High-pressure study on borides, nanocrystals and negative thermal expansion materials

By the use of Mao-Bell diamond anvil cell, employed with x-ray diffraction, optical absorption, Raman and Fourier Transform Infrared (FTIR) spectroscopy, iron borides, nanocrystalline Ni, Fe, Ni₃Fe, Al₂O ₃, Negative Thermal Expansion (NTE) materials such as HfW₂O ₈, ZrW₂O₈, ZrMo₂O₈ have been studied under high pressure.; The results of a synchrotron x-ray diffraction study of Fe₂B under quasi-hydrostatic conditions from 0 to 50 GPa are reported. Over this pressure range, no phase change or disproportionation has been observed. A value of the bulk modulus, K, of 192 ± 14 GPa and the first pressure derivative of the bulk modulus, K′, of 2.6 ± 0.6, are obtained. The compression is found to be anisotropic, with the a-axis being more incompressible than the c-axis.; X-ray diffraction data of nanocrystalline Ni, Fe and Ni₃Fe, using a synchrotron source, was collected under nonhydrostatic and quasi-hydrostatic conditions up to 60 GPa. The bulk moduli, of 185.4 ± 10 GPa, 171 ± 5 GPa, 179.4 ± 8.1 GPa, 168.3 ± 2.6 GPa, are determined from quasi-hydrostatic compression data of nanocrystalline Ni, α-Fe, ϵ-Fe and Ni₃Fe, respectively, which are found similar to those of large-grained counterparts. Their phase transformations are studied and compared with their bulk counterparts.; A new phase of Al₂O₃ formed by compression of the nanocrystalline γ-phase has been detected. This high-pressure phase is metastable upon decompression to ambient pressure, and has a bulk modulus of 251 ± 10 GPa for Al₂O₃ of 67 nm. From hydrostatic compression, bulk moduli of K₆₇ = 238 ± 3 GPa and K₃₇ = 172 ± 3 GPa are obtained for the 67 nm and 37 nm γ-Al₂O₃ particles, respectively, which are significantly higher than that found in a previous study of smaller sized nanocrystals of γ-alumina (K₂₀ = 162 ± 14 GPa for 20 nm crystallites).; High pressure optical absorption, Raman and FTIR measurements are carried out on negative thermal expansion materials: HfW₂O₈, ZrW₂O₈ and ZrMo₂O₈. The changes under pressure of their stretching and bending modes as well as lattice modes have been studied. Phase transitions are examined. From the measured pressure dependence of their band gaps, the metalization pressures are estimated.