Lanthanoid contraction effects

The Ln ions exhibit large ionic radii ranging from 117 pm for La to 100 pm for Lu, Table 2-1. Also well known, the Ln radii steadily decrease throughout the series as a result of the so-called lanthanoid contraction effect. These are very characteristic chemical features of the lanthanoid elements. 

The Post Transition Metal Effect, Lanthanoid Contraction and... 

It was also proposed that the significant s and p orbital contraction at row four (Cu) is caused by the post-transition metal effect (d contraction), caused by an increase of the effective nuclear charge for the 4s electrons due to filling the first d shell (3d). A similar interpretation is possible for the row six (Au). This effect is commonly called lanthanoid contraction due to the effect of filling the 4f shell. The traditional explanation for the smaller size of gold (compared to Ag) is the lanthanoid contraction. However, this effect is only sufficient to cancel the shell-structure expansion, to make Au (nonrelativistic) similar to Ag (nonrelativistic). 

Since much of the chemistries of Cd and Hg are distinct, we shall deal with the two metals separately. In making this decision, we are effectively saying that the consequences of the lanthanoid contraction are of minor significance for the heavier metals of the last group of the J-block. 

Oxidation number (IV), as in Z1O2 or Hf02. The oxidation states for Rf calculated relativistically to occur in water (III) and (IV), especially (IV), and possibly (II). Zirconium and Hf are so similar, with the lanthanoid contraction and relativistic effects canceling the otherwise expected larger size of the latter in its ions, that we treat them together. The same goes for many subsequent elements. In what follows here, the rarer Hf can often be substituted for Zr, except in comparisons or separations. 

On the contrary, a quite systematic behavior with the lanthanoid ion contraction is observed. A plot of the energy of CTI band versus the ionic radius at coordination number 8 of the Ln ions (16) indicates a linear decrease of this energy as the ionic radius increases. This is seen in Figure 3, and illustrates a bare size effect of the energy variation. The phenomenon is called "optical detection of the lanthanoid ion contraction by internal charge transfer absorption". 

Another explanation for the effect of the lanthanoid ion contraction on the energy of the NIR-CTI band may be found in the extent of doming of the porphinato ligands in the Ln(P)2 structures. A comparison of the inclination angles of the pyrrole units in and... 

Lanthanoid manganites, such as LaMnOj, NdMnOj and GdMnOj, are of potential value in solid oxide fuel cell cathodes. However, many of these phase show thermal contraction because of the diminishing Jahn-Teller distortion of the Mtf " cations as the temperature is increased. Such effects tend to rule out these materials for real cell applications, although A- and B-site substitution, as demonstrated for PbTiOj earlier, can ameliorate the problem. 

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