Making Yb2Hf2O7 Defect Fluorite Uncompressible by Particle Size Reduction

In this work, we show that particle size reduction has a drastic effect on the compressibility of defect fluorite Yb2Hf2O7, as the bulk modulus radically increases by 80% when particle size is smaller than 20(4) nm. We have reached this conclusion after characterizing the high-pressure behavior of Yb2Hf2O7 nanoparticles using synchrotron powder X-ray diffraction and ab initio simulations. Our studies show a strong influence of particle size on the high-pressure behavior of Yb2Hf2O7 nanoparticles. Nanoparticles of size larger than 20(4) nm exhibit a comparable bulk modulus to that of other rare-earth hafnates (≈220 GPa). At 17(0.1) GPa, they undergo a phase transition to a monoclinic structure, which has been predicted by our simulations. The transition is not reversible, as Yb2Hf2O7 becomes amorphous under decompression. In contrast, nanoparticles smaller than 20(4) nm are highly incompressible (bulk modulus ≈ 400 GPa) and undergo a non-reversible amorphization beyond 15(0.1) GPa.