Internal electric fields and structural instabilities in insulating transition metal compounds: Influence on optical properties

This review is focused on new ideas developed in the last two decades which play a key role for understanding the optical properties of insulating materials containing transition metal (TM) cations. Initially, we deal with compounds involving d4 and d9 ions where the local structure of the involved MX6 complexes is distorted, a fact widely ascribed to the Jahn‐Teller (JT) effect. Nevertheless, that assumption is very often wrong as the JT coupling requires an orbitally degenerate ground state in the initial geometry a condition not fulfilled even if the lattice is tetragonal. For this reason, the equilibrium geometry of d4 and d9 complexes in low symmetry lattices, is influenced by two factors: (i) The internal electric field due to the rest of lattice ions (ii) The existence of structural instabilities that lead to negative force constants. Then, we explore stable systems involving d3, d5 or d9 cations, where the internal electric field, ER, is responsible for some puzzling phenomena. This is the case of ruby and emerald. A similar situation holds when comparing the normal and the inverted perovskites. The role of ER is particularly remarkable looking for the origin of the color in the historical Egyptian Blue pigment