Temperature quenching of Cr3+ in ASc(Si1-xGex)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(Si1-xGex)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 TQ. With increasing Ge content TQ 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 TQ decreases. An optimum Ge-content in LiSc(Si1-xGex)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.