Substitution of Two Different Cations

Two different cations may be substituted into Fe304 for a variety of purposes. The experiments of Lotgering and Van Diepen for example, provided an interesting example of how judicious substitutions could be used to establish reaction (32) for B-site ions, and Mn substitutions into LiFc50g to suppress dielectric loss illustrates the use of a second-cation substitution to optimize a material for a specific application. In this final section on spinels, we choose to discuss the solid solution between Zn[Fc2]04 and Ge[Fe2]04, which would appear to offer the opportunity to study the evolution of the character of the B-site charge carriers with Fes -ion concentration. Ideally, the system would be Znil,GeJ [Fe2i2xFe2t2x]04- 

The cubic (ao = 8.441 A) end member Zn[Fe2]04 was seen to be n-type with mobile electrons moving at the bottom of a narrow 3d band. Transport properties 

Unfortunately it appears difficult to prepare the ideal mixed system. Miyahara and SaP observed large saturation magnetizations and high ferrimagnetic Curie temperatures indicative of the presence of FeA-O-Fee interactions. They used X-ray and magnetization data to estimate that 30 percent of the A-sites are Fe in Zno.5Geo.5Fe204. A study by Robbins has also indicated that Fe ions will tend to displace Ge ions on A-sites. 

Support for this tentative interpretation comes from a study by Tricker et al. on the system 2 [Zn(ilxV2Sn(i x)/2Fci-xFci ] in which they also find evidence from Mossbauer data for thermal excitation of the mobile ferrous electron to an unspecified conduction band. In this case the conduction band is probably associated with the Shb ions, which would have a 5s band edge close to the Fel 3d redox energy. 

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