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Ceramics and glasses

In the perovskite structure (ABO₃), as in BaTiO₃, what is the oxygen coordination number around the B-site cation (Ti⁴⁺)?

  1. A4
  2. B12
  3. C6
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Answer: C. 6

In the ideal perovskite structure, the small B-site cation (Ti⁴⁺) is surrounded by 6 oxide ions, forming TiO₆ octahedra that share corners and link up in three dimensions. The large A-site cation (Ba²⁺) sits in the space between the octahedra, surrounded by 12 oxide ions. Twelve is the A-site coordination number, an easy value to confuse. Four is the tetrahedral coordination number of the wurtzite and zinc blende structures. In BaTiO₃, a slight displacement of Ti from the center of the octahedron produces spontaneous polarization, the origin of ferroelectricity, a large dielectric constant and piezoelectricity.

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