Binary rare-earth compounds

Binary rare-earth compounds such as carbides, sulfides, nitrides, and hydrides have been used to prepare anhydrous trihalides, but they offer no special advantage. Treating these compounds at a high temperature with a halogen (98) or hydrogen halide (115) produces the trihalide, e.g.,... 

Figure 5.15. Binary rare earth compounds. According to a suggestion by Gschneidner and Daane (1988), the bonding character (percentage of metallic, covalent and ionic bonding) is shown for the R compounds with the elements of the various groups from the 7th (Mn group) to the 17th...

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. 

Solid state physical and chemical properties of binary rare earth compounds involving non-metallic elements have attracted considerable scientific interest in recent years (see volumes 3 and 4 of this series). Some of the compounds exhibit... 

Fig. 17. The bonding character in binary rare earth compounds formed with the elements from the Vll A (Mn, Tc, Rh) group to the VIIB (halides) group of the periodic table.

With over 3000 binary rare earth compounds some efforts have been made to ... 

The number of all possible magnetic rare earth compounds is too vast to attempt a complete discussion in this chapter. We shall, instead, attempt to describe the results for the simplest and most well-characterized (mainly binary) rare earth compounds, and concentrate on systems for which neutron diffraction... 

Although the binary rare earth compounds are not the subject of our survey, we have considered a unique one-YGe (CrB structure type). The DEDD for germanium chains is shown in fig. 207. Any maximum between the germanium pairs is not observed. The conclusion (about the absence of the covalent Ge-Ge interaction for the binary YGe compound contrary to that found in the ternary YNi2Gc2 phase) is reasonable because of ... 

The crystal chemical concept of structural series has been used before to correlate the structures of certain binary rare earth compounds (see for example Parthe and Lemaire, 1975). For ternary structures the concept of intergrown structure segments is particularly useful (see for example tables 3, 10, 12, 13, 15, 16, 21). 

Similar to other binary /(-p-clcment systems, the formation of binary rare earth - antimonides with a simple stoichiometry is a characteristic feature of these systems. The largest number of structure types formed was encountered for the group of RSb2 compounds (4 members). The polymorphic modifications were observed for GdSb2 and TbSb2 as well as the RSb (R = La, Ce) and RsSb (R = Yb, Y and Sc) compounds were noted to undergo the solid state transformations. 

With the hope that there exist copper oxide compounds with higher Tc, we have undertaken an extensive matrix experiment, in which composition and processing conditions were varied for a wide variety of rare earth-alkaline earth-copper-oxide combinations. Most of the higher Tc, reports have involved Y-Ba-Cu-O compositions, and so this system received the major portion of our attention. We also examined other isoelectronic elements such as Sc, La, and Lu substituting for Y and, to a lesser extent, Sr for Ba. Binary rare earth-... 

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. 

Catalysis on other binary, non-metallic rare earth compounds... 

A more thorough evaluation of surface chemistry on binary, non-metallic rare earth compounds, excluding the oxides, is not possible at present because of the lack of available data. We note, however, that investigation of surface reactivities on some of these compounds, in particular those showing intermediate valence phenomena (Campagna et al., 1976), should provide an exciting new field of activity in the surface chemistry of rare earth materials. 

The formation of the binary rare earth carbides has been summarized in table 2. The crystal structure and lattice parameter data listed in this table were quoted from the review Critical evaluation of binary rare earth phase diagrams (Gschneidner and Calderwood 1986). The listed lattice parameters were assessed by them and are the mean values when more than one acceptable set of data were presented for an individual compound. In this section, the crystal structures of each binary rare earth carbide will be evaluated in detail. 

As noted earlier there are over 3000 binary rare earth intermetallic compounds, many of which exhibit interesting behaviors, but there are only a few, in addition to those noted earlier, which exhibit sufficiently unusual properties to be mentioned here. Undoubtedly in the eyes of many of the readers there will be compounds with which they are familiar and which they believe deserve mention here. Indeed the reader may be correct but in the authors limited views and experience the following have had the greater impact on us. 

G. Meyer and M.S. Wickleder have described the synthesis and structures of the many types of rare-earth halides. They have classified them as simple, complex, binary, ternary, quaternary, multinuclear complex, and other categories needed to deal with this most studied of the rare-earth compounds. The structure types are skillfiilly illustrated to show the elementary architecture of each type. 

Numerous magnetic, binary, rare earth (RE) transition metal (TM) compounds exist, of which the C05RE... 

While perhaps with not as wide variety as the borides, the rare earth elements also form interesting compounds with carbon to form the rare earth carbides. The phases are particularly rich for the relatively metal-rich carbide compounds. Adachi etal. have written a detailed, 129-page long review on the rare earth carbides, while Gschneidner and Calderwood have comprehensively reviewed phase diagrams and lattice constants of binary rare earth carbides. ... 

Formation of superstructures of the binary rare-earth germanides The crystal structure of Ho26Pd4(Pd,Ge)i9 jc represents a substitutional variant of the Er26Ge23-j structure type. The difference between the two structures lies only in the replacement of germanium atoms by palladium atoms in positions 2(c) and 8(j) and the presence of a statistical distribution of Ce and Pd atoms in positions 2(c) and 8(i). Other isotypical ternary phases are not known. Only the compound Ce26Li5Gc23 -y, where the Li atoms occupy the 2(b) and 8(i) positions which are vacant in the previous phases, has a similar structure (table 5). 

We present below a short list of representative intermetallic compounds grown from various solvents. Binary compounds grown from one of their eonstituent elements have not been included. For example, these include USuj from Sn, UGHj from Ga, CelUj from In and TiBcj from the middle of the Ti-Be phase diagram. In our list, R means rare earth, although the rare earth compounds referred to cannot always be grown for all the rare earths. 

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)... 

Among the activated binary rare earth germanates, only the luminescence properties of Y2(Ge04)0 Tb have been recorded. In this compound, terbium shows its usual intense yellow-green emission at wavelength 550 nm (Bondar, 1979). 

Structures of binary rare earth-3d transition metal compounds... 

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