Valence ytterbium compounds

Figure 5.8. Lanthanide Ln203 oxides (cubic cI80-Mn2O3 type, on the left side) and Pb alloys (LnPb3, cubic cP4-type, on the right). The trends of the lattice parameter and of the heat of formation are shown (see the text and notice the characteristic behaviour of Eu and Yb). A schematic representation of the energy difference between the divalent and trivalent states of an ytterbium compound is shown. Apromff represents the promotion energy from di- to trivalent Yb metal, A,//11, and Ar/Ynl are the formation enthalpies of a compound in the two cases in which there is no valence change on passing from the metal to the compound the same valence state (II or III) is maintained.

A similar tendency of decreasing valence is observed for ytterbium compounds. In most compounds Yb atoms are in the normal trivalent state and only in compounds with 40 at.% of P an intermediate valence between 3+ and 2+ was found. 

The lattice parameter of the compounds R3 MC in the lanthanide series for a given element M changes linearly with the radius of the trivalent rare earth ions, following the lanthanide contraction regularity, except for europium and ytterbium which appear to have a variable-valence state. In the ytterbium-M (Al, Ga, In, Tl)-carbon system, the unit cell volume of the compound YbgMC, is larger than the value expected from the relationship mentioned above. In the europium-M(Al, Ga, In, Tl)-carbon system no compound was found. For the compounds of yttrium, as expected, the lattice parameter falls between those of the respective terbium and dysprosium compounds. 

Closely related to the heavy fermions and spin fluctuators are the valence fluctuation/intermediate valence materials. The origin of this phenomenon starts with cerium and its a 7 transformation (see sections 3.3.4 and 3.7.2). Today it involves many cerium materials and also compounds of samarium, europium, thulium and ytterbium. Because of the breadth of the subject matter and space limitations in this chapter we refer the reader to the following reviews Jayaraman (1979), Lawrence et d. (1981), de Chatel (1982), Coqblin (1982), Nowik (1983), Brandt and Moshchalkov (1984), Varma (1985) and Stassis (1986). 

Today the lanthanide eontraction is still one of the most important tools available to the scientist in applying systematics to the behavior of lanthanide materials. Deviations from the lanthanide contraction established for a given compound series gives a measure of anomalous valences for cerium, samarium, europiun, thulium and ytterbium (see section 3.2) which are important in evaluating the nature of these elements in valence fluctuation, heavy fermion, and spin fluctuation behaviors (see section 4.4.4). 

In addition to using the lattice parameter(s) of the RM compound to estimate the valence state of europium and ytterbium one can use magnetic susceptibility data (Gschneidner 1969b, Wohlleben 1981) and Lm absorption edges, XPS and UPS (Wohlleben 1981) to distinguish between the 4f" and 4f" configurations and intermediate valences. For europium one can also use Mossbauer isomer shift data (Brix et al. 1964, Clifford 1967, van Steenwijk and Buschow 1977, de Vries et aL 1984). 

The last chapter (134) in this volume is an extensive review by Colinet and Pasturel of the thermodynamic properties of landianide and actinide metallic systems. In addition to compiling useful theiTnodynamic data, such as enthalpies, entropies, and free eneigies of formation and of mixing, the authors have made an extensive comparative analysis of the thermodynamic behavior of the rare earths and actinides when alloyed with metallic elements. They note that when alloyed with non-transition metals, the enthalpies of formation of uranium alloys are less negative than those of the rare earths while those of thorium and plutonium are about the same as the latter. For transition metal alloys the formation enthalpies of thorium and uranium are more negative than diose of the rare earths and plutonium (the latter two are about the same). The anomalous behaviors of cerium, europium and ytterbium in various compounds and alloys are also discussed along with the effect of valence state changes found in uranium and plutonium alloys. 

The binuclear samarium and ytterbium complexes with mixed valence of Ln(II, IQ) (Me5C5)2Sm( i-Cp)Sm(C5Me5)2 [37] and (Me5C5)2Yb(ji-F)Yb(C5Me5)2 [46], are close to the being considered class of compounds in their nature. The complex with cyclopentadienyl bridge can be isolated from the mixture containing cyclopentadiene and... 

Lanthanide atoms are known to most likely exist in halogen compounds in a stable trivalent state. The thermodynamics of vaporization of LnX3 was recently studied fairly completely [1-4], Europium, ytterbium, and samarium are exceptions for which reliable thermodynamic characteristics of the vaporization process have virtually not been published. This primarily accounts for the incongruent character of evaporation [5,6] and the valence transformation Ln(III) Ln(II) in these compounds at high temperatures, which is in accord with the general tendency toward decreasing stability of the trivalent state in the lanthanide series [7, 8] La, Lu, Gd, Ce, Tb, Pr, Er, Nd, Ho, Pm, Dy, Tm, Sm, Yb, and Eu. Their thermal decomposition occurs due to the decreased stability of the state of Ln(III) in tribalogenide compounds [1] ... 

R2M12P7. The unit-cell plot of R2M12P7 compounds (table 80 fig. 91) probably indicates a different valence state fi-om R " for cerium (in compounds with Fe, Co, Ni, Rh), europium (with Ni) and ytterbium (with Rh and Ni) atoms. The magnetic moment on the transition-metal atoms is observed only in cobalt compounds, magnetic properties of all other R2M12P7 compounds are caused by rare-earth atoms. 

In the compound YbZn2P2 partial substitution of zinc atoms by copper leads to an increase of the ytterbium valence which rises to a value of 3+ at a composition of YbCuZnP2 ( Ueff = 4.71 (Zwiener et al. 1981). 

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