Thermal-conductance measurements on thin Mylar and glass plates indicate that abrasion of the plate surfaces does not suppress specular reflection of phonons, contrary to the behavior in crystalline materials. This reinforces the suggestion that nonspecular reflection at crystal surfaces is caused primarily by lattice defects—defects which are absent in amorphous materials.

• Received 28 April 1975

DOI:https://doi.org/10.1103/PhysRevB.12.4487