Structural confinement-induced highly efficient deep-red emission and negative thermal quenching performance in Mn4+-activated Ca7Mg2Ga6-yAlyO18:Mn4+ phosphors

High quantum efficiency (QE) and thermally stable emission are indispensable for the application of phosphors. Owing to the strong coupling between the lattice and naked d-orbitals of Mn4+, Mn4+-activated oxide red-emitting phosphors usually exhibit a low QE and serious thermal quenching (TQ), which strictly inhibit their prospective application. Herein, we rationally designed a novel phosphor series Ca7Mg2Ga6-yAlyO18:0.02Mn(4+) (CMGA(y)O:Mn4+, 0 = y = 1.5), which adopts a high structural symmetry of the space group F432. Highly efficient and thermally stable deep-red emissions (?(em) = 721 nm) were achieved simultaneously by structurally confining the Mn4+-emitting centres at the isolated octahedrally coordinated sites. Specifically, the highest internal and external quantum efficiencies of 90.2 and 75.9%, respectively, were obtained for CMGA(1.5)O:Mn4+ due to Al3+-alloying-induced local structure modification and suppression of non-radiative transitions. Excellent thermal stabilities of 99, 102.2, and 87.9% of room-temperature photoluminescence intensities were retained at 423 K for CMGA(y)O:Mn4+ with y = 0, 0.5, and 1.5, respectively. CMGA(0.5)O:Mn4+ exhibits an abnormal negative TQ behaviour over the measured temperature range (298-473 K), which is attributed to the energy gain via the electron-phonon interactions of the isolated MnO6 octahedra. Moreover, in situ high-pressure emission spectra of CMGO:0.02Mn(4+) showed that the emission position is highly sensitive to the external pressure with redshift coefficients of 2.8(2) and 1.16(4) nm GPa(-1) in different pressure regions. These excellent photoluminescence properties of CMGA(y)O:Mn4+ phosphors signify their great application potential in red LEDs and optical pressure sensors. The findings of this work have paved an avenue for the design of high-performance Mn4+-activated phosphors.