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

Highlights: • Recently discovered new compounds in the MMeR(BO₃)₂ borate family. • MMeR(BO₃)₂ borate-based materials with down-conversion luminescence. • Colour-tunable phosphor materials were developed by controlling the fraction of the rare earth ions. • Replacement of alkali and alkali earth elements in KBaR(BO₃)₂ result in the transformation of the crystal structure. -- 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 Some borates have not been synthesized because the structure is no longer maintained for relatively large R cation sizes (La, Ce for KBaY(BO₃)₂ borate family).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.