Challenges in the rational design of intense inorganic pigments with desired colours – Nature Reviews Materials

The d-d transition is the one relevant for YInMn blue and is the most common origin of colour in transition metal oxides and minerals. It can occur when a transition metal cation with partially filled d orbitals (d to d) is placed in the centre of a polyhedron (such as an octahedron or a tetrahedron) formed by negatively charged ligands (for example, O in oxides). An isolated transition metal ion on its own has five d orbitals of the same energy, but when ligands are attached to make a coordination complex, the d orbitals split into different energy levels. The resulting energy levels depend on the geometry of the structure (for example, octahedral or tetrahedral). Electrons in the ground state orbitals absorb light in the visible range and jump into a higher energy level, reflecting the complimentary colour. The observed colour depends on the energy difference between ground states and excited states, also known as the crystal field splitting energy.

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