Mechanically induced phase transformation of zinc sulfide

• MD simulations predict transformation of wurtzite to cubic ZnS by compression.
• Powder compression and wet milling experiments agree well with MD simulations.
• Stress number governs degree of phase transformation and defect formation.
• Mechanically induced microstructural changes lower photoluminescence of ZnS.

Molecular dynamics (MD) simulations of the consecutive compression–decompression cycles of hexagonal zinc sulfide (wurtzite) nanoparticles predict an irreversible phase transformation to the cubic polymorph. The phase transformation commences at the contact area between the particle and the indenter and proceeds with the number of compression cycles. Dislocations are visible for a particle size above 5 nm.Results from wet grinding and dry powder compression experiments on a commercial wurtzite pigment agree qualitatively with MD simulation predictions. X-ray diffraction patterns reveal that the amount of cubic polymorph in the compressed samples increases with pressure applied to the powder. In comparison with powder compression, wet milling leads to a more pronounced phase transformation. This occurs because the particles are exposed to a large number of stress events by collision with the grinding media, which leads to the formation of defects and new surface crystallites by particle fracture. According to the MD simulations, phase transformation is expected to occur preferentially in surface crystallites because they experience the highest mechanical load.Because of the phase transformation, the wet ground and compressed samples exhibit a lower photoluminescence intensity than the feed material. In comparison with powder compression, milling reduces the photoluminescence intensity more substantially. This occurs because a higher defect concentration is formed. The defects contribute to the phase transformation and photoluminescence quenching.

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