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

Abstract

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 450nm 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∼2222cm−1, B∼794cm−1, C∼3232cm−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.