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

Abstract

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 Ca₇Mg₂Ga_6−yAl_yO₁₈:0.02Mn⁴⁺ (CMGA_yO:Mn⁴⁺, 0 ≤ y ≤ 1.5), which adopts a high structural symmetry of the space group F432. Highly efficient and thermally stable deep-red emissions (λ_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_1.5O: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_yO:Mn⁴⁺ with y = 0, 0.5, and 1.5, respectively. CMGA_0.5O: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:0.02Mn⁴⁺ 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_yO: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.