements yields compounds with trivalent americium which are isostructural with the analogous rare earth compounds. In the last types of reactions americium exhibits a typical actinide behavior. 

Experimentally ferro- and antiferroelectricity of this type was discussed for rare earth compounds in [20-22]. Unoki and Sakudo [20] were the first who found the antiferroelectric anomaly in the DyV04 crystal that is simultaneously ferroeleastic and antiferroelectric. 

Pure rare-earth compounds are unknown in nature minerals (see also appendix I) usually contain groups of rare earths because of their nearly identical chemical character. It is not surprising, therefore, that a systematic investigation of the individual binary silicate systems RE2O3—Si02 revealed a large number of new phases and unknown structural types. These experiments were started about 15 years ago, when modern methods of ion separation were developed and provided very pure rare-earth elements. 

The term mixed valence is widely used in the literature to describe a phenomenon rather different from that considered here. Typically it refers to a metal (or a rare-earth alloy, or a compound such as SmS) which features a broad conduction band (formed by the overlap of s, p, or d orbitals), and a very narrow band such as an /-band, slightly above the Fermi level. This band becomes partially populated, hence the mixed, or fractional, valence. Fluctuations with time, and various degrees of localization in space, result from electron-phonon interaction. A useful review appeared recently.Many of the ideas used in this field parallel those used in other more chemical types of electron transfer. Recent articles on mixed-valence as a polaronic effect and on local polaronic effects exemplify this, and the dynamical properties have been discussed. Other recent reviews of this area deal with spectroscopic techniques and with mixed valency in rare-earth compounds. ... 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

Among the various classes of rare-earth compounds the fee (NaCl type) R-monosulphides are the simplest from a structural and magnetic-exchange point... 

Deformation potential coupling constants are of the order of fip, (Ziman 1960). To observe deformation potential effects in the temperature dependence of elastic constants several conditions have to be met as discussed above dpA(,(0) must be large and - Eq has to be of the order of k T. This excludes normal metals and only d-band metals with rather narrow bands can exhibit this behavior. Typical examples have been given above. In intermetallic rare-earth compounds simple density of states arguments show why elastic constant effects can be observed only for CsCl-type and Th3P4-type materials. In table 4 electronic specific heat values are listed for various rare earth compounds. This is an updated list of a previous work, see Liithi et al. (1982). This table indicates that monopnictides and monochalcogenides have smaller values of y than CsCl- and Th3P4-structure materials, i.e., the 5d band of the former structure is more hybridized than in the latter. 

De Haas-van Alphen (dHvA)-type quantum oscillations as observed in the sound velocity and sound attenuation provide important information about the Fermi surface and the electron-phonon interaction (Roberts 1968, Fawcett et al. 1980). This technique has been successfully applied to intermetallic rare-earth compounds as discussed below. Recent progress in dHvA techniques for heavy-fermion materials (Taillefer et al. 1987, Reinders et al. 1986) should make similar MAQO experiments also possible. Compounds studied so far are LaAg, LaB5, LaAlj, RBj, CeSn3, CeB, CeCu and CePbj. 

---