from amorphization to recrystallization, at critical sizes as reported for metallic (e.g., silver (15)) and semiconductor nanoparticles (e.g., Si, CdSe, PbS, TiO 2, and SnO 2 (16−21)). This can lead to qualitative changes in the behavior of nanoparticles under pressure, e.g. In nanoparticles, structural changes may nucleate at or be frustrated by the presence of surfaces, whose influence is most simply measured by the surface to volume ratio that is size-dependent. In type II transformations, the pathway between the two stable phases involves a common lower-symmetry intermediate. Here the lattice distortions and atomic displacements may therefore be correlated with the softening of one or more phonons of the corresponding symmetry. In type I transformations, the lower-symmetry phase is related to the higher-symmetry phase by a unique distortion corresponding to a nontrivial irreducible representation of the higher-symmetry group. An objective classification based upon symmetry (14) distinguishes between two fundamental types (with a third type consisting of multistage combinations of the first two). Reconstructive transformations tend to nucleate heterogeneously, whereas displacive transformations may involve homogeneous nucleation. (1−13) The additional degrees of freedom associated with size, shape, and surfaces make pressure-induced phase transformations in nanoparticles much more complex and versatile in comparison to those in the bulk.īulk structural phase transformation pathways have long been classified as reconstructive or displacive, according to whether or not the breaking and making of interatomic bonds is involved. For example, size-dependent phase transformation pressures and pressure-induced shifts in Raman peak positions, optical absorption, and kinetic barriers, as well as a tendency to amorphization, have been observed by experimental and computational methods. Much of the interest in nanomaterials derives from their size-dependent structural and functional properties, including their behavior under pressure.
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