Crystal Structures and Luminescence Properties of the Oxonitridolithosilicates RE2[LiSi3N5O2] (RE = Ce, Pr, Nd, Sm, Gd, Tb).

Abstract

The rare-earth oxonitridolithosilicate series RE2[LiSi3N5O2] (RE = Ce, Pr, Nd, Sm, Tb) is reported as isotypic representatives of the previously discovered compound Gd2[LiSi3N5O2], accessed via high-temperature solid-state synthesis. All compounds crystallize in the non-centrosymmetric tetragonal space group P 4 ¯ 2 1 m (no. 113), as determined by single-crystal X-ray diffraction and confirmed by Rietveld refinement. The crystal structure is related to the melilite structure type and is built up of a tecto-lithosilicate comprising corner- and edge-sharing [LiN4]-, [SiN4]-, and [SiN3O]-tetrahedra, with the rare-earth ions occupying channels within the resulting network. The lanthanoid contraction-induced decrease of all unit cell parameters across the series is consistent with the structural analysis and is further supported by thermal measurements. These trends indicate a hierarchical distribution of rigidity and flexibility within the framework, thereby establishing this compound class as a model system for structure-property relations in rare-earth oxonitridolithosilicates. On the rare-earth site, the intra-configurational 4f-4f transitions of Tb3+ and Nd3+ are identified. For Eu2+-activated Tb2[LiSi3N5O2], an orange 5d → 4f emission at λmax = 618 nm is observed, establishing the potential of this compound class for solid-state lighting applications.

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