Abstract
The structural phase transition and compressibility of CaF2 nanocrystals with size of 23 nm under high pressure were investigated by synchrotron X-ray diffraction measurement. A pressure-induced fluorite to α-PbCl2-type phase transition starts at 9.5 GPa and completes at 20.2 GPa. The phase-transition pressure is lower than that of 8 nm CaF2 nanocrystals and closer to bulk CaF2. Upon decompression, the fluorite and α-PbCl2-type structure co-exist at the ambient pressure. The bulk modulus B0 of the 23 nm CaF2 nanocrystals for the fluorite and α-PbCl2-type phase are 103(2) and 78(2) GPa, which are both larger than those of the bulk CaF2. The CaF2 nanocrystals exhibit obviously higher incompressibility compare to bulk CaF2. Further analysis demonstrates that the defect effect in our CaF2 nanocrystals plays a dominant role in the structural stability.
Highlights
Calcium fluoride (CaF2 )—with low absorption coefficient, high transmittance, anionic conductivity, and high resistivity—has become the focus of fundamental scientific research and industrial applications [1,2,3]
Selected-area electron diffraction (SAED) pattern shows that the major diffraction rings can be indexed into a cubic structure
The structural phase transition and compressibility of CaF2 nanocrystals with size of 23 nm under high pressure were investigated by synchrotron X-ray diffraction measurement
Summary
Calcium fluoride (CaF2 )—with low absorption coefficient, high transmittance, anionic conductivity, and high resistivity—has become the focus of fundamental scientific research and industrial applications [1,2,3]. Because of its typical crystal structure, CaF2 becomes the best material for high pressure research. Many theoretical and experimental studies on structural phase transition, optical, and electronic properties of CaF2 under high pressure have been reported [4,5,6,7,8,9,10,11,12,13]. CaF2 undergoes two structural phase transitions to highly coordinated structures. The first phase transition from the fluorite structure to an orthorhombic α-PbCl2 -type structure (Fm3m to Pnma) is reported to occur at 8–10 GPa [4,5,6]. Theoretical studies of CaF2 predicted that the second phase transition from α-PbCl2 -type structure to Ni2 In-type structure (Pnma to P63 /mmc) takes place at
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