Invited Talks: Transition Metal Oxides - University Blog Service - The ...

The John B. Goodenough Symposium in Materials Science & Engineering –
In Honor of His 90 th Birthday
The University of Texas at Austin, Austin, Texas
October 26-27, 2012
Covalency influence on crystal chemistry of some inorganic compounds with
octahedral-site cations
Y. Shirako 1
1 Department of Chemistry, Gakushuin University, 1-5-1, Mejiro, Toshima-ku, Tokyo, 171-8588, Japan
Abstract Body in Space Below:
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The purpose of this presentation is to explain the local structures which is a network of octahedral
ionic clusters in inorganic compounds using a modification of the methods to predict a shape of severalatomic
molecules [1], and then to restrict the adopted inorganic crystal structure of each composition. This
way is a qualitative rule and yet easily available. In particular, the improved method may be effective to
classify pressure-induced structural phase transition sequences of various compounds.
In the covalency-dominant small molecules such as CO 2 and NH 3 , shape of the molecule depends on
number of electrons in valence shell orbitals of molecule (Walsh’s rule) [1]. Essence of the rule is
delocalization of the lone-pair electrons. The author extends the Walsh’s rule to be able to apply to
inorganic crystal structures with frameworks of the octahedra. The modified rule can be called “extended
Walsh’s rule”. In the extended rule, number of electrons in outer shell orbitals can be used as a qualitative
parameter to relate stability of the inorganic crystal structures with those compositions. For example, all
M-X-M angles in post-perovskite compounds AMX 3 are more bent to 90 º than those in the perovskite
which is less dense than the post-perovskite. The extended Walsh’s rule suggests that, if all 4-electrons on
p orbitals of closed-shell X can be delocalized to t 2g orbitals on M’s, linear M-X-M fragments are stable,
or else bent M-X-M fragments are stable since the electrons can be delocalized through e g orbitals on M’s.
Therefore, the M-X-M fragments in which the M ions have at least one of d orbitals with inability of
formation of -bond (filled orbitals), unlike Ti 4+ , Nb 4+ and Re 4+ , are expected to be bent by covalent
effect. Actually, post-perovskite compounds containing the -bond ability cations such as Ti 4+ and Zr 4+ in
octahedral-sites are almost not reported.
The previous articles have been reported on the similar topic [2, 3, 4]. Goodenough and Kafalas (1973)
[2] described discussion about contribution of covalency, electrostatic energy, and ionic size for crystal