Cationic pair substitution in LaAlO3:Mn4+ for octahedral-tilting-dependent zero-phonon line

Abstract

Zero-phonon line (ZPL) emission of Mn⁴⁺, without the participation of phonons, is tightly related to the host crystal structure. However, the intensity of the intrinsic ZPL is much weaker than that of Stokes and anti-Stokes vibrational bands, and it always leads to a discontinuous emission peak. Regulating the ZPL of Mn⁴⁺ for a strong emission is very important but remains a challenge for perovskite-type oxides. Here, novel La_1−xBa_xAl_1−xTi_xO₃:0.001Mn⁴⁺ (LBAT:0.001Mn⁴⁺, x = 0–0.2) and La_1−yY_yAl_1−yGa_yO₃:0.001Mn⁴⁺ (LYAG:0.001Mn⁴⁺, y = 0–0.2) samples were successfully synthesized through a high-temperature solid-state reaction, and a tunable ZPL of Mn⁴⁺ was found by cationic pair substitution of Ba²⁺–Ti⁴⁺ and Y³⁺–Ga³⁺ for La³⁺–Al³⁺ in LaAlO₃:Mn⁴⁺. The ZPL intensity is related to the local symmetry around Mn⁴⁺ and the ZPL energy corresponds to the Mn–O bond distance and the O–Mn–O bond distortion. Through co-doping Ba²⁺–Ti⁴⁺, the ZPL at 710 nm is enhanced and the intensity increases continuously with increasing the x value, due to the local symmetric degree of Mn⁴⁺ decreasing slowly. However, Y³⁺–Ga³⁺ co-doping induces a linear and quick increase in the intensity of ZPL at 704 nm with increasing y value, due to the local symmetric degree of Mn⁴⁺ decreasing quickly. The octahedral tilting distortion is very important for the local symmetry. Ba²⁺–Ti⁴⁺ co-doping reduces octahedral tilting distortion, but Y³⁺–Ga³⁺ co-doping induces a serious octahedral tilting distortion. Consequently, the ZPL emission exhibits an octahedral-tilting dependent behavior. Mainly due to the larger distortion of the O–Mn–O bond, the energy of ZPL for LYAG:0.001Mn⁴⁺ is higher than that for LBAT:0.001Mn⁴⁺. The outcomes of this work provide a promising way to regulate the ZPL intensity and energy by tuning the local structure around Mn⁴⁺, and may have wide implications for Mn⁴⁺-doped phosphors and solid-state lighting.