Fig. 1: Optical properties of octahedrally coordinated Cr³⁺ ions in AlB₄O₆N.

a Representative emission spectra of Al_0.993Cr_0.007B₄O₆N at selected temperatures depicting both narrow-line ²T_1g, ²E_g(²G) → ⁴A_2g(⁴F)- and broad-band ⁴T_2g(⁴F) → ⁴A_2g(⁴F)-based emission (break at y axis for better visualization of both emission bands). b Effective Tanabe-Sugano diagram based on estimated Racah parameters B and C for Al_0.993Cr_0.007B₄O₆N. The vertical dashed line depicts the Dq/B ratio for Al_0.993Cr_0.007B₄O₆N. The d³ micro-configurations in an octahedral crystal field related to the multielectron states are depicted on the right side of each diagram with the respective emission denoted as a colored arrow. c Structure of Al_0.993Cr_0.007B₄O₆N with view along the crystallographic a axis emphasizing the octahedral coordination of the Al³⁺/Cr³⁺ ions (in grey) embedded in a condensed network of [BO₃N]^6- tetrahedra (O atoms in blue, N atoms in green)