Na_(1‑x)Li_x(Gd_0.39Y_0.39Yb_0.2Er_0.02)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)Li_x(Gd_0.39Y_0.39Yb_0.2Er_0.02)­F₄ (0 ≤ x ≤ 1) (denoted as Na_(1‑x)Li_xReF₄) 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)Li_xReF₄ 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)Li_xReF₄ microcrystals transformed into tetragonal structure and their morphology changed into octahedron except for the sample Na_(1‑x)Li_xReF₄ with x = 0.9. Predictably, the obtained Na_(1‑x)Li_xReF₄ microcrystals at the moderate substitution concentration of Li⁺ ions (x = 0.4 and 0.5) own the coexistent phases of hexagonal and tetragonal and display the obvious symbiotic morphologies of hexagonal prism and octahedron. All the samples showed characteristic emission peaks in green and red region with center wavelengths at 522 nm, 542 nm, and 652 nm respectively, and their upconversion intensities initially lift and then drop with the increasing x, giving the maximum at x = 0.5. Specifically, for the codoped Na_0.5Li_0.5ReF₄ microcrystal, its UC intensity of the strongest green emission at 542 nm is about 18 times and 73 times greater than that of the codoped NaReF₄ and LiReF₄ microcrystals, respectively. Moreover, the variation amplitude of intensity for the green emission bands is much greater than that of the red emission with the increasing x. It could obtain a superior host matrix for upconversion luminescence by means of the formation of Na_(1‑x)Li_xReF₄ solid solution at the feasible substitution concentration of Li⁺ (0.2 < x ≤ 0.6).