Na(1-x)Lix(Gd0.39Y0.39Yb0.2Er0.02)F4 (0 ≤ x ≤ 1) Solid Solution Microcrystals: Li/Na Ratio-Induced Transition of Crystalline Phase and Morphology and Their Enhanced Upconversion Emission

Na(1-x)Lix(Gd0.39Y0.39Yb0.2Er0.02)F4 (0 ≤ x ≤ 1) (denoted as Na(1-x)LixReF4) solid solution microcrystals with different microstructures have been synthesized by a facile hydrothermal method. The influences of Li+ ion concentration on the morphology, crystal structure, grain size, and upconversion emission of the microcrystals are deeply investigated. It is found that the substitution concentration of Li+ not only causes the phase transition but also induces variation in morphology and size. When the substitution concentration of Li+ ions is lower (x ≤ 0.3), the obtained Na(1-x)LixReF4 microcrystals possess hexagonal structure and exhibit hexagonal prism in morphology. Whereas at the higher substitution concentration of Li+ ions (x ≥ 0.6), the crystalline phase of the final Na(1-x)LixReF4 microcrystals transformed into tetragonal structure and their morphology changed into octahedron except for the sample Na(1-x)LixReF4 with x = 0.9. Predictably, the obtained Na(1-x)LixReF4 microcrystals at the moderate s...