Polymorphism and polymorph-dependent luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2+

Building on studies of monoclinic Li3SiNO2, a polymorph, beta-Li3SiNO2, with a previously unknown structure type was synthesized. The beta-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) angstrom, and a cell volume of V = 1057.2(1) angstrom(3). Using high-temperature solid-state reactions in sealed tantalum tubes, it was possible to obtain high purity samples (<5 wt% of side phase LiSi2N3 according to Rietveld analysis) containing exclusively one or the other polymorph, depending solely on the cooling rate. In contrast to the monoclinic phase, orthorhombic beta-Li3SiNO2 additionally contains a third layer and shows a layer-sequence of the type ABCB. Doped with the activator ion Eu2+, the new polymorph exhibits an intense yellow emission (lambda(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.

Automated summary

A previously unknown polymorph, beta-Li3SiNO2, was synthesized and found to have an orthorhombic structure with a new layer sequence. This polymorph showed a significant blue-shift in emission compared to the monoclinic phase, and exhibited intense yellow luminescence when doped with Eu2+ ions.