Softening versus hardening transition in surface bilayer bonding of bismuth nanofilm

We have found that the surface bilayer of Bi(001) undergoes an unusual reversible transition at 350 K, well below the bulk melting temperature. Surface Debye temperature abruptly decreases without an anharmonic signature and the vertical-to-lateral ratio of surface thermal expansion increases in a critical manner, as revealed by high-resolution diffraction measurement. Because of the metallic-covalent bilayer structure of semimetal Bi, the most feasible explanation for the result is that bond softening occurs between the first and second bilayers at 350 K. This softening transition leads to the simultaneous hardening of the topmost intrabilayer bonds.