Despite the complex structure, a number of chemical variations are known with the A 14 MX 11 structure (Figure ), many with similar electronic properties. The A atom is generally a +2 cation such as Yb , Ca , and Eu where +3 Rare Earth elements (e.g. La and Y ) can be substituted. The M atom can be a variety of transition metals suitable for centering the MX 4 tetrahedron (such as M = Mg 15 , Mn 8 , Al 21 , Zn , or Cd ).