X-Ray diffraction and SAED characterisations of RE4O4[PO4]Cl (RE = La, Pr, and Nd) and photoluminescence properties of Eu3+-doped La4O4[PO4]Cl

Abstract

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 ORE_4/4 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.