This volume is devoted to the electron and phonon energy states of
inorganic layered crystals. The distinctive feature of these
low-dimensional materials is their easy mechanical cleavage along planes
parallel to the layers. This feature implies that the chemical binding
within each layer is much stronger than the binding between layers and
that some, but not necessarily all, physical properties of layered
crystals have two-dimensional character. In Wyckoff's Crystal
Structures, SiC and related com- pounds are regarded as layered
structures, because their atomic layers are alternately stacked
according to the requirements of cubic and hexagonal close-packing. How-
ever, the uniform (tetrahedral) coordination of the atoms in these
compounds excludes the kind of structural anisotropy that is fundamental
to the materials dis- cussed in this volume. An individual layer of a
layered crystal may be composed of either a single sheet of atoms, as in
graphite, or a set of up to five atomic sheets, as in Bi2 Te3' A layer
may also have more complicated arrangements of the atoms, as we find for
example in Sb S . But the unique feature common to all these materials
is 2 3 the structural anisotropy, which directly affects their
electronic and vibrational properties. The nature of the weak interlayer
coupling is not very well understood, despite the frequent attribution
of the coupling in the literature to van der Waals forces. Two main
facts, however, have emerged from all studies.