Lanthanoids magnetism

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

The neglect of the ligand field in Eq. (10.1) leads one to expect no satisfactory account of the experimental magnetism of lanthanoid complexes either. It is an empirical fact, however, that Eq. (10.1) accounts extremely well for observed magnetic moments in most lanthanoid compounds. We compare typical experimental moments for lanthanoid complexes with those calculated from Eq. (10.1) in Fig. 10-3. Significant discrepancies occur for/ and/ species and we will comment on these shortly. 

Altogether, we can say that the success of Eq. (10.1) in reproducing the magnetic moments of lanthanoid complexes is due entirely to the very small magnitude of the ligand-field splittings and so, in turn, to the contracted nature of the/orbitals. 

Overall, then, the magnetic moments of all lanthanoid complexes are well reproduced without reference to the ligand field inter alia, we can infer that the ligand-field splittings in /-block complexes are no greater than about kT at room temperature. 

In yttrium iron garnet Y3Fe5012 ( YIG ) a ferrimagnetic coupling (superexchange) is active between the octahedral and the tetrahedral sites. Since the tetrahedral sites are in excess, the magnetic moments do not compensate each other. The magnetic properties can be varied by substitution of yttrium by lanthanoids. 

However, there does not seem to be sufficient experimental data of sufficient accuracy to have made that elaborate procedure worthwhile. It has proved possible to use the diagrams published for the lanthanoid ion case with some success for treating the magnetic properties of actinoid complexes in at least some cases.49-85... 

Table 7 The Magnetic Properties of Some Lanthanoid Element Compounds...

Exhaustive compilations of magnetic properties of transition metal complexes are available.166-192-194 The following sections describe very briefly the magnetic behaviour of a selection of complexes or groups of complexes, from the various electronic configurations which arise for the transition metals. They are intended to be illustrative of the points made in the earlier sections no attempt at completeness of coverage is made. More detailed and intensive accounts of magnetic properties of transition metal complexes are available.99 166 169-174-175 186-195-197 No mention is made of results for lanthanoid and actinoid elements, nor of ESR g-values. 

Andrews, PC., Beck, T., Fraser, B.H., etal. (2009) FunctionaUsed P-diketonate polynuclear lanthanoid hydroxo clusters synthesis, characterisation, and magnetic properties. Polyhedron, 28, 2123-2130. 

Atom and ion sizes and the lanthanoid contraction Spectroscopic and magnetic properties Sources of the lanthanoids and actinoids... 

Worked example 24.2 Calculating the effective magnetic moment of a lanthanoid ion... 

The spectroscopic and magnetic properties of actinoids are complicated and we mention them only briefly. Absorptions due to 5/-5/ transitions are weak, but they are somewhat broader and more intense (and considerably more dependent on the ligands present) than those due to 4f-4f transitions. The interpretation of electronic spectra is made difficult by the large spin-orbit coupling constants (about twice those of the lanthanoids) as a result of which the RusseU-Saunders coupling scheme partially breaks down. 

Magnetic properties show an overall similarity to those of the lanthanoids in the variation of magnetic moment with the number of unpaired electrons, but values for isoelec-tronic lanthanoid and actinoid species, e.g. Np(VI) and Ce(III), Np(V) and Pr(III), Np(IV) and Nd(III), are lower for the actinoids, indicating partial quenching of the orbital contribution by crystal field effects. 

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