Cerium subgroup

Rare Earths are produced primarily from three ores, monazite, xenotime, and bastnasite. Monazite is a phosphate mineral of essentially the cerium subgroup metals and thorium -(light rare Earths, Th) P04. The composition of monazite is reasonably constant throughout the world, with almost 50% of its rare Earth content as cerium and most of the remaining 50% as the other members of the cerium subgroup. Xenotime, like monazite, is a rare Earth orthophosphate but contains up to 63% yttrium oxide and also a markedly higher propor-... 

Thus the energy of formation of complexes in the rare earth series should vary in the same manner as the molar volumes and basic properties of hydroxides should vary in the opposite manner. It is clear from Fig. 3.14 that the ionic radii, molar volumes and the basic properties of hydroxides show negative deviations. The central members of the cerium subgroup show maximum deviations. The deviations shown by the yttrium subgroup are smaller than the cerium subgroup. This situation is analogous to the behaviour of transition... 

The elements 57-62 are known as the cerium subgroup, and 63-71 as the yttrium subgroup. Yttrium, atomic number 39, although not a rare-earth element, is found associated with the rare earths and is separated only with difficulty. 

Compounds of equiatomic composition with rare-earth metals of the yttric and cerium subgroups crystallize with the ZrNiAl type whereas scandium germanide of equiatomic composition forms the MgAgAs type. 

The CeosSCd sGed compoxmd has no equivalent in the Nd-Sc-Ge system. The Ri.25Sc3Ge4 compound exists with all the rare-earth metals of the cerium subgroup (La, Ce, Pr, Nd, Sm, Eu). 

Considering the crystal structures of the compounds it is possible to conclude that the presence of scandium leads to the formation of various kinds of superstructures to the structure types of the binary compounds (Sm5Gc4, Ca2As). RScGe ternary germanides are isostructural and are formed with the elements of the cerium subgroup (La, Ce, Pr, Nd, Sm, Eu), i.e. at the ratio r /rsc 1.1 for R=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu the compounds of this composition are not observed. 

Babizhets ky, V.S., 1995, Interaction of rare earth metals of cerium subgroup with nickel and phosphorus, Thesis for obtaining scientific degree of Candidate of Chemistry (Ivan Franko L viv State University) 176pp. In Ukrainian. 

Hordijenko, S.P, and VF. Hol nik, 1977, Thermo-dynamie funetions of solid phosphides of cerium subgroup rare earth metals and gadolinium, in Obtaining, Properties and Utilization of Phosphides (Naukova Dumka, Kiev) pp. 66-71. In Russian. Hordijenko, S.P, and K.E. Mironov, 1983, Izv. Akad. 

Ammino-derivatives op Subgroup A—Derivatives of Titanium Kilts, Zirconium Salts, Cerium, and Thorium Halides. 

Lanthanide elements (referred to as Ln) have atomic numbers that range from 57 to 71. They are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). With the inclusion of scandium (Sc) and yttrium (Y), which are in the same subgroup, this total of 17 elements are referred to as the rare earth elements (RE). They are similar in some aspects but very different in many others. Based on the electronic configuration of the rare earth elements, in this chapter we will discuss the lanthanide contraction phenomenon and the consequential effects on the chemical and physical properties of these elements. The coordination chemistry of lanthanide complexes containing small inorganic ligands is also briefly introduced here [1-5]. 

Note that the radii of the first four elements (La, Ce, Pr, Nd) are relatively large and also are more similar to each other than to the remainder of the series. This group is sometimes called the cerium group while the others are lumped as the yttrium group. Recall that the relative distributions of REs in the various ore types, described in the preceding section, seem to mirror this subgrouping scheme. Note also that the radius of Y is between Ho and Er. 

R-R -Ge systems. The formation of compounds with the Ceo.8Ro.2G composition (structure type CrB) is characteristic of elements of the yttric subgroup (Y, Dy, Ho, Er, Tm). An increase in the volume of the unit cell in the series of the isostructural compounds is coimected with the ability of the cerium atoms to adopt an intermediate state. 

The complete group of rare earth metals consists of the three elements scandium Sc, yttrium Y and lanthanum La and the fourteen lanthanides cerium to lutetium. The total number of REMs is thus 17. They are, for practical reasons, divided into two subgroups, the cerium group with the lighter elements and the yttrium group with the heavier ones (Table 17.1). 

The only practical chemical methods for the determination of any of the individual rare earths from each other and, especially, adjacent lanthanides are based on the oxidation of cerium(III) to cerium(lV) and the determination of Ce(IV) by gravimetric or volumetric methods as discussed in previous sections. Fractional crystallization methods can be used to separate the rare earths into subgroups, but clean separations of adjacent rare earths are not feasible. As mentioned previously, Eu, Sm, and Yb can be reduced to the plus two oxidation state, but only Eu(II) can really be satisfactorily determined by chemical methods. 

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