  Dependence of structural parameters on the size of nanoparticles is a topic
of general interest where the effect of shape is often neglected. We report a
comprehensive study on size dependent structural parameters of ZnO
nanostructures (NS) having a wide range of aspect ratios (length/diameter). It
reveals that with increase in size, ZnO NS undergo a shape transition from
spherical to rod like morphology that induces sudden change in internal
parameter (u) which represents relative position of two hexagonal close-packed
sublattices. The change in u introduces concomitant changes in anion-cation
(Zn-O) bond lengths as well as bond angles and thereby bears a linear
dependence with the aspect ratio (AR). Further, the unit cell volume and
microstrain decrease with increase in particle size and show a drastic
reduction when flat crystal faces begin to appear at the spherical surface
(AR~1.3). The significant change in structural parameters associated with the
shape transition arises due to surface dipole induced electrostatic relaxation
that may be further influenced by interaction with the ambient gases as
evidenced from the Extended X-ray Absorption Fine Structure (EXAFS)
measurement. The present study addresses the underlying reasons of shape
induced change in structural and electronic properties of ZnO NS.
