Unusual Compression Behavior of Nanocrystalline CeO2

Abstract

The x-ray diffraction study of 12 nm CeO2 was carried out up to ~40 GPa using an angle dispersive synchrotron-radiation in a diamond-anvil cell with different pressure transmitting medium (PTM) (4:1 methanol: ethanol mixture, silicone oil and none) at room temperature. While the cubic fluorite-type structure CeO2 was retained to the highest pressure, there is progressive broadening and intensity reduction of the reflections with increasing pressure. At pressures above 12 GPa, an unusual change in the compression curve was detected in all experiments. Significantly, apparent negative volume compressibility was observed at P = 18–27 GPa with silicone oil as PTM, however it was not detected in other circumstances. The expansion of the unit cell volume of cubic CeO2 was about 1% at pressures of 15–27 GPa. To explain this abnormal phenomenon, a dual structure model (hard amorphous shell and relatively soft crystalline core) has been proposed.

Size dependent compressibility of nano-ceria: Minimum near 33 nm

“…The concept of shorter and stronger Ce-O bonds at the surface of ceria is supported by firstprinciple electronic calculations. 8 But at large crystallite sizes, this counts little overall because of the small surface area to volume ratio. As crystallite size decreases, the shell becomes important, as the surface area to volume ratio increases, and the consequence is an increased effective bulk modulus up to a peak near 33 nm.…”

“…The data, although neither perfectly linear nor a perfect fit with a Birch-Murnaghan (BM) equation of states (EoS), are free from drastic discontinuities and sharp slope changes sometimes seen in other reports on nanoceria. 8 We attribute this to selection of the pressure-transmitting medium (PTM) as helium, neon, or argon. Experimenters who have chosen a mixture of methanol and ethanol as a PTM sometimes see significant and sharp deviations from linearity (especially above 15 GPa), which may come from the nonhydrostaticity of the PTM solidifying.…”

“…Wang 8 and colleagues reported in 2014 that the compression curve for their nanoceria was irregular above 12 GPa using silicon oil as the PTM. They supposed that good PTMs like helium or neon would be too soft to hinder the grains from touching each other and thus would be functionally equivalent to no PTM.…”

Structural phase transition of ternary dielectric SmGdO3: Evidence from angle dispersive x-ray diffraction and Raman spectroscopic studies

“…[1][2][3][4][5][6] The partial filling of the inner 4f electron shells, leading to well known lanthanide contraction, affects the physical properties of rare earth metals as well as rare earth sesquioxides. [3][4][5][6] However, 4f electrons do not participate in bonding and behave like core electrons. The screening of the 4f electrons by the Xe core makes the bulk properties of lanthanides somewhat insensitive to addition of electrons in the 4f shell.…”

“…Furthermore, the exotic and rich physical properties of these oxides could be tuned by external pressure and temperature, which induce significant crystallographic and electronic changes. [5][6][7] Recently, some of the rare-earth sesquioxides (REOs) have attracted considerable attention as high-k dielectric materials in order to replace the conventional gate dielectric material, silicon dioxide, in complementary metal-oxide semiconductor (CMOS) technology. [8][9][10] Interestingly, some of REOs were found to possess variable dielectric constant depending on their crystal structure.…”