Crystal-field regulation enables broadband-to-line emission switching in Cr3+-activated pyroxenes

Abstract

Cr³⁺ ions as a common activator in inorganic phosphors is well-known and exhibit two distinct emissions, i.e. the broadband (⁴T₂ → ⁴A₂) and the narrow-line (²E → ⁴A₂) emission, both of which have wide applications in red/NIR lasers and pc-LEDs. Controllable switching between these two emission states has great potential in applications such as switches, sensors, and regulators. However, most current studies using chemical modulation of host lattices remain at the level of static regulation. In this work, we report the realization of a pressure-driven dynamic and reversible broadband-to-line emission switching in Cr³⁺-activated pyroxenes LiScSi₂O₆ and LiScGe₂O₆. Under compression, LiScSi₂O₆ undergoes sequential phase transitions C2/c → P2₁/c → HP-C2/c at 0.8 GPa and 21.3 GPa, while LiScGe₂O₆ has a partially irreversible phase transition P2₁/c → HP-C2/c at around 16.2 GPa. Remarkably, a pressure-induced energy level-crossing between ⁴T₂ and ²E is observed in both materials, resulting in dramatic switching of the broadband emission from the spin-allowed ⁴T₂ → ⁴A₂ transition to the sharp-line emission from the spin-forbidden ²E → ⁴A₂ transition. The calculated crystal field strength (D_q/B) of LiScSi₂O₆ and LiScGe₂O₆ increases greatly from 2.10 and 2.26 under ambient conditions to 3.89 and 4.07 at around 30 GPa, respectively. Particularly, the narrow-line emission (²E → ⁴A₂) becomes 44.5-fold more intense than the broadband emission (⁴T₂ → ⁴A₂) at 22.0 GPa in LiScSi₂O₆:0.04Cr³⁺, compared to the initial state, which can be attributed to both the enhancement of crystal field strength and local symmetry reduction. Finally, the relationship between local coordination structures and the optical property evolution is thoroughly studied using XRD, Raman spectroscopy, PL spectroscopy, excitation spectroscopy and Eu-probes. These results demonstrate static pressure as a powerful tool to regulate the crystal field strength, and thus to achieve interesting optical phenomena including broadband-to-line emission switching and intense, laser-like deep-red emissions.