Lanthanide-doped disordered crystals: Site symmetry and optical properties

Lanthanide ions (Ln3+) doped disordered crystals, which exhibit excellent downshifting/upconversion luminescent properties, are widely used in a variety of optical and optoelectronic fields. The optical properties of Ln3+ ions are critically dependent on the local structure around Ln3+ ions. Therefore, by employing Ln3+ (e.g., Eu3+) as the sensitive structural probe, the local structure and site symmetry of Ln3+ dopants in the disordered crystals can be determined. Furthermore, through modification of the local site symmetry of Ln3+ ions, various highly efficient Ln3+-doped luminescent materials have been designed over the past decade. As such, it is urgent to renew the knowledge about the huge family of Ln3+-doped disordered crystals. Rather than being exhaustive, this review aims to highlight the most recent advances in the site symmetry and optical properties of Ln3+-doped disordered crystals, with an emphasis on probing the local site symmetry and tuning downshifting/upconversion luminescent properties based on modification of the microstructures around Ln3+ dopants. Finally, the challenges and future perspectives of this important kind of phosphors are proposed.