The rare-earth crystal structure sequence ${\mathrm{hcp}}^{\to }$ Sm-${\mathrm{type}}^{\to }$ double-${\mathrm{hcp}}^{\to }$ fcc observed for decreasing atomic number and increasing pressure is quantitatively correlated with the $d$-band occupancy through the $d$-band energy contribution to the total energy. Self-consistent calculations show that the number of $d$ electrons is very sensitive to the relative size of the ion core. The theory thus accounts for La's anomalous double-hcp structure and the apparent deviations from the electron-per-atom rules exhibited by certain rare-earth Laves phases.

• Received 9 December 1976

DOI:https://doi.org/10.1103/PhysRevLett.38.564