Thermal expansion in harmonic potential

[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!

 

[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.