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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 (β55% at 6 nm). One reason is a β230
K offset in the bulk melting temperature which is apparently caused
by the Ξ²-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<sup>2</sup> 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. <i>J. Phys. Chem. C</i> <b>2014</b>, 10.1021/jp412531t],
a size-dependent phase diagram is proposed for this oxide material