Topotactic Transformation Route to Monodisperse β-NaYF4:Ln3+ Microcrystals with Luminescence Properties

A novel nonorganic wet route for direct synthesis of uniform hexagonal beta-NaYF4:Ln(3+) (Ln = Eu, Tb, Ce/Tb, Yb/Er, and Yb/Tm) microcrystals with various morphologies has been developed wherein the intermediate routine cubic-hexagonal (alpha -> beta) phase transfer process was avoided. The morphology can be effectively tuned into hexagonal disc, prism, and novel hierarchical architectures by systematically fine manipulating the Na2CO3/F- feeding ratio. It has been found that the routine alpha -> beta phase transfer for NaYF4 was not detected during the growth, while NaY(CO3)F-2 emerged in the initial reaction stage and fast transformed into beta-NaYF4 via a novel topotactic transformation behavior. Detailed structural analysis showed that beta-NaYF4 preferred the [001] epitaxial growth direction of NaY(CO3)F-2 due to the structural matching of [001]NaY-(CO3)F-2//[0001]beta-NaYF4. Besides, the potential application of the as-prepared products as phosphors is emphasized by demonstrating multicolor emissions including downconversion, upconversion, and energy transfer (Ce-Tb) process by lanthanides doping.