LATTICE EXPANSION AND GRAIN BOUNDARY EXCESS VOLUME OF NANOCRYSTALLINE Se PREPARED BY MECHANICAL MILLING

Lattice constant is one of the most principle crystal parameters, which critically determines physical, chemical, mechanical and thermodynamic, and other properties. Conversely, material properties could be tuned for specific applications by tailoring the lattice constants. Practically, the lattice constants were modified by controlling surface/boundary/interface, lattice solute/defect and external conditions (e.g. temperature, stress, etc.). Literature inspections on the lattice constants of nanocrystalline (nc) materials show ambiguous results indicating either lattice contraction or expansion. To clarify the contradictory results, in this work, nc Se specimens with different grain sizes were prepared by mechanical milling. Quantitative x-ray-diffraction measurements revealed that the nc Se specimens exhibit large lattice expansions with decreasing grain size. The measured lattice expansion in nc Se can be understood on the basis of the stress field generated at grain boundary (GB) due to the excess GB free volume. The measured unit cell volumes of nc Se were well fitted using a GB stress field model from which the excess GB volume also shows processing method dependence.