THE TERNARY TITANIUM TRANSITION-METAL BISMUTHIDES TI4TBI2 WITH T = CR, MN, FE, CO, AND NI

The title compounds were prepared by reaction of the elemental compone nts and with the exception of the isotypic chromium compound their tet ragonal V4SiSb2-type crystal structures (I4/mcm, Z = 4) were determine d and refined from single-crystal X-ray data, Ti4CrBi2: a = 1051.6(1), c = 506.7(1) pm; Ti4Mn Bi-2: a = 1019.1(1), c = 497.8(1) pm, R = 0.03 1 for 176 structure factors; Ti4FeBi2: a = 1048.6(1), c = 493.3(1) pm, R = 0.013 (274 F values); Ti4CoBi2: a = 1050.6(2), c = 488.2(1) pm, R = 0.038 (472 F values); Ti4NiBi2: a = 1055.4(1), c = 481.4(1) pm, R = 0.020 (373 F values), and 14 variable parameters each, The compounds are isotypic with V4SiSb2, a structure which is isopointal with U6Mn a nd closely related to the structures of W5Si3 and TITe, All atoms have high coordination numbers, Unusual features of the structure are chan nels formed solely by the bismuth atoms and linear chains of the heavi er transition metal ions with bond distances varying between 253.3 (Cr -Cr) and 240.7 pm (Ni-Ni). The electrical conductivities of Ti4TBi2 (T = Fe, Co, Ni)-determined with a four-probe technique for sintered pol ycrystalline samples between 4 K and room temperature-indicate metalli c behavior, The magnetic susceptibilities of the five compounds were d etermined with a SQUID magnetometer, Ti4CrBi2, Ti4FeBi2, and Ti4NiBi2 are Pauli paramagnetic, The magnetic susceptibilities of Ti4MnBi2 and Ti4CoBi2 are strongly temperature dependent, The evaluation of these d ata according to a modified Curie-Weiss law suggests that both compoun ds contain one unpaired electron per formula unit, A brief discussion of chemical bonding in these compounds leads to the conclusion that co nsiderable Ti-Ti bonding must be present in these bismuthides, in spit e of the fact that the shortest Ti-Ti bonds are as long as 299 pm. (C) 1997 Academic Press.