  Freezing and melting of Ar condensed in a granular packing of template-grown
arrays of linear mesopores (SBA-15, mean pore diameter 8 nanometer) has been
studied by specific heat measurements C as a function of fractional filling of
the pores. While interfacial melting leads to a single melting peak in C,
homogeneous and heterogeneous freezing along with a delayering transition for
partial fillings of the pores result in a complex freezing mechanism
explainable only by a consideration of regular adsorption sites (in the
cylindrical mesopores) and irregular adsorption sites (in niches of the rough
external surfaces of the grains, and at points of mutual contact of the powder
grains). The tensile pressure release upon reaching bulk liquid/vapor
coexistence quantitatively accounts for an upward shift of the
melting/freeezing temperature observed while overfilling the mesopores.
