Rare Earth compoundsThe most common electron counts are 18 or 19 valence electrons per formula unit for REME (RE=rare earth or early transition metal, M=late transition metal, E=main-group element) compounds containing anionic [ME]n- substructures with wurtzite-type or zincblende-type arrangements. In contrast to the zinc-blende structure, the wurtzite structure has a free c/a ratio and a free z parameter for one atom, which determines the puckering of the lattice: the two extremes are the graphite sheets of the AlB2 structure, or the wurtzite lattice. The f electrons of the rare earths are localized and are, therefore, not considered as valence electrons. Depending on the ionic radius of the rare earth ion, the REME compound crystallizes in either a cubic structure, or a hexagonal structure containing a more or less puckered anionic sublattice.
It was recently shown that 18-valence-electron compounds with the stuffed wurtzite-type or zincblende-type structure (LiGaGe or C1b structure) are closed-shell species, which are semiconducting, or at least have a pseudogap at the Fermi energy.
For further information see:
Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds
Frederick Casper, Claudia Felser, Ram Seshadri, C. Peter Sebastian and Rainer Pöttgen
Accepted J. Phys. D 2008 arXiv:0710.5769
Covalent bonding and the nature of band gaps in some half-Heusler compounds
Hem Chandra Kandpal, Claudia Felser and Ram Seshadri, J. Phys. D: Appl. Phys. 39 (2006) 776, doi:10.1088/0022-3727/39/5/S02