High-pressure Study On Nanocrystals

We have studied the high-pressure behavior of nanocrystalline Ge-â… (cubic diamond structure),q-GeO₂(α-quartz structure),andβ-Ga₂O₃ using in situ high-pressure synchrotron x-ray diffraction and diamond anvil cell techniques.The effects of grain size on transition pressure and bulk modulus have been discussed by analyzing the data of nanocrystals and comparing with that of bulk counterpart.And the high-pressure data ofâ…£A group elements,â…¢A-VA andâ…¡B-â…¥A binary compounds were summarried in order to explore the relationship between transition pressure P_tr and energy gap E_g.First,energy-dispersive synchrotron x-ray diffraction measurements of nanocrystalline Ge with grain sizes of 100 nm,49 nm,and 13 nm were carried out up to 35 GPa.Transition pressures from Ge-â… to Ge-â…¡were 11.5 GPa,12.4 GPa,and 16.4 GPa,and the bulk modulus were 88 GPa,92 GPa,and 112 GPa for Ge-â… with grain size of 100 nm,49 nm,and 13 nm, respectively.It is confirmed that the values of bulk modulus and transition pressures would increase with decreasing particle size.A model was established to calculate the bulk modulus with consideration of grain size, and Jamieson equation was used to calculate the transition pressures of samples with different sizes.Theoretical results corresponded well with the tendency of experimental data.Second,high-pressure behaviors of nanocrystalline q-GeO₂ with average crystallite sizes of 40 nm and 260 nm have been studied by in situ high-pressure synchrotron radiation x-ray diffraction measurements up to about 51.5 GPa at ambient temperature.Two phase transformations,from q-GeO₂ to amorphous GeO₂ and from amorphous GeO₂ to monoclinic GeO₂ phase,were detected.The ranges of transition pressure from q-GeO₂ to amorphous GeO₂ were found to be approximately 10.8-14.9 GPa for the 40 nm q-GeO₂ sample,and 9.5-12.4 GPa for the 260 nm q-GeO₂ sample. The mixture of amorphous and monoclinic GeO₂ phases remained up to 51.5 GPa during compression and even after pressure release.This result strongly suggested that the difference of free energy between the amorphous phase and the monoclinic phase might be small.Consequently, defects in the starting material,which could alter the free energies of amorphous phase and the monoclinic phase,may play a key role for the phase transformation of q-GeO₂.Third,free-standing nanocrystallineβ-Ga₂O₃ particles with an average grain size of 14 nm prepared by chemical method was investigated by angle-dispersive synchrotron x-ray diffraction in diamond anvil cell up to 64.9 GPa at ambient temperature.It was found thatβ- toα-Ga₂O₃ transition began at about 13.6-16.4 GPa,and extended up to 39.2 GPa.At the highest investigated pressure,onlyα-Ga₂O₃ was present,which remained after pressure release.Based on the first principle calculation and the relationship between the transition pressure and energy gap,we concluded that the transition pressure of nanocrystallineβ-Ga₂O₃ is higher than that of bulk counterpart.The enhancement of the transition pressure was mainly due to the difference of surface energy betweenβ-Ga₂O₃ andα-Ga₂O₃ phases,which was determined to be about 4.6-4.7 J/m².A Birch-Murnaghan fit to the P-V data yielded a zero-pressure bulk modulus at fixed Bo'= 4:B₀ = 228 GPa and B₀ = 333 GPa forβ-Ga₂0₃ andα-Ga₂O₃ phases,respectively.It could be concluded that the phase transition pressure and bulk modulus of nanocrystallineβ-Ga₂O₃ were higher than that of bulk counterpart.Finally,the high-pressure data ofâ…£A group elements,â…¢A-â…¤A andâ…¡B-â…¥A binary compounds were summarried.It was found that the binary compounds with wurtzite structure will transform from direct to indirect band-gap semiconductor under high pressure.But almost all semiconductors with zinc-blende structure could transform to six-fold or eight-fold coordinated metallic phase,except ZnS and ZnSe.As to ZB-structured semiconductors with metallic high-pressure phase,a linear relationship was obtained:P_trâ–³V=0.14+0.12E_g.And for semiconductors that could not transform into metallic phase during compression,the appropriate relationship tended to be P_trâ–³V＜0.14+0.12E_g.