Size dependence of cubic to trigonal structural distortion in silver micro- and nanocrystals under high pressure Page: 4 of 11
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Figure 2 shows X-ray diffraction patterns, up to ~30 GPa, for the micro- and nano-Ag powders.
From first analysis, it appeared that no phase transformations or distortions occurred because no splits,
disappearances, or appearances of the diffraction peaks were observed. But when the d-spacing of the
(200) peak was carefully compared with that of the (111) peak, it became apparent that some sort of
distortion was induced at high pressures. For convenience, we introduce the concept of apparent
lattice parameter (ALP), ah,.' Here, ak,(P) can be derived from an individual hkl Bragg reflection
(e.g., hkl = 111 or 200) at a given pressure, P, in GPa by the equation ah ,(P) = dhk(P) * (h2 + k2 +
/12 for the cubic crystal system. It was found that ahk,(P) was strongly dependent upon the Miller
index and crystal size.
The apparent lattice parameters a200 and ami and their ratio a200/am u for the nano-Ag powder are
shown in Figure 3a and Figure 3b, respectively. It is well-known that ahk/ of Ag with the idealized fcc
structure is not dependent upon the Miller index; however, according to Figure 3, it becomes apparent
that ahk(P) in the nano-Ag is actually strongly dependent upon the Miller index when P > 1.9 GPa. In
general, a2oQ was always larger than the corresponding a1m1 and the higher the pressure, the larger the
relative difference (within the pressure range of 0 to 11.2 GPa). Specifically, the relative difference at
ambient pressure was less than 0.03% but increased to 0.37% at a pressure of 11.2 GPa. Beyond 11.2
GPa, this difference decreased slightly and then became relatively constant at ~ 0.31 % between 19.9
and 32.0 GPa, which is one order of magnitude larger than the 0.03% at ambient pressure.
Based on the results for the nano-Ag powder, we decided to do similar analyses for the micro-Ag
powder. The a2oo/am ratio as a function of pressure for the micro-Ag powder is shown in Figure 3b,
which is compared to the results obtained from the nano-Ag powder. By comparing the two curves,
we can conclude that the a200/am1I ratio under high pressure is dependent upon the crystal size, with
the ratio increasing with decreasing crystal size.
These results suggest that some kind of phase distortion is occurring in the micro- and nano-Ag
samples under high pressure. It is well-known that fcc metals have three 4-fold rotational axes and
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Guo, Qixum; Zhao, Yusheng; Zin, Zhijun; Wang, Zhongwu; Skrabalak, Sara E & Xia, Younan. Size dependence of cubic to trigonal structural distortion in silver micro- and nanocrystals under high pressure, article, January 1, 2008; [New Mexico]. (https://digital.library.unt.edu/ark:/67531/metadc925939/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.