A Novel Cyan‐Emitting Phosphor K₃Ca0.3Y0.7Si₂O₇:Ce³⁺ Enables Full‐Spectrum Lighting and Fingerprint Detection via Phase‐Structure Alteration

Crystal‐field engineering accelerates the discovery of Ce 3+ cyan phosphors, yet structure–property links remain unclear. We exploit the K 3 YSi 2 O 7 phase transition to shift Ce 3+ emission from green to ideal cyan while boosting thermal stability from 5% @ 423 K to 63.6% and internal quantum efficiency from 44.8% to 61.9%. The transition relocates Ce 3+ into a more rigid site that suppresses electron–phonon coupling and non‐radiative loss. The optimized composition K 3 Ca₀. 3 Y₀. 7 Si 2 O 7 :Ce 3+ fills the cyan gap in white LEDs, raising the color‐rendering index from 84.9 to 97.1, and is further fabricated into a thermally stable cyan polydimethylsiloxane film and applied to latent fingerprint imaging. This work delivers a high‐performance cyan phosphor and clarifies how crystal‐field engineering governs Ce 3+ luminescence.