STRUCTURE OF TACO4SI3, A NEW SILICIDE CLOSELY-RELATED TO THE Y13PD40SN31 STANNIDE

A new ternary compound ''TaCo4Si3'' has been studied using single-crys tal X-ray diffraction data. The crystal structure is closely related t o that of Y13Pd40Sn31 with space group P6/mmm and Z=21, a=b=17.141(3) Angstrom, c = 7.906(2) Angstrom, D-x=9.14 g cm(-3), mu (MoK alpha)=54. 5 mm(-1), F(000)=4924, R=0.049 for 308 contributing unique reflections and 36 refined parameters. In the five compounds, Y13Pd40Sn31, Li13Ni 40Si31, Sc12.7Ni40.7Ge31, NbCo4Si3 and TaCo4Si3, the structures can be described as a stacking along the c-axis of two elemental ''Zr4Al3'' and ''CaCu5''-type blocks, but the nature and the z-coordinate of the atom on the 2(e) site (0 0 z) are not well determined. In the case of TaCo4Si3, these uncertainties are solved.