Study of Germanium Spinels (Part 3)

The cation distribution in Co2GeO4-CoM2O4 and Co2GeO4-Co2RO4 systems (Co-I group), and in Ni2GeO4-NiM2O4 and Ni2GeO4-Ni2RO4 systems (Ni-I group), where M=Al or Cr, and R=Sn or Ti, has been studied by lattice constant variation, reflectance spectra and pattern fitting method of X-ray diffraction diagrams. The cation distribution in Co1.2Zn0.8GeO4-Co0.6Zn0.4M2O4 and Co1.2Zn0.8GeO4-Co1.2Zn0.8RO4 (Co-II group), and in Ni1.2Zn0.8GeO4-Ni0.6Zn0.4M2O4, Ni0.8Zn1.2GeO4-Ni0.8Zn1.2RO4 (Ni-II group), where M=Al or Cr, and R=Sn or Ti, has also been studied.These results are summarized as follows.(1) Spinel type solid solutions with the 6-coordinated sites occupied by Ge4+ ions are formed over the entire ranges of Co-II and Ni-II groups.(2) In the Cr-, Al- and Sn-systems of Co-I and Ni-I groups, no spinel type solid solutions near Co2GeO4 and Ni2GeO4 compositions are observed. But in the Ti-system of Co-I and Ni-I groups, spinel type solid solutions in which Ti4+ ions are substituted for Ge4+ ions in the 4-coordinated sites are formed. In the composition range near the other end members of CoAl2O4, CoCr2O4, Co2TiO4 and Co2SnO4 (Co-I group), spinel type solid solutions in which Ge4+ occupy the 6-coordinated sites are formed over the wide range. In the composition range near the end members of NiCr2O4 or NiAl2O4 (Ni-I group), spinel type solid solution are formed over narrow range. The cation distiributions in the spinel type solid solutions were accounted for by the site preference of cations and Pauling's electrostatic valence rule. We proposed that this rule is one of the most important factors for determining the cation distribution in the spinel structure.