Novel compounds in the MMeR(BO3)2 borate family (M = alkali metal, Me = alkaline earth metal, R = rare-earth element): Syntheses, crystal structures and luminescent properties

Abstract

This review focuses on the recently discovered new compounds in the MMeR(BO3)2 borate family (where M = alkali metal, Me = alkaline earth metal, and R = rare-earth element), describing their crystal structures and luminescent properties. The basic building block for this structure is the two-layer {R[A(BO3)2]2} package (where A is an alkali metal and/or alkali earth metal). The features of the A-cation distribution in the structure determine the unit-cell lattice parameters and symmetry of the compounds. Some borates have not been synthesized because the structure is no longer maintained for relatively large R cation sizes (La, Ce for KBaY(BO3)2 borate family). Many new rare-earth doped borates with down-conversion luminescent properties have been discovered and various colour-tunable phosphor materials were developed by changing the doping concentration and by controlling the fraction of the rare earth ions. A review of the literature clearly shows that the mechanism of the energy transfer between the active ions in the co-doped borates is preferably via electric multipolar interaction, while the exchange interaction is negligible. Emission intensity of Tb3+, Eu3+, and Mn2+-doped MMeR(BO3)2 borates can be effectively sensitized by Ce3+ co-doping, making these borate materials promising colour-tunable light emitting phosphor materials for pc-wLED application.