Size-Dependent Structural Stability and Tuning Mechanism: A Case of Zinc Sulfide

Synchrotron X-ray diffraction and thermodynamic calculation combined with particle size dependent defect ratio, stress distribution, and surface energy have been used to tackle the structural stability and phase transformation of bulk and nano zincblende (ZnS). X-ray diffractions demonstrate that bulk zincblende-type ZnS transforms to a rocksalt phase at ∼15.4 GPa associated with an in situ fracture. The transition pressure remains constant upon decreasing particle size to 15 nm. Below 15 nm, this phase transformation occurs at an elevated pressure; the particle does not break down and instead transforms to a smaller rocksalt grain with 18% volumetric shrinkage relative to zincblende. The two defined critical sizes of 3.4 and 15 nm represent the largest size for a particle that starts a defect extinction and homogeneous stress distribution, respectively, thus enabling interpretation of a combined effect of pressure and particle size. When the particle is larger than 15 nm, the hosted defect (or dislocation) acts to behave similar to that in bulk; the resultant stress concentration under compression is responsible for the deformation of zincblende and subsequent nucleation and fracture of rocksalt. Below 15 nm, a series of defect activities turns silent; alternatively, surface energy and multiparticle interaction begin directing the enhancement of structural stability. The surface energy of rocksalt is calculated as ∼2.33 J/m 2, 0.57 J/m 2 greater than that of zincblende. The proposed mechanism enables not only a reconciliation of incompatible results but also a reasonable interpretation of the reversed Hall−Petch relation observed in materials at a critical size of 10−20 nm. This study provides important information for optimizing feasible routes for fabrication of multifunctional engineering materials with tailored properties.