One often overlooks the delicate balance that prevails between the crystal structure and the properties of condensed matter: tiny variations in the local symmetry and interatomic spacing may cause huge changes in the way a solid behaves. This is confirmed by the response of a solid to changes in temperature and pressure. Variations in the particle size, however, also cause small but regular changes in the unit cell dimensions, and can therefore provide an effective route for exploring the property landscape of a solid. Size-driven lattice distortions may affect physical properties more drastically than temperature and pressure, and are less likely to cause ‘collateral damages’. I shall first discuss the microscopic basis for size-dependent variation in the lattice parameters in different types of solids. This effect not only leads to the stabilization of novel crystal structures, but also provides a simple physical mechanism for irreversible structural transformations in solids.
Our work over the years has shown that size-induced structural changes play a crucial role in several interesting situations, such as: (a) persistence of superconductivity down to unexpectedly small sizes, (b) appearance of a magnetic moment in isolated Fe atoms embedded in a nanocrystalline metals, (c) destruction of ferroelectricity in nanocrystalline oxides, and so on.