Polymorphism and polymorph-dependent luminescence properties of the first lithium oxonitridolithosilicate Li 3 SiNO 2 :Eu 2 .

Building on studies of monoclinic Li 3 SiNO 2 , a polymorph, β-Li 3 SiNO 2 , with a previously unknown structure type was synthesized. The β-phase crystallizes in the orthorhombic space group Pbca (no. 61) with lattice parameters of a = 18.736(2), b = 11.1267(5), c = 5.0897(3) Å, and a cell volume of V = 1057.2(1) Å 3 . Using high-temperature solid-state reactions in sealed tantalum tubes, it was possible to obtain high purity samples (<5 wt% of side phase LiSi 2 N 3 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-Li 3 SiNO 2 additionally contains a third layer and shows a layer-sequence of the type ABCB . Doped with the activator ion Eu 2+ , the new polymorph exhibits an intense yellow emission ( λ max = 586 nm, fwhm = 89 nm, 0.33 eV, 2650 cm -1 ) 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.