ATOMIC MEAN-SQUARE DISPLACEMENTS IN FCC METALS

In two recent letters, one by Zoli in 1991, and the other by Schober i n 1992, the evaluation of the quasiharmonic and anharmonic contributio ns to the atomic mean-square displacement (MSD) for f.c.c. metals has been discussed. In Zoli's work, the difference in the two contribution s is found to be 91%. Schober, on the other hand, has not evaluated th e explicit anharmonic contribution to MSD. The huge difference in Zoli 's work is shown here to be due to an inaccurate evaluation of the exp licit anharmonic contribution to MSD. A proper self-contained method a s presented here, which employs the same model in the quasiharmonic an d anharmonic calculations of MSD or Debye-Waller factor, indeed shows that the two contributions differ from each other by 10-15% depending on the temperature. Larger differences exist at higher temperatures. S ome numerical results are given for a model of the fc.c. lattice, name ly a nearest-neighbour central force model employing a Lennard-Jones p otential (applicable to rare-gas solids) and the Morse potential as a model for Cu.