On the optical properties of the Mn4+ ion in solids

A review of the optical properties of Mn4+ ions in a number of host lattices is presented. As an example, detailed crystal field calculations of the Mn4+ energy levels in Na2SiF6 and K2SiF6 are reported and compared with the literature experimental data. Special attention is paid to the crystal field analysis of the observed features in relation to the structural data and peculiar features of the chemical bonding. A simple criterion is proposed, which can effectively and easily describe ionicity/covalency of the Mn4+-doped crystals. This criterion is based on the position of the sharp spin-forbidden 2Eg→4A2g emission line transition: if it is located above 15500 cm−1 (below ∼645 nm), the bonding is essentially ionic, while its location below 15500 cm−1 (above ∼645 nm), is suggestive of covalent bonding.