Size-Dependent Phase Transformations in Bismuth Oxide Nanoparticles. II. Melting and Stability Diagram

Melting of nanocrystalline bismuth oxide particles between 6 and 50 nm was investigated in situ in the transmission electron microscope (TEM). It revealed a size-dependent melting behavior with a strong melting point reduction (-55% at 6 nm). One reason is a -230 K offset in the bulk melting temperature which is apparently caused by the b-phase in which the nanomaterial resides. As a second reason, a strong size dependency was observed from which an approximate solid-surface energy of 0.3 J/m(2) was determined. Yet, the conditions in the TEM could cause a lowering of the transition temperatures compared to chemically neutral conditions, although theoretical considerations predict reduction in the solid state to be negligible. Everything indicates that no stable, liquid surface layer forms prior to melting. In spite of the covalent-ionic bonds in this oxide material the qualitatively same size dependence shows like in metals. Combined with size-dependent evaporation examined in a companion study [Guenther et al. J. Phys. Chem. C 2014, 10.1021/jp412531t], a size-dependent phase diagram is proposed for this oxide material.