Crystal structure and defects of Zr 4 Co 4 Si 7 (V-phase) investigated by high resolution transmission electron microsco
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Crystal structure and defects of Zr4 Co4 Si7 (V-phase) investigated by high resolution transmission electron microscope J. F. Mao, H. Q. Ye, X. G. Ning, and L. L. He Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, China
D. Z. Yang School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China (Received 8 February 1996; accepted 9 July 1996)
The results of high resolution transmission electron microscope (HRTEM) observation and image simulation show that Zr4 Co4 Si7 possesses the same structure type of Zr4 Co4 Ge7 . Addition of Fe or Ni into the Zr4 Co4 Si7 compound, except that the dimensions changed slightly, does not change the lattice type and coordination in the crystal structure, maintaining the V-phase structure. Also, twins with coherent boundaries and partially coherent interfaces are observed. The image conditions of Zr4 Co4 Si7 and the structure differences between Zr4 Co4 Si7 and tetrahedral close-packed phases are also discussed.
I. INTRODUCTION
V-phase was first reported by Westbrook1 in the ternary system of titanium-nickel-silicon, the composition of which was designated as Ti4 Ni4 Si7 . Since then, many researchers2–4 have investigated Vphases in related ternary systems of transition-metal silicides (and germanides) at the composition ratio 4 : 4 : 7. In the meantime, Jeitschko5 determined the detailed crystal structure of the V-phase in the compound Zr4 Co4 Ge7 using the x-ray diffraction technique, and then proposing the atomic unit cell of Zr4 Co4 Ge7 . Jeitschko and co-workers have also found thirteen Vphases in 1969.2 Recently, Chabot and Parth´e6 reported in a summary at least twenty-four V-phases in the ternary systems of (scandium, titanium, zirconium, hafnium, niobium, tantalum)–(chromium, manganese, iron, cobalt, nickel)–(silicon, germanium) with possible combinations of elements. In geometric examinations, they classified the V-phase as one of the typical structures of ternary transition-metal silicides (and germanides) in which the structures are characterized by two kinds of infinite columns parallel to the shortest cell axis. Those infinite columns are of face-shared square antiprisms of transmission-metal atoms with a Si (and Ge) atom in each antiprism and of face-shared octahedra of Si (and Ge) atoms with a transition-metal atom in each octahedron. To study atomic structures and defects of complex structures, the high resolution transmission electron microscope technique should be applied. In the last decades, the resolution of electron microscopes has improved to such an extent that atomic imaging becomes J. Mater. Res., Vol. 12, No. 2, Feb 1997
possible and the projection of a crystal structure in the direction of an electron beam can be directly seen. Moreover, it gives the atomic configuration of local defects, microdomain or grain boundaries, surfaces, etc.7 Therefore, the high resolution electron microscope technique has drawn m
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