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Structural data, retrieved from the Inorganic Crystal Structure Database, have been used to study the effective shape and sizes of six coordinated Cu 2+  and Mn 3+  ions in oxides and fluorides. The distortions of the coordination octahedra for these two metal ions, essentially due to the structural implications of the Jahn-Teller effect, are assumed to be interpretable in terms of a nonspherical (ellipsoidal) effective shape of the metal ions. For octahedra with approximate tetragonal  D  4 h   symmetry (rotational ellipsoid) a reasonable one-parameter correlation function is shown to be 2/ r  4  2  + 1/ r  2  2  = 3/ r  0  2 , where  r  4  is the ionic size perpendicular to and  r  2  the size along the rotation axis. The single parameter  r  0  to be determined, estimated to 0.66(1) Å for Cu 2+  and 0.59(1) Å for Mn 3+ , represents the size of the spherical case. From the correlation function, possible estimates of the effective sizes for square planar four coordination (and linear two coordination) can be obtained by extrapolation, giving 0.54 Å and 0.48 Å for four coordinated Cu 2+  and Mn 3+ , respectively. According to the correlation function the radii decrease with the square root of the coordination number.

The effective shapes and sizes of Cu 2+ and Mn 3+ ions in oxides and fluorides