Mechanical behaviour is decided by the structure and kinetics of defects in materials. External forces play an important role in determining the growth, decay and motion of defects. In addition there is an inherent fluctuation in the various microscopic characteristics of the system as the latter acts as a ‘heat bath’. A disturbance that is set up in the system is affected by these fluctuations. The response of the system to external forces can be related to the behaviour of the system due to intrinsic fluctuations in the absence of impressed forces. This is the basis of relation between study of fluctuations in the system and various relaxation phenomena observed. It is proposed to discuss certain features of this subject in relation to various material properties.

Molecular-dynamics computer-simulation of an ionic molecular solid LiKSO₄ has been carried out at 300 and 1000 K using the atom-atom potentials obtained from lattice dynamical studies. We observe hopping of lithium ions to interstitial positions which is related to reorientations of sulphate tetrahedra.

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl₂CaBa₂Cu₂O₈ (Tl-2122,T_c=107 K) and YBa₂Cu₃O₇ (Y-123,T_c=92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.

Rigid-ion model calculations of phonon dispersion relations, densities of states and partial densities of states of the highT_c superconductor La₂CuO₄ and its isostructural compound La₂NiO₄ have been carried out both in the tetragonal and orthorhombic phases of La₂CuO₄ and the tetragonal phase of La₂NiO₄. The calculations are in fair agreement with reported experiments. The computed phonon dispersion in the tetragonal phase of La₂CuO₄ reproduces the soft mode behaviour for the lowest Σ₄ TO branch which is found to harden in the orthorhombic phase, consistent with experimental data.