Polymorphism and polymorph-dependent luminescence properties of the first lithium oxonitridolithosilicate LiSiNO:Eu.

Building on studies of monoclinic LiSiNO, a polymorph, β-LiSiNO, with a previously unknown structure type was synthesized. The β-phase crystallizes in the orthorhombic space group (no. 61) with lattice parameters of = 18.736(2), = 11.1267(5), = 5.0897(3) Å, and a cell volume of = 1057.2(1) Å. Using high-temperature solid-state reactions in sealed tantalum tubes, it was possible to obtain high purity samples (<5 wt% of side phase LiSiN according to Rietveld analysis) containing exclusively one or the other polymorph, depending solely on the cooling rate. In contrast to the monoclinic phase, orthorhombic β-LiSiNO additionally contains a third layer and shows a layer-sequence of the type . Doped with the activator ion Eu, the new polymorph exhibits an intense yellow emission ( = 586 nm, fwhm = 89 nm, 0.33 eV, 2650 cm) under irradiation with UV to blue light. Hence, the structural difference between the two polymorphs goes along with a significant blue-shift of 16 nm. The results from single-crystal diffraction and single-grain luminescence measurements were confirmed by Rietveld analysis of bulk samples and powder luminescence data.