Unprecedented Deep-Red Ce³⁺ Luminescence of the Nitridolithosilicates Li_38.7RE_3.3Ca_5.7[Li₂Si₃₀N₅₉]O₂F (RE = La, Ce, Y)

Ce3+ doped solids find broad application, e.g. in phosphor converted light emitting diodes, utilizing the usually broad blue to yellow-orange emission of the respective phosphors. The red to infrared spectral range was not yet accessible with the activator Ce3+, even in nitride host materials. Here, we report on the nitridolithosilicates Li38.7RE3.3Ca5.7[Li2Si30N59]O2F (RE = La, Ce, Y) with unique, red-shifted Ce3+ luminescence. The materials were synthesized by solid-state metathesis reactions in tantalum ampules. The isotypic crystal structures exhibit a highly condensed three-dimensional network made up of SiN4 and LiN4 tetrahedra. Crystal structures were refined from single-crystal and powder X-ray diffraction data. The results are supported by energy-dispersive X-ray spectroscopy as well as charge distribution, lattice energy, and bond valence sum calculations. Optical band gaps of ≈4 eV were determined from diffuse reflectance UV/vis data using Tauc plots. The nitridolithosilicates are highly excitable from the UV to the green-yellow spectral range with a preferred excitation wavelength of λexc ≈ 540 nm. The emission spectra peak is in the deep-red with λem = 638–651 nm (fwhm ≈ 3600 cm–1). According to the unique absorption and emission properties, application as luminescent solar concentrators or in horticultural lighting appears promising.