Neutron Rietveld refinement of the structure of the ternary silicide Ti 4 Ni 4 Si 7
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The structure of Ti4Ni4Si7 is studied using powder neutron diffraction and Rietveld analysis. Precise determination of the crystalline parameters, atomic positions, and temperature factors is achieved. A criterion previously elaborated for the phase TiFeSi2 is used to determine the coordination number. From the arrangement of the atoms in the unit cell and their interbond distances, we find that Si atoms are nearest neighbors to Ti and Ni. An extension of the same criterion shows that Ti-Si and Ni-Si hybridization is likely to be strong while Ti-Ni hybridization is weak. A comparison with mono- and disilicides is made.
I. INTRODUCTION
Binary silicides containing refractory or near noble metals are of great technological importance in VLSI technology as gates, interconnects, and contacts.1 In principle, ternary silicides or pseudobinary solid solutions of Si and two metals provide additional diversity, including optimization of electrical properties by adjusting the degree and nature of Si/metal orbital hybridization.2 A large number of Si and Ge ternary compounds have been reported, but their structural and physical properties are poorly understood. A typical Si-rich ternary compound is the so-called V phase, exemplified by Ti4Ni4Si7 first reported by Westbrook et al.3 and later on by Markiv et al.4 The structure is tetragonal, space group I4/mmm.5 Analogous Ge compounds also exist.6 Table I lists the V-phase silicides obtained by permuting the choices for two metals, as well as other, less prevalent ternary silicides. We have prepared Ti4Ni4Si7 by two methods: rapid thermal annealing of rf sputtered Ti/Ni or Ni/Ti bilayer films on Si(100),9 or arc melting of stoichiometric mixtures of the elements. In this paper we report high-precision lattice parameters, atomic positions, and temperature factors for Ti4Ni4Si7 obtained by Rietveld refinement of powder neutron diffraction data. The results should help to understand the differences in bonding and properties between binary and ordered ternary silicides. II. EXPERIMENTAL PROCEDURE
Guided by the ternary phase diagram,3'4 15 g of Ti4Ni4Si7 were prepared by arc melting Ti sponge, Ni rod (99.99%), and Si crunch (99.99%). The sample was then annealed for 5 days in an evacuated quartz tube at a)Current
address: Department of Geological Sciences, University College London, London WC1E 6BT, England. J, Mater. Res., Vol. 5, No. 9, Sep 1990
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1000 °C. An x-ray diffraction pattern was then taken, and the agreement between the experimental and calculated d values was good. The neutron diffraction measurements were then performed on the H4-S triple axis powder diffractometer at the High Flux Beam Reactor at Brookhaven National Laboratory (BNL). A Si(220) monochrornator was used, and the neutron wavelength was 1.388 A. A Pyrolitic Graphite (004) analyzing crystal was used. The pattern was taken from 20 = 10° to 20 = 102.4° in steps of 0.15°. A vanadium sample can was used to minimize absorption. The powder profile refineme
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