Temperature quenching of Cr³⁺ in ASc(Si_1-xGe_x)₂O₆ (A=Li/Na) solid solutions

Blue absorbing near infrared (NIR) emitting phosphors are a promising class of materials for phosphor converted NIR LEDs, which can be used in compact NIR spectrometers. Preferably, these phosphors have a broad emission spectrum and show negligible luminescence quenching at LED operating temperatures (100 °C). Here, we investigated ASc(Si_1-xGe_x)₂O₆ (A = Li/Na, x = 0,0.2,0.4,0.6,0.8,1) solid solutions doped with Cr³⁺ to tune and optimize the emission maximum and bandwidth to cover the full 700–1100 nm range. With increasing Ge content an emission redshift was observed, along with emission band broadening at intermediate Ge/Si ratio, which is explained by disorder around Cr³⁺ in the second coordination sphere (mixed Si/Ge). Temperature dependent emission spectra and luminescence decay curves were measured between 90 K and 670 K to determine the quenching temperature T_Q. With increasing Ge content T_Q drops from 550 K to below 400 K. Interestingly, Cr³⁺ emission in the highly symmetric site in LiScSi₂O₆ shows a strongly temperature dependent lifetime before thermal quenching sets in. DFT calculations on LiScSi₂O₆ indicate that asymmetric vibrations at the Sc site are involved and calculated phonon energies were confirmed by measuring FTIR. Our study indicates that a solid solution is a promising way to increase the emission bandwidth. However, with increasing Ge content T_Q decreases. An optimum Ge-content in LiSc(Si_1-xGe_x)₂O₆:Cr³⁺ is x = 0.2–0.4 as it redshifts the NIR band maximum close to 900 nm and offers a FWHM bandwidth around 180 nm, while keeping the thermal quenching temperature high enough for application in NIR-LEDs.