Structures of Transition Metal Oxides in the Solid State

Can be viewed as a Primitive Cubic Lattice

Replace each lattice point with an MO₆ octahedron

ReO₃ / Rhenium Trioxide

Can be viewed as the ReO₃ structure, then putting a cation (A) in the centre of each unit cell/

A; larger, low charge, 12 co-ordinate e.g. Ca, Sr, Ba, Lanthenides

B; small, high charge, 6 co-ordinate e.g. TMs, Gallium, Al

For an ideal perovskite; r_A + r_X = sqrt(2)(r_B + r_A)

Tolerance factor; t = (r_A + r_X) / (sqrt(2)(r_B + r_A))

For an ideal cubic perovskite, t = 1. In practice, it usually deviates from 1.

0.9 < t < 1.0    Cubic,   0.75 < t < 0.9     Lower Symmetry (e.g. tetragonal or orthorombic)

BaTiO₃ adopts a distorted perovsktie structure - Ti diplaced off centre, due to high tolerance factor (1.07), to achieve better bonding.

If t >1.06, you get a hexagonal perovskite, which introduces some face sharing of octahedra (instead of just the usual corner sharing). 

Ilmentie (FeTiO₃)- structure adopted when both A and B are small. HCP of O, with Fe and Ti occupying 2/3 of octahedral holes in alternate layers. 

Similar to perovskites - have general formula A_n+1B_nO_3n+1.

Like perovskites, A = large cation, B = small cation.

n = 1 - single perovskite layer e.g La₂CuO₄, Sr₂TiO₄

n = 2 - double perovskite layer e.g. Sr₃Ti₂O₇, Sr₃Ru₂O₇, La₂SrCo₂O₇

n = 3 - triple perovskite layer e.g. Sr₄Ti₃O₁₀

Based on a 2x2x2 FCC array, with 1/2 octahedral and 1/2 tetrahedral holes filled. 

• Normal Spinels - A in tetrahedral sites, B in octahedral sites
• Inverse Spinels - B in tetrahedral sites, A in octahedral sites

Factors that influence the structure type;

• Attractive/Repulsive Forces; Maximise attractive forces between oppositely charged ions and minimise forces between like charged ions. (can be done by maximising CN, but this is restricted by relative sizes)
• Radius Ratios;
• Shared Edges/Faces; Presence of shared edges/faces in unfavourable
• Non-Ionic Effects; 
  • Covalency; favours lower CN - orbital overlap more efficient as ions are closer
  • Van der Waals forces; higher CN for more polarisable ions
  • Metal-Metal bonding; stabilises structures containing edge/face sharing octahedra
  • CFSEs; influences structure between O_h and T_d sites, plus Jahn-Teller exists

Found in

• Discrete clusters; organometallics, low OS 2^nd/3^rd series e.g. ReCl₅/Re₃Cl₉, MoCl₅/[Mo₆Cl₁₄]^2-
• Extended lattice structures; 1^st series, early 2^nd/3^rd, e.g. Sc₇Cl₁₀, ZrCl
• Metals themselves.

Can also be seen in oxide materials - M^ll oxides have NaCl/rocksalt structure. MO diagram shows we  place electrons into t_2g or e_g bands, depending on T.M. present. 

Copper and Gold coloured, yet silver isn't. Looking at energy diagram, colour is caused by excitation of electrons from 3d band (configuration 3d¹⁰4s¹). This is in agreement with the UV-Vis spectrum, as the absorbance is towards the blue end of the spectrum, giving an orange colour. Different colours due to difference in relative energies of d and s orbitals. Silver  energy gap is bigger, due to 4d orbitals being relatively stable, meaning absorption doesn't appear in the visible region.