Polymorphism of Ti 3 SiC 2
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We investigated the crystal structure of Ti3SiC2 by means of high-resolution electron microscopy (HREM). Two polymorphs, ␣– and –Ti3SiC2, were identified. The amount of the ␣ phase was larger than the  phase, indicating that the former has lower energy than the latter. We also found that the bright spots in HREM images of Ti3SiC2 do not necessarily correspond to the atomic columns; thus an intuitive interpretation of the image contrast in terms of the stacking sequences of the close-packed layers should be made cautiously.
The ternary carbide Ti3SiC2 has a unique combination of mechanical, electrical and thermal properties and has been the subject of numerous studies in recent years.1–8 It has high elastic moduli and easy machinability,9 good thermal and electrical conductivity,10,11 and excellent oxidation properties12 and fatigue resistance.13 Very recently it was shown that the thermopower of Ti3SiC2 is essentially zero over an extended temperature range (from 300 to 850 K).14 Ti3SiC2 was determined to be a hexagonal structure with the space group P63/mmc (194) and the lattice parameters a ⳱ 0.3068 nm, c ⳱ 1.7669 nm.15 The atomic positions of Ti correspond to the 2a and the 4f (zTi ⳱ 0.1350), Si to the 2b, and C to the 4f (zc ⳱ 0.5675) Wyckoff positions. The structure is shown in Fig. 1 as ␣–Ti3SiC2. It can also be described as a layer stacking sequence of the Ti and Si atoms: ABABACAC
.
(1)
The underlined letters refer to the layers of the Si atoms, the remainder are the layers of the Ti atoms. The carbon atoms occupy the octahedral interstitial sites of Ti atoms and are not shown in this representation. Recently, Farber et al.16 reported that the structure of Ti3SiC2 observed using high-resolution electron microscopy (HREM) was inconsistent with that determined from x-ray diffraction data. They found that the HREM image of Ti3SiC2 has the following stacking sequence of bright spots: ABCBACBC
.
(2)
According to a one-to-one correspondence between the bright spots in the image and atomic columns, Farber et al.16 attributed the bright spot stacking sequence (2) to the structure with the atomic layer stacking sequence (2). 948
J. Mater. Res., Vol. 17, No. 5, May 2002
Thus, they concluded that a polymorphic phase transformation occurred in the transmission electron microscope (TEM) samples during the thinning process. In their transformation model, the transition occurs by a shear of consecutive Si layers in opposite directions while the neighboring Ti3C2 layers remain static. However, in such a close-packed structure, keeping the Ti3C2 layers static will make the shear of the sandwiched Si layers very difficult, if not impossible. In the present work, the structure was investigated in detail by HREM. We first found that the bright spots in the HREM images do not necessarily correspond to the atomic columns of Ti3SiC2; thus an intuitive interpretation of the image contrast in terms of the stacking sequences of the close-packed layers should be made cautiously. On the other hand, we found that the structure
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