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

Cr3+ ions as a common activator in inorganic phosphors is well-known and exhibit two distinct emissions, i.e. the broadband (T-4(2) -> (4)A(2)) and the narrow-line (E-2 -> (4)A(2)) 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 -> P2(1)/c -> HP-C2/c at 0.8 GPa and 21.3 GPa, while LiScGe2O6 has a partially irreversible phase transition P2(1)/c -> HP-C2/c at around 16.2 GPa. Remarkably, a pressure-induced energy level-crossing between T-4(2) and E-2 is observed in both materials, resulting in dramatic switching of the broadband emission from the spin-allowed T-4(2) -> (4)A(2) transition to the sharp-line emission from the spin-forbidden E-2 -> (4)A(2) 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 (E-2 -> (4)A(2)) becomes 44.5-fold more intense than the broadband emission (T-4(2) -> (4)A(2)) at 22.0 GPa in LiScSi2O6:0.04Cr(3+), 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.