I Thermal expansion in harmonic potential

1. Sep 10, 2016

dam

It is generally said that thermal expansion is a process determinated by the anharmonic terms in the potential of a crystalline solid. However, in the Course of Theoretical Physics by Landau Lifshitz, Statistical Physics part 1, paragraph 67, a form for the coefficient of thermal expansion is derived from the free energy of a harmonic solid. How's that possible if in an harmonic potential no thermal expansion should be present? Thanks in advance!

2. Sep 13, 2016

tkyoung75

Ashby suggests that the atomic bond exhibits linear elasticity for only small relative displacement of atoms. I presume that this is the range for which the expansion coefficient is valid.