The effect of electron cloud expansion on the red luminescence of Sr4Al14O25:Mn4+ revealed by calculation of the Racah parameters

• The coexistence of Mn ions with +4, +3 and +2 valences in Sr4Al14O25 was detected.
• The zero-phonon transition is determined from cryogenic spectrum measured at 10 K.
• The deep red luminescence is attributed to the Mn4+ occupying the AlO6 center.
• The crystal field stabilization energy and the Racah parameters were calculated.
• The nephelauxetic ratio β about 0.32 indicates a strong electrostatic repulsion.

In this work, the crystal field stabilization energy (Dq) and the Racah B and C parameters were calculated quantitatively to evaluate the nephelauxetic effect of Mn4+ suffering from the Sr4Al14O25 host, aiming for developing novel highly efficient red phosphor for white light-emitting diodes. The phosphor of Mn4+ activated Sr4Al14O25 was synthesized via a high temperature solid reaction in air ambient. The crystallinity was investigated using the X-ray diffraction; and the photoluminescence properties were characterized with the spectrometer. The red emission was attributed to the Mn4+ which occupies the center of AlO6 octahedron. Meanwhile, the Mn2+ and Mn3+ were detected by using the X-ray absorption near-edge structure spectroscopy (XANES) assisted with Electron Paramagnetic Resonance (EPR) techniques. The energy levels of 4A2, 4T2g and 4T1g were determined according to excitation spectra, corresponding to the 4A2 (t23) → 4T1 (4F) and the 4A2 (t23) → 4T2g (t22e) transitions at 330 and 450 nm respectively, whereby the calculation was carried out according to the Tanabe–Sugano diagram for octahedral complex with the d3 electron configuration. The nephelauxetic ratio β about 0.32 suggests a strong cloud expansion of Mn4+ in the Sr4Al14O25 crystal lattice with Dq ∼ 2222 cm−1, B ∼ 794 cm−1, C ∼ 3232 cm−1 and C/B ∼ 4.07. In the perspective of physics, these parameters are helpful to perceive the inter-repulsion of the outer electrons in 3d orbit of Mn4+ and the effect of local circumstance, in essential to understand the variant spectral configurations of Mn4+ luminescence observed in different hosts.

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