The structural phase transition and compressibility of CaF 2 nanocrystals with size of 23 nm under high pressure were investigated by synchrotron X-ray diffraction measurement. A pressure-induced fluorite to α-PbCl 2 -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 CaF 2 nanocrystals and closer to bulk CaF 2 . Upon decompression, the fluorite and α-PbCl 2 -type structure co-exist at the ambient pressure. The bulk modulus B 0 of the 23 nm CaF 2 nanocrystals for the fluorite and α-PbCl 2 -type phase are 103(2) and 78(2) GPa, which are both larger than those of the bulk CaF 2 . The CaF 2 nanocrystals exhibit obviously higher incompressibility compare to bulk CaF 2 . Further analysis demonstrates that the defect effect in our CaF 2 nanocrystals plays a dominant role in the structural stability.
The high-pressure transport behavior of CaF 2 nanoparticles with 3 mol% Tb concentrations was studied by alternate-current impedance measurement. All of the electrical parameters vary abnormally at approximately 10.76 GPa, corresponding to the fluorite-cotunnite structural transition. The substitution of Ca 2+ by Tb 3+ leads to deformation in the lattice, and finally lowers the transition pressure. The F − ions diffusion, electronic transport, and charge-discharge process become more difficult with the rising pressure. In the electronic transport process, defects at grains play a dominant role. The charge carriers include both F − ions and electrons, and electrons are dominant in the transport process. The Tb doping improves the pressure effect on the transport behavior of CaF 2 nanocrystals.
We investigate the carrier behavior of HgTe under high pressures up to 23 GPa using in situ Hall effect measurements. As the phase transitions from zinc blende to cinnabar, then to rock salt, and finally to Cmcm occur, all the parameters change discontinuously. The conductivity variation under compression is described by the carrier parameters. For the zinc blende phase, both the decrease of carrier concentration and the increase of mobility indicate the overlapped valence band and conduction band separates with pressure. Pressure causes an increase in the hole concentration of HgTe in the cinnabar phase, which leads to the carrier-type inversion and the lowest mobility at 5.6 GPa. In the phase transition process from zinc blende to rock salt, Te atoms are the major ones in atomic movements in the pressure regions of 1.0–1.5 GPa and 1.8–3.1 GPa, whereas Hg atoms are the major ones in the pressure regions of 1.5–1.8 GPa and 3.1–7.7 GPa. The polar optical scattering of the rock salt phase decreases with pressure.
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