Mode Grüneisen parameters and thermal expansion of GaP and InP

Abstract

The mode Grüneisen parameters γ i ( q) and those γ i ( q→0) for long-wave phonons of GaP and InP are studied from the electronic theory of solids by using the lattice dynamical method and the homogeneous deformation method. The agreement of the obtained results for γ i ( q) and γ i ( q→0) with the experimental data available for GaP and InP is excellent. The temperature dependence of the average Grüneisen constant γ( T) and the thermal expansion coefficient α( T) for GaP and InP obtained for the calculated Grüneisen parameters is also presented. The obtained α( T) for GaP becomes negative at low temperatures by summing the phonon mode over the irreducible 1 48-th part of the Brillouin zone, contrary to the previous prediction of the positive thermal expansion. The simplified treatment by the weighted averaging in the symmetry directions such as [100], [110] and [111] does not give the correct results for γ( T) and α( T).