The solid solution BaLi1–xCuxPO4(x⩽ 0.5): an example of Cu+ single-ion luminescence in oxide insulators

Abstract

The crystallographic and spectroscopic properties of the solid solution BaLi_1–xCu_xPO₄(0 ⩽x⩽ 0.5) have been investigated as a function of temperature and monovalent copper concentration. A structural description of the compounds is proposed, on the basis of an isotypy with the hexagonal form of β-SiO₂ trydimite. This description involves the existence of a 1:1 ordering between the cross-linked LiO₄ and PO₄ tetrahedra, in a way such that the replacement of lithium by copper cannot involve any direct Cu⁺–Cu⁺ interaction in the network. This argument is supported by the spectroscopic results. The materials exhibit a unique blue emission under UV excitation, whatever the temperature and the composition. This luminescence possesses a time-constant which is independent of the copper concentration. It is attributed unambiguously to Cu⁺ single ions.