The transition metal ions with the 3d unfilled electron shells can be stabilized in various oxidation states in solids, which explains diversity of their spectroscopic properties, variation of their emission spectra and possible applications in solid state lasers, lighting etc. The ions with the 3d³ electron configuration make a special group, whose emission spectra can be divided into three categories: i) sharp spin-forbidden ²E→⁴A₂ emission, which does not depend on the crystal field strength; ii) broad spin-allowed ⁴T₂→⁴A₂ emission, which depends on the crystal field strength; iii) coexistence of both emissions.

In the present work we focus on two representatives of these group of ions: Mn⁴⁺ and Cr³⁺ and crystalline solids doped with these impurities.

The Mn⁴⁺ ions in solids exhibit only sharp emission between 620 nm and 700 nm [1]. The phosphors emitting red light between 620 nm and 650 nm (such as K₂SiF₆, Na₂SiF₆ etc) are used in white LEDs to produce warm white light. The phosphors emitting at longer wave lengths (e.g. double perovskites Ba₂LaNbO₆, La₂LiSbO₆ etc) are used for applications in white LEDs for agriculture, to enhance plant growth [2]. Recently, we successfully explained tunability of the Mn⁴⁺ emission in solids [3] and the ways of enhancement of its intensity [4], as well as influence of the host composition on the emission spectra [5].

The Cr³⁺ ions in solids can produce emissions of all above-mentioned three kinds. The applications of Mn⁴⁺ and Cr³⁺ ions for optical thermometry are discussed in detail [6, 7], with the interplay of the strong and intermediate crystal effects being a very important factor in this regard. Further perspectives of thermometric materials with the transition metal ions are highlighted.

References:

[1] M. G. Brik and A. M. Srivastava J. Lumin. 133 (2013) 69.

[2] A.M. Srivastava, M.G. Brik, H.A. Comanzo, W.W. Beers, W.E. Cohen, T. Pocock, ECS J. Sol. State Sci. & Technol. 7 (2018) R3158.

[3] M.G. Brik, C.-G. Ma, A. M. Srivastava, M. Piasecki, Chin. J. Lumin. 41(9) (2020) 1011.

[4] M.G. Brik, W.W. Beers, W. Cohen, S.A. Payne, N.J. Cherepy, M. Piasecki, A.M. Srivastava, Opt. Mater. 91 (2019) 338.

[5] A.M. Srivastava, M.G. Brik, H.A. Comanzo, W.W. Beers, W. Cohen, N.J. Cherepy, S.A. Payne, Opt. Mater. 95 (2019) 109196.

[6] M. D. Dramićanin, B. Milićević, V. Đorđević, Z. Ristić, J. B. Zhou, D. Milivojević, J. Papan, M. G. Brik, C.-G. Ma, A. M. Srivastava, M. M. Wu, Chemistry Select 4 (2019) 7067.

[7] Z. Ristić, V. Đorđević, M. Medić, S. Kuzman, M.G. Brik, Ž. Antić, M. D. Dramićanin, Opt. Mater. 120 (2021) 111468.