“…Some potential upconversion materials, such as Er 3+ -doped RbGd 2 C1 7 and RbGd 2 Br 7 ; Cs 3 Yb 2 Cl 9 and Cs 3 Yb 2 Br 9 ; Cs 3 Y 2 Br 9 (10% Dy 3+ and Cs 3 Dy 2 Br 9 ); Cs 3 Er 2 X 9 (X = Cl, Br, I); and Er 3+ in Cs 3 Lu 2 Br 9 , were synthesized by means of the Bridgman technique − or improved Meyer method . In order to prepare the alkali metal bromide/rare-earth metal bromide compounds by phase equilibrium method and acquire the solubility of cesium bromide and rare-earth bromide at 298.15 K in H 2 O or HBr (∼12 mass %)–H 2 O medium, the phase equilibria for CsBr + REBr 3 + H 2 O (RE = La, Ce, Pr, Nd, Sm, Er, and Tm) ternary systems and CsBr + REBr 3 + HBr (∼13%) + H 2 O (RE = La, Ce, Pr, Nd, Sm, Dy, Er, and Tm) quaternary systems were reported continuously, − and the related research on rare-earth halide in aqueous media was compiled exhaustively . These phase diagrams were highly valuable for researchers to learn the phase equilibrium relationship of CsBr and REBr 3 in aqueous systems.…”