Low-temperature heat capacity and heat conductivity of single-crystal synthetic opals

The heat capacity at constant pressure (in the range 3-50 K) and the lattic e heat conductivity (from 5 to 75 K) of a single-crystal synthetic opal are measured. It is shown that the heat capacity of the opal behaves at these temperatures in a way similar to porous amorphous materials. The data on th e heat conductivity suggest that single-crystal opals can be related to a c lass of semicrystalline (partially crystallized amorphous) materials. Howev er, because of specific features of their crystal structure, the opals form a nonstandard type of semicrystalline material which we termed semiamorpho us. (C) 2001 MAIK "Nauka/Interperiodica".