Investigations of the effect of halogen size on structure stability have been conducted in well-reduced and heavily interbridged zirconium chloride-iodide cluster systems. The title compounds are obtained in good yields from reactions of Zr, ZrCl4, ZrI4, and B or Be in sealed Ta tubes for similar to 4 weeks at 850 degrees C. Single-crystal diffraction at room temperature established these as Zr6Cl1.65(4)I-10.35(4)Be and Zr6Cl1.27(3)I-10.73(3)B [R (3) over bar, Z = 3, a = 14.3508(8), 14.389(1) Angstrom, c = 9.8777(9), 9.915(2) Angstrom, respectively] and Zr6Cl11.47(2)I-1.53(2)B [P4(2)/mnm, Z = 2, a = 12.030(1) Angstrom, c = 7.4991(8) Angstrom]. These are derivatives of the Zr6I12C and orthorhombic Zr6Cl13B structures, respectively, the latter containing unusual linear chains of clusters interbridged by Cl-i-i that are in turn interconnected by three-bonded Cl-a-a-a atoms. The random substitution of fractional Cl at specific I sites in the first two, and I for certain Cl in the third, was positionally resolved in all cases. The replacement always occurs at two-bonded X(i), so that single types of halogen are left in sites that interconnect clusters and generate the three-dimensional array. Structural changes seen in both structures are specifically related to relief of X ... X crowding in the parent structure (matrix effects). Substitution of Cl for I-i in the Zr6I12C type greatly reduces intercluster I ... I repulsions and allows, among other things, a 0.20 Angstrom (5.8%) reduction in Zr-I-a-i intercluster bond lengths. Increased Cl ... I repulsions caused by I substitution in orthorhombic Zr6Cl13B (Pnnm) convert the twisted chains and angular Cl-a-a-a interchain bridges to planarity in tetragonal Zr6Cl11.5I1.5B. Phase widths found are 0 less than or equal to x less than or equal to 1.4 for Zr(6)Cl(x)I(12-x)Z (Z = B, Be) and 0 less than or equal to x less than or equal to 1.5 for Zr6Cl13-xIxB. The limit for iodine substitution in the chlorine-rich rhombohedral Zr6Cl12-xIxBe is about x = 2.5.