Study of melting and crystallization of Bi nanoparticles in glass 72B2O3-28Na2O

_Full text: SAXS and WAXS techniques were used to study the crystal-to-liquid and liquid-to-crystal transitions of bismuth nanoparticles (NPs) embedded in a sodium-borate glass. The study aimed to determine the dependence of the melting T_f and crystallization T_c temperatures on the radius R of Bi spherical NPs. For that purpose, samples were prepared containing Bi NPs with different average radius and size dispersion. The results confirm previous experimental observations and are also in agreement with theoretical models that predict that there is a linear dependence between melting and crystallization temperatures on the inverse of NPs radius. The values of melting and crystallization temperatures determined from the extrapolation of T_f × 1/R and T_c × 1/R curves to 1/R →0 are in good agreement with the experimental values reported in the literature for the bulk Bi. It was also observed that for Bi NPs having radius smaller than 18 Å the melting and crystallization temperatures coincide. This suggests that for Bi NPs with radius smaller than this value the existence of a permanent crystalline structure is not possible. This behavior confirms previous experimental results for Bi NPs in the same glass, and is in agreement with theoretical studies by molecular dynamics. The lattice parameters of Bi nanocrystals (a and c for hexagonal cell), calculated from the diffraction data for several samples, are smaller than the reported for bulk Bi, this contraction being larger for the lattice parameter c. These contractions are expected by theoretical models that predict this behavior as a result of relatively high fraction of atoms on the surface of NPs. From the dependence between the lattice parameters and the temperature the linear thermal expansion coefficients of Bi nanocrystals was determined. The results show the thermal expansion coefficients of Bi nanocrystals are larger than for bulk Bi. The same behavior was independently obtained from the dependence between the radius of gyration of Bi nanocrystals and the temperature, determined from SAXS. Moreover, as occurs in the case of bulk Bi, it was observed that the expansion of the c lattice parameter is larger than the expansion of the a lattice parameter. On the other hand, differently of nanocrystals the Bi nanodroplets have a thermal expansion coefficient about 40% lower than the value reported for bulk liquid Bi. (author)