High-resolution transmission electron microscopy of some Tin+1AXn compounds (n=1, 2; A=Al or Si; X=C or N)

The crystal structures of Ti2AlN, Ti3SiC2, and Ti3AlC1.8, studied by high-resolution transmission electron microscopy confirmed the P6(3)/mmc space group and the layered nature of these ternaries. The structure of Ti2AlN agrees with that previously determined from analysis of x-ray and neutron diffraction data. Conversely, the crystal structures of thinned Ti3SiC2 and Ti3AlC1.8 samples differ from the bulk structure as determined from x-ray and neutron diffraction analysis. Since both structures have identical symmetries and lattice parameters, the differences-which involve the shearing of the Si or Al planes in opposite directions-are indistinguishable by conventional transmission electron microscopy. This polymorphic phase transformation in the Ti(n+1)AX(n) compounds, for n greater than or equal to 2, is discussed in relation to dimensionally induced hexagonal close-packed to face-centered-cubic phase transformations in Ti-based thin multilayers. (C) 1999 American Institute of Physics. [S0021-8979(99)02217-3].