Electron–phonon coupling mechanisms of broadband near-infrared emissions from Cr ³⁺ in the Ca ₃ Sc ₂ Si ₃ O ₁₂ garnet

Cr3+ in the Ca3Sc2Si3O12 garnet (CSSG) has the ability to convert blue light to broadband near-infrared (NIR) emissions, which is a promising strategy for next-generation smart NIR light sources based on blue LEDs. The Cr3+ luminescence strongly depends on temperature due to electron-phonon coupling (EPC). We reveal the EPC mechanism of Cr3+ in CSSG for the first time by temperature-dependent photoluminescence measurement from 77 to 573 K and cathodoluminescence using a scanning electron microscope. Cr3+ occupies the Sc3+ site and experiences a weak crystal field in CSSG, manifesting a broad NIR emission in the 700-900 nm range that originates from the 4T2g→4A2g transition. The zero phonon line (ZPL) of the 4T2 state is observed at ∼713 nm with a vibrational energy of ∼310 cm-1. A strong EPC leads to a large Stokes shift (∼2900 cm-1). The Huang-Rhys parameter (S = 4), crystal field strength (Dq/B), and Racah parameters (B and C) are estimated.