The cation distribution in Co
2GeO
4-CoM
2O
4 and Co
2GeO
4-Co
2RO
4 systems (Co-I group), and in Ni
2GeO
4-NiM
2O
4 and Ni
2GeO
4-Ni
2RO
4 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 Co
1.2Zn
0.8GeO
4-Co
0.6Zn
0.4M
2O
4 and Co
1.2Zn
0.8GeO
4-Co
1.2Zn
0.8RO
4 (Co-II group), and in Ni
1.2Zn
0.8GeO
4-Ni
0.6Zn
0.4M
2O
4, Ni
0.8Zn
1.2GeO
4-Ni
0.8Zn
1.2RO
4 (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 Ge
4+ 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 Co
2GeO
4 and Ni
2GeO
4 compositions are observed. But in the Ti-system of Co-I and Ni-I groups, spinel type solid solutions in which Ti
4+ ions are substituted for Ge
4+ ions in the 4-coordinated sites are formed. In the composition range near the other end members of CoAl
2O
4, CoCr
2O
4, Co
2TiO
4 and Co
2SnO
4 (Co-I group), spinel type solid solutions in which Ge
4+ occupy the 6-coordinated sites are formed over the wide range. In the composition range near the end members of NiCr
2O
4 or NiAl
2O
4 (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.
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