Abstract
The phase evolution of as-prepared NaYF(4):Yb,Er upconversion nanoparticles (UCNPs) with a metastable cubic structure is studied based on in situ heating experiments via transmission electron microscopy (TEM). The atomistic behavior on the single NaYF(4):Yb,Er UCNP is observed during the phase transition. The formation and evolution of voids on the NaYF(4):Yb,Er UCNP appear at a temperature below 420 °C. Small circular voids are transformed at the initial stage to a large, hexagonal-pillar shaped single void. Two different routes to reach the stable α-phase from the metastable cubic structure are identified on a single NaYF(4):Yb,Er UCNP. The first is via a stable β-phase and the second is a direct change via a liquid-like phase. The specific orientation relationships, [110](cubic)//[11[Formula: see text]0](hexagonal) and {002}(cubic)//{2[Formula: see text]00}(hexagonal), between the cubic and hexagonal structures are confirmed. Additionally, a few extra-half planes terminated in the cubic structures are also observed at the cubic/hexagonal interface.