Reversible colorless-cyan photochromism in Eu2+-doped Sr3YNa(PO4)3F powders

Abstract

Photochromic materials have attracted increasing interest as optical switches and erasable optical memory media. Here, we report on a novel reversible colorless-cyan photochromic powder material Sr₃YNa(PO₄)₃F:Eu²⁺. The photochromic properties including coloring and bleaching are characterized by reflectance spectra after the powder is irradiated by different wavelengths of light for different irradiation times or processed by thermal treatment. The results show that it can be colored by short wavelength light (200–320 nm) and bleached by longer wavelength light (320–800 nm) or thermal treatment at 240 °C. The optimal doping concentration of Eu²⁺ is determined to be about 0.5 mol%. It shows high fatigue resistance in photochromism performance. Electrons in 4f ground levels excited by short wavelength irradiation to higher 5d states of Eu²⁺ and then captured by traps are responsible for the coloring process. Correspondingly, the release of trapped electrons from two kinds of traps with different depths causes the initially fast and later slow bleaching processes. The critical role of charge carrier motions between two different traps is discussed. The colorless-cyan photochromism can be explained semi-quantitatively on the basis of obtained results. A tentative model was proposed to illustrate the PC mechanism.