Zhejiang Univ, Dept Mat Sci & Engn, Lab New Struct Mat, Hangzhou 310027, Peoples R China
; Zhejiang Univ, Ctr Anal, Hangzhou 310027, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Beijing, Peoples R China
; Tech Univ Denmark, Dept Chem, DK-2800 Lyngby, Denmark
The high-pressure behaviour of nanocrystalline alpha-quartz GeO2 (q-GeO2) with average crystallite sizes of 40 and 260 nm has 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, q-GeO2 to amorphous GeO2 and amorphous GeO2 to monoclinic GeO2, are detected. The onset and end of the transition pressures for the q-GeO2-to-amorphous GeO2 phase transition are found to be approximately 10.8 and 14.9 GPa for the 40 nm q-GeO2 sample, and 9.5 and 12.4 GPa for the 260 nm q-GeO2 sample, respectively. The mixture of amorphous and monoclinic GeO2 phases remains up to 51.5 GPa during compression and even after pressure release. This result strongly suggests that the difference of free energy between the amorphous phase and the monoclinic phase might be small. Consequently, defects in the starting material, which alter the free energies of the amorphous phase and the monoclinic phase, may play a key role for the phase transformation of q-GeO2.