Many Kf-HfCl4-Z systems have been explored to establish the stability and structures of Hf-6(Z)Cl12Cln cluster phases relative to the large variety known for zirconium. The slowness of Hf-HfCl4 reactions and the limited number of new phases required special care in synthesis to avoid excessive HfCl4 pressures. Only B and C interstitials (Z) gave cluster phases, namely (1) AHf(6)Cl(14)Z with A = Li, Na, K, or nothing and Z = B or C, in stuffed versions of the orthorhombic Nb6Cl14 structure; (2) orthorhombic AHf(6)Cl(15)C With A = K or Cs separately occupying one of the two different cation lattice sites in the CsKZr6Cl15B structure type; (3) cubic Nasimilar to 0.8Hf6Cl15B (stuffed Ta6Cl15) plus Na0.8Hf6Cl15B in a tetragonal subgroup when prepared with more sodium. The Na1.0Zr6Cl15B analogue of the last was also prepared and structurally refined [Na-0.8(2)Hf6Cl15B, Ia ($) over bar 3d, Z = 16, a =21.428(2) Angstrom, R/R(w) = 5.3/4.3%; Na-0.76(6)Hf6Cl15B Na-1.04(5)Zr6Cl15B, I4(1)/acd, Z = 16, a = 21.540(2), 21.622(2) Angstrom, c = 21.387(3), 21.793(3) Angstrom, R/R(w) = 5.5/4.1, 5.6/5.4%, respectively]. The tetragonal distortions in the last are in the opposite sense (c/a = 0.9929 for Hf, 1.0079 for Zr). Both distortions from cubic seem to give smaller clusters, slightly longer M-Cl-a-a distances, and an additional secondary chlorine neighbor about one of the split, fractional sodium positions. Volumes of hafnium cluster phases are 1.6-2.9% smaller than those of the analogous zirconium compounds and, in one comparable case, d(M-B) is similar to 0.03 Angstrom less for Hf than for Zr.