In this paper we review the spectroscopic properties of the Mn4+ ion (3d electronic configuration) and establish a relationship between the energy of the 2Eg → 4A2g emission transition and the covalence of the “Mn4+-ligand” bonding. A new parameter β1=(normalBB0)2+(normalCC0)2 (where (B, C (B0, C0) are the Racah parameters of the Mn4+ ions in a crystal (free state), respectively) was recently introduced that allows for establishing a linear relationship between β1 and the energy of the 2Eg level. This makes it possible to understand the large variation in the energy of the 2Eg → 4A2g emission transition that is frequently encountered in the spectroscopy of the Mn4+ ions in solids.

In this paper we review the spectroscopic properties of three transition metal ions - Mn4+, Cr3+ and Ni2+ - in crystals and establish a relationship between the energies of the lowest in energy spin-forbidden transitions and covalence of the “metal –ligand” chemical bonds. A new parameter β1=(BB0)2+(CC0)2 (where (B, C (B0, C0) are the Racah parameters of the ions in a crystal (free state), respectively) is shown to determine the energy of the above-mentioned transitions. The considered ions can be used as reliable probes of the covalent effects in various hosts. Several practical recommendations on how to tune the spin-forbidden transitions energy to meet specific needs are suggested.

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The spectroscopic properties of the Mn4+ ion (3d3) in the double perovskite NaLaMgTeO6 are reported in this work. Evidence is presented for the occupation by the Mn4+ ion of both the six coordinated Mg2+ and Te6+ sites in the host structure. The Mn4+ energy levels are calculated using the exchange charge model of crystal field theory for both occupied sites. The results of our calculations yield the crystal field splitting and Racah parameters of Dq = 2008 cm−1, B = 790 cm−1, C =2881 cm−1, with C/B=3:65 (Mg2+ site) and Dq=2008 cm−1, B=790 cm−1,C =2949 cm−1, with C/B = 3:73 (Te6+ site). A cross-cutting comparative study of the variations in the crystal field splitting and the Racah parameters of the six-coordinated Mn4+ ion in a series of materials with the perovskite structure are presented.