Determination of the elastic tensor in low-symmetry structures
Institute for Materials Research,
Tohoku University, Aoba-ku - 980-77 Sendai, Japan
2 Department of Physics, Brookhaven National Laboratory Upton, NY 11973-5000, USA
Accepted: 4 June 1998
A method for determining the elastic constants of low-symmetry crystal structures with internal coordinates through the computation of the Hellmann-Feynman forces is described. The approach applies to cases where previous methods were impractical, namely cases where the macroscopic strains are coupled to the internal degrees of freedom. The approach is illustrated for triclinic , which has 12 internal degrees of freedom. In non-cubic structures invariances must be imposed on the force constants, regardless of whether those interatomic force constants were obtained from a supercell- or from a linear-response approach. Moreover, it is shown that important aspects of crystal bonding are easily analyzed in terms of the force constants.
PACS: 62.20.-x – Mechanical properties of solids / 62.20.Dc – Elasticity, elastic constants / 63.20.-e – Phonons in crystal lattices
© EDP Sciences, 1998