X-Ray diffraction and SAED characterisations of RE ₄ O ₄ [PO ₄ ]Cl (RE = La, Pr, and Nd) and photoluminescence properties of Eu ³⁺ -doped La ₄ O ₄ [PO ₄ ]Cl

The new compounds RE4O4[PO4]Cl (RE = La, Nd, Sm and Gd) were synthesised by solid state reaction via a salt flux route and selected compositions were investigated by SAED, photoluminescence, and powder- and/or single crystal-X-ray diffraction. All the crystal structures consist of an alternation of [RE4O4]4+ and [ClPO4]4− layers. The [RE4O4]4+ layer contains ORE4/4 tetrahedra which share common edges. In the tetragonal RE4O4[PO4]Cl (RE = Pr and Nd) compounds, the PO43− and Cl− anions are located between these layers in a disordered manner. This is reflected by the presence of diffuse streaks parallel to the c* axis on the SAED patterns. These lines disappear for larger rare earth metals and keep the place to well defined superstructure reflections in La4O4[PO4]Cl, with a*superstructure = 4/5a*tetragonal − 2/5b*tetragonal and b*superstructure = 2/5a*tetragonal + 4/5b*tetragonal. The presence of these superstructure reflections is due to a new ordering of the PO43− and Cl− anions between the [RE4O4]4+ layers. The Eu3+-doped La3PO7 and La4O4[PO4]Cl samples show the characteristic Eu3+-related 4f → 4f lines and intense emission of red light upon excitation via the O2− → Eu3+ charge-transfer band at 280 nm. Due to the efficient energy transfer from the charge-transfer state to the 4f-levels, especially, La4O4[PO4]Cl:Eu3+ (5 mol%) may be of potential interest as a red phosphor.