X-ray and resistance studies of the melting and freezing phase transitions for gallium in an opal

The melting and freezing processes and the structure of gallium in an opal were studied using x-ray powder diffraction within the temperature range 10 to 320 K. Four different modifications of solid confined gallium were found. Two of these modifications did not coincide with any known gallium structures; another two coincided with beta-Ga and disordered alpha-Ga. The broadening of the total melting and freezing processes and reduction of the phase transition temperatures compared to bulk alpha-Ga phase were obtained. The size of confined gallium crystallites corresponding to the different modifications was estimated for cooling and warming. The reproducible freezing was treated as a result of the steep temperature dependence of the nucleation rate in supercooled melts. Additional measurements of resistance for the opal filled with gallium revealed that it was sensitive mainly to the melting and freezing within the gallium modification coinciding with disordered alpha-Ga. Melting-freezing hysteresis for this modification was found to depend on temperatures of pre-warming. It was suggested that the freezing of this modification is driven by the amount of frozen crystallites of the phase with the highest onset of freezing. The melting broadening and the lowering of the phase transition temperatures for the confined gallium modifications were discussed.