Manipulating the oxidation state of Cu ions in BaAlBO3F2 via X-rays and thermal treatment

Abstract

The oxidation state of Cu in BaAlBO₃F₂ (BABF) is reduced to Cu⁺ by ionizing radiation. In the as-prepared state, the higher oxidation state of Cu is considered to occupy two different crystallographic sites (A and B), which are possibly the five-coordinate Al³⁺ and the twelve-coordinate Ba²⁺ site, respectively. Upon X-ray ionization of the host matrix, electrons are selectively captured by the Cu ion at the B site, reducing the oxidation state to the monovalent Cu⁺ state. This reduction leads to a photoluminescence (PL) band peaking at 520 nm under UV excitation, corresponding to the 3d⁹4s¹ → 3d¹⁰ transition of Cu⁺. The evolution of the PL band (or the formation of a luminescent center) induced by ionizing radiation is referred to as radiophotoluminescence (RPL). The RPL emission intensity is proportional to the X-ray irradiation dose and can be reversed by heat treatment. During heat treatment at temperatures ranging from 100 to 200 °C, electrons from the Cu⁺ center at the B site are transferred to the higher oxidation state of Cu at the A site. Consequently, another Cu⁺ center is formed at the A site, resulting in a PL band peaking at 580 nm, while the intensity of the 520 nm peak decreases due to the depopulation of Cu⁺ at the B site. Furthermore, increasing the treatment temperature above 350 °C reverses the electron transfer at the A site. The formation and annihilation of Cu centers with different oxidation states are reproducible multiple times.