The Fermi energy is at the valence band maximum when M in A 14 M X 11 is trivalent (e.g., Al 3+ , see Figure ), since there are 14 nonbonding Yb 2+ atoms, 4 nonbonding Sb 3- atoms, a single (M Sb 4 ) 9-tetrahedral unit, and a single (Sb 3 ) 7-trimer per formula unit.