From Table 12.3 in the link below : http://questions.transtutors.com/Transtutors

Question

From Table 12.3 in the link below :

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Give two cations that combine with oxide ion (O^-2) in octahedral geometry; which cation(s) can have tetrahedral, octahedral geometries or eventually both. Justify mathmatically your answer.

From Table 12.3 in the link below : Give two cations that combine with oxide ion (O^-2) in octahedral geometry; which cation(s) can have tetrahedral, octahedral geometries or eventually both. Justify mathmatically your answer.

Explanation / Answer

We have to check the r+/r- ratio.

If r+/r- is in the range of 0.414 to 0.732, it is said to be in octahedral geometry.

If r+/r- is in the range of 0.225 to 0.414, it is said to be in tetrahedral geometry.

r- = 0.14 nm; so, r+ can be in the range = 0.14(0.414 to 0.732) = 0.057 to 0.102 nm. (for octahedral)

r- = 0.14 nm; so, r+ can be in the range = 0.14(0.225 to 0.414) = 0.031 to 0.057 nm. (for tetrahedral)

So, cations which can exist as tetrahedral = Ca2+, Fe2+, Fe3+, Mg2+, Mn2+, Na+ (borderline), Ni2+, Ti4+ (borderline), Al3+ (borderline)

So, cations which can exist as tetrahedral = Si4+, Al3+ (borderline), Ti4+ (borderline)

So, cations which can exist as both = Al3+, Ti4+ (possibly)

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