  The binary (simple metal) phase diagram Au-Cd contains a number of
intermetallic compounds with various distortions, superlattices and vacancies.
To understand the reasons of these structural complexities and their phase
stability, we analyze these crystal structures within the nearly free-electron
model in the frame of Fermi sphere - Brillouin zone interactions. Examination
of the Brillouin-Jones configuration in relation to the nearly-free electron
Fermi sphere provides insights for significance of the valence electron energy
contribution to the phase stability. Representation of these complex structures
in the reciprocal space clarifies their relationship to simple basic cells.
This approach shows the importance of the additional planes for the stability
of superslattices. The AuCd-hP18, AuCd3-hP24 and AuCd4-hP273 structures are
shown to be related to the AuCd-cP2 via rhombohedral distortion with
superlattices and vacancies.
