Intermetallic valence changes

This intermetallic crystallizes in hexagonal Fe2P structure and is an established intermediate valence compound (v w 2.8) at ambient pressure which was suspected to move towards the full Yb configuration under volume reduction. We have said before that the use of the °Yb resonance does not allow the precise enough measurement of isomer shift. Thus a possible valence change can only be traced via... 

Finally we note that the cellular model also provides a hint as to the direction of possible valence changes in the intermetallics RM upon hydrogenation, when R is one of the elements Ce, Eu or Yb. Hydrogen absorption results in the formation of an appreciable area of contact between R atoms and H atoms at the expense of R-M contacts. The R atoms therefore will tend to adopt the same valences they have in the binary hydrides, i.e., Ce tends to be trivalent, Eu and Yb tend to be divalent. This agrees with experimentally observed valence changes (Buschow et al., 1977 Buschow, 1980b). 

The discussion of interatomic distances is less simple for intermetallic compounds than for pure metals among the complicating factors are the partial ionic character of bonds, the transfer of electrons, with consequent changes in valency, and the preferential use of the valencies of an atom in the formation of strong bonds rather than weaker ones. These factors, which of course participate in minimizing the energy of the system, usually operate to decrease the interatomic distances. Then-effects may be illustrated by some examples. 

An extensive system of metallic radii has been formulated on the basis of this equation. It is evident that there is some uncertainty about this empirical equation in particular, the value 0.60 A for the factor of the logarithmic term is somewhat uncertain but, in fact, the conclusions about electronic structure, bond numbers, and valence in metals and intermetallic compounds that have been reached through use of the equation would not be significantly changed by some change in the value of this factor. <... 

In this section, we show some examples of structural phase transitions of intermetallic compounds containing rare earth elements that show Kondo effect. The volmne and the valence of rare earth elements are often changed through a change in the characteristic temperature, the so-called Kondo temperature Tr, which is influenced significantly by appl)hng... 

Intermetallic compounds, various alloys, and additional semimetallic compounds of berkelium should be prepared and characterized to extend the knowledge of the physicochemical behavior of berkelium in these kinds of solids. Studies of such materials under pressure would be of interest in determining the effects of the non-berkelium component on physical properties such as bulk modulus (compressibility), pressure for the onset of 5f-electron delocalization, and possible volume collapse associated with a change in the metallic valence of Bk from 3 to 4. 

The existence of surface valence transitions in the rare earths and their compounds is well established, however it was not until quite recently that analogous effects were discovered for Ce and Ce intermetallics (Laubschat et al. 1990, Weschke et al. 1991, 1992). In the context of Ce the term valence transition will refer to the difference in hybridization strength that occurs between the a and y phases, rather than 4f population changes which can occur for the pure metals as noted above. The discovery of this behavior in Ce has important consequences for the spectroscopy of heavy fermion compounds inasmuch as it may help explain some of the discrepancies between model calculations and experiment noted above. 

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