Topochemical synthesis of lanthanide phosphor-doped La ₂ O ₂ S with a metastable orthorhombic polymorph

The layered oxysulfide La₂O₂S is well known as a host matrix for a wide range of luminescence applications over the past five decades. While La₂O₂S adopts a hexagonal primitive (hP) lattice as the lowest-energy polymorph, we recently discovered a metastable form of La₂O₂S with an orthorhombic A-centred lattice (oA) through the topochemical deintercalation of sulfur atoms from the La₂O₂S₂ precursor. Compared to the conventional structure of hP-La₂O₂S containing a single inequivalent lanthanum environment, the novel polymorphic form oA-La₂O₂S is characterized by the presence of two inequivalent lanthanum environments. This paper reports on the preparation and optical characterisation of Ce-, Pr-, and Eu-doped oA-La₂O₂S phases. The principal contributions to the luminescence spectra of these doped materials are situated in the green, green and red, and red regions of the visible spectra for Ce, Pr, and Eu, respectively. In contrast, the emissions observed for the corresponding doped hP-La₂O₂S phases occur in the violet, green and red, and orange regions, respectively.