In hexaferrites, a Ba2+ or Sr2+ ion substitutes an oxygen ion (O2-) in some of the hexagonal close-packed oxygen layers; this is possible seemingly due to the similarity of the ionic radii (1.35 A for Ba,1.18 A for Sr, and 1.4 A for O). Also, Ca2+ with an ionic radius of 1 Angstrome (in 6-coordinated site) was reported to substitute Ba or Sr in the layers of, for example, BaM and SrM hexaferrites. Nd2+ has the right charge and appropriate radius to replace Ba or Sr in the layer. However, this ion was not reported to occupy 6-coordinated sites according to Shannon, Acta Cryst. A32 (1976) 751. On the other hand, Nd3+ ion can occupy a 6-coordinated site, and has a radius of 0.98 A, which could be high enough to substitute a Ba ion in BaM layers.However, such a substitution would introduce charge imbalance. To maintain charge balance, Nd3+ replacing an Fe3+ at octahedral interstitial sites seems to be an option.However, the radius of this ion seems to be somewhat too high to fit into such an interstitial site. So, in light of reported results on Nd-substituted hexaferrites, one may ask whether Nd really inter the hexaferrite lattice, and if so, in what valency, and at what sites?
Dr Sammy , at presently, all the high quality permanent magnet are made up of Nd substituted hexaferrites. So, you need not worry about all these.
Surely Nd will increase the coercivity as it contibute strongly to the anisotropy Any RE for that matter except Gd would do that.
Thank you all for the answers. But I was really puzzled with the valence state of Nd in the hexaferrite lattice, and the preferential site occupation. Since if Nd occupies specific sites in the hexaferrite lattice, the magnetocrystalline anisotropy would be impacted depending on the preferential sites. In light of the argument raised in the original question regarding the ionic radius and the coordination of sites occupied by Nd, could any one explain the correlation between the increase in coercivity with Nd-doping and the substitution at specific sites of the hexaferrite, or (for example) the presence of secondary phases which could, for some reason, enhance the coercivity?
This is a good question. In reality one cannot replace Ba a divaalent wit a trivalent Nd. It is possible that some Fe trivalent could get replaced. But the site is too small. therefore in a polycrystalline material may a small doping would do but foe more then it should be a precipitate in the grainboundaries. it is a conjecture. Also any such impurity in the grain boundary could also impede wall motion which in turn could increase coercivity. In any case a detailed analysis using microscopes etc might reveal something.
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