Structures of Transition Metal Oxides in the Solid State

  • Created by: E.H13
  • Created on: 24-05-20 14:24

AX3 Structrue

Can be viewed as a Primitive Cubic Lattice

Replace each lattice point with an MO6 octahedron

ReO3 / Rhenium Trioxide

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ABX3 - Perovskite Structure

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; rA + rX = sqrt(2)(rB + rA)

Tolerance factor; t = (rA + rX) / (sqrt(2)(rB + rA))

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)

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Distorted perovskites

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. 

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Ruddlesdon Popper Phases

Similar to perovskites - have general formula An+1BnO3n+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

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AB2O4 Structure - Spinel

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 Oand Td sites, plus Jahn-Teller exists
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Metal-Metal Bonding

Found in

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

Can also be seen in oxide materials - Mll oxides have NaCl/rocksalt structure. MO diagram shows we  place electrons into t2g or eg bands, depending on T.M. present. 

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Colour in Group 11 Metals

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.

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