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 temperatur e range 10 to 320 K. Four different modifications of solid confined ga llium were found. Two of these modifications did not coincide with any known gallium structures; another two coincided with beta-Ga and diso rdered alpha-Ga. The broadening of the total melting and freezing proc esses and reduction of the phase transition temperatures compared to b ulk alpha-Ga phase were obtained. The size of confined gallium crystal lites corresponding to the different modifications was estimated for c ooling and warming. The reproducible freezing was treated as a result of the steep temperature dependence of the nucleation rate in supercoo led melts. Additional measurements of resistance for the opal filled w ith gallium revealed that it was sensitive mainly to the melting and f reezing within the gallium modification coinciding with disordered alp ha-Ga. Melting-freezing hysteresis for this modification was found to depend on temperatures of pre-warming. It was suggested that the freez ing of this modification is driven by the amount of frozen crystallite s of the phase with the highest onset of freezing. The melting broaden ing and the lowering of the phase transition temperatures for the conf ined gallium modifications were discussed.