Acquisition of Emissive Single-Crystalline AEu(MoO4)2 (A = Na, K, Rb, and Cs) Double Molybdates by One-Pot Hydrothermal Synthesis: Relationship between Particle Morphology and Photoluminescence Properties

Emissive double molybdates, AEu(MoO4)2 (A = Na, K, Rb, Cs), have garnered significant attention due to the diversity of crystal structures depending on the ionic radius of A+ ions. However, the relationship between particle morphology and photoluminescence properties has not been reported for them. In this study, single-crystalline double molybdates were successfully synthesized by varying the molar ratio of A/Eu through a one-pot hydrothermal route without an annealing process. This approach allowed for the creation of a variety of crystal structures and particle morphologies. The two-dimensional (2D) layered materials of KEu(MoO4)2, RbEu(MoO4)2, and CsEu(MoO4)2 exhibit a luminescence intensity stronger than that of the scheelite-type NaEu(MoO4)2 with a three-dimensional (3D) framework structure because the layered structure can activate a higher concentration of Eu3+ ions due to the suppression of migration of excitation energy between Eu3+ ions. By varying the amount of A sources added, the morphologies were drastically changed, and their luminescence intensity was improved. Particularly, the particle morphologies of RbEu(MoO4)2 changed from rhombic nanoplate-like to rod-like and, finally, to hexagonal plate-like particles. The luminescence intensity achieved up to 5.8 times higher, attributed to the crystal growth along the in-plane direction of RbEu(MoO4)2 including the EuO8 layer.