  Anomalous thermal expansion behaviour of several open frame-work compounds
has been extensively investigated using the techniques of inelastic neutron
scattering and lattice dynamics. These compounds involve increasing level of
structural complexity and flexibility, which leads to increased values of
thermal expansion coefficients approaching colossal values. In several
compounds, neutron inelastic scattering experiments have produced quantitative
estimates of the anharmonicity of phonons over a range of low energies, and
thereby explained the observed thermal expansion quantitatively. The
anharmonicity is found to be an order of magnitude larger than that in usual
materials. Lattice dynamical calculations have correctly predicted the observed
anharmonicity in the neutron experiments and revealed the overall nature of
phonons involved. In compounds showing negative thermal expansion, the phonons
responsible have rather low energies up to 10 meV. In most compounds, the
anharmonic phonons span all over the Brillouin zone, while in some cases the
specific phonons are limited to certain wave-vectors. The nature of specific
phonons responsible for anomalous behavior is found to be different in all
these compounds. These phonons generally involve transverse vibrations,
librations and internal distortions of the polyhedral units. The paper reviews
recent advances in the understanding of anomalous thermal expansion behaviour.
