Title: Comment on "Eshelby twist and correlation effects in diffraction from nanocrystals" [J. Appl. Phys. 117, 164304 (2015)],
Abstract: The aim of this comment is to show that anisotropic effects and image fields
should not be omitted as they are in the publication of A. Leonardi, S. Ryu, N.
M. Pugno, and P. Scardi (LRPS) [J. Appl. Phys. 117, 164304 (2015)] on Pd <011>
cylindrical nanowires containing an axial screw dislocation. Indeed, according
to our previous study [Phys. Rev. B 88, 224101 (2013)], the axial displacement
field along the nanowire exhibits both a radial and an azimuthal dependence
with a twofold symmetry due the <011> orientation. As a consequence, the
deviatoric strain term used by LRPS is not suitable to analyze the anisotropic
strain fields that should be observed in their atomistic simulations. In this
comment, we first illustrate the importance of anisotropy in <011> Pd nanowire
by calculating the azimuthal dependence of the deviatoric strain term. Then the
expression of the anisotropic elastic field is recalled in term of strain
tensor components to show that image fields should be also considered. The
other aspect of this comment concerns the supposedly loss of correlation along
the nanorod caused by the twist. It is claimed for instance by LRPS that : "As
an effect of the dislocation strain and twist, if the cylinder is long enough,
upper/lower regions tend to lose correlation, as if the rod were made of
different sub-domains.". This assertion appears to us misleading since for any
twist the position of all the atoms in the nanorod is perfectly defined and
therefore prevents any loss of correlation. To clarify this point, it should be
specified that this apparent loss of correlation can not be ascribed to the
twisted state of the nanowire but is rather due to a limitation of the X-ray
powder diffraction. Considering for instance coherent X-ray diffraction, we
show an example of high twist where the simulated diffractogram presents a
clear signature of the perfect correlation.