The intermetallic zirconium compounds ZrNiAl, ZrRhSn, and ZrPtGa - Structural distortions and metal-metal banding in Fe2P related compounds

ZrNiAl, ZrPtGa, and ZrRhSn have been prepared by reacting the elements in a n are-melting furnace and subsequent annealing at 970 K. These compounds ha ve previously been investigated only by use of X-ray powder data for ZrPtGa and ZrRhSn and single cry stal film data for ZrNiAl. Precise single crysta l diffractometer data are reported in the present paper. The structure of Z rNiAl is confirmed. It adopts a substitution variant of the Fe2P type: P (6 ) over bar 2m, a = 691.5(2), c = 346.6(1) pm, Z = 3, wR2 = 0.0538, 320 F-2 values, 14 parameters. The structure refinement reveals a large displacemen t parameter U-33 for one nickel position, indicative of a slight nickel dis location. Weak superstructure reflections could be found on Guinier powder patterns for ZrPtGa and ZrRhSn. These compounds crystallize with the HfRhSn type structure (Z = 6, space group P (6) over bar 2c), a superstructure of the Fe2P type: a = 714.5(1), c = 706.3(2) pm, wR2 = 0.0651, 594 F-2 values , 18 parameters for ZrPtGa and a = 734.2(2), c = 721.8(2) pm, wR2 = 0.0349, 355 F-2 values, 18 parameters for ZrRhSn. Structural motifs of these compo unds are transition metal centered trigonal prisms formed by the zirconium, aluminium, gallium, and tin atoms. While these trigonal prisms are regular in ZrNiAl, significant distortions occur in the structures of ZrPtGa and Z rRhSn. Due to the distortions of the trigonal prisms in the superstructures , some platinum and rhodium atoms are dislocated from the subcell mirror pl anes towards Pt-Pt and Ph-Ph pairs with distances of 310 and 313 pm, respec tively. The formation of the superstructure is most likely due to packing r easons. Chemical bonding in ZrRhSn was investigated on the basis of an exte nded Huckel calculation.