Yttrium Oxide (Y₂O₃) Nanoparticle Crystallization in Gas-Phase Synthesis: A Molecular Dynamics Study

Y₂O₃ particles are widely used as the host materials for phosphors, which have shown great potential in energy and biology applications owing to their unique structure and properties. The crystal structure of Y₂O₃ nanoparticles is important for their properties and performance. In gas-phase synthesis, it is of particular interest how the crystalline phase initially forms from amorphous nascent Y₂O₃ nanoparticles, because this process provides the seed for the later evolution of the crystalline phase and thus may exert significant influence on the crystal structure of the final products. In this study, we use molecular dynamics simulations to study the initial crystal formation of Y₂O₃ nanoparticles at different temperatures. First, a phase diagram is derived for Y₂O₃ nanoparticles below 3.5 nm as a function of temperature and size. High-temperature environments favor the formation of the monoclinic phase, and large particle sizes favor the formation of the cubic phase. The large internal pressure of small nanoparticles dramatically decreases the cubic-monoclinic transformation temperature, from 1548 K for 3.5 nm to 673 K for 2.3 nm. The size enlargement during particle coalescence is an important mechanism for the crystallization of nascent Y₂O₃ nanoparticles in the early stage of gas-phase synthesis. Nucleation starts from the surface and quickly sweeps over the entire particle, accompanied by a temperature rise. The crystallization only occurs for an initial temperature below a critical value, e.g., 1373 K for 2 nm Y₂O₃ particles. For the initial temperature above the critical value, another crystallization mechanism is related to cooling after particle coalescence. In this case, the increase of the kinetic energy during coalescence is quickly dissipated to the surrounding gas molecules. A fixed cooling rate simulation shows that the crystallization can occur in this process, featured by an integral decay of atom mobility and nucleation from the core.