# Comment on "Eshelby twist and correlation effects in diffraction from nanocrystals" [J. Appl. Phys. 117, 164304 (2015)]

## TL;DR

This comment emphasizes the importance of considering anisotropic effects and image fields in analyzing diffraction from nanocrystals, correcting misconceptions about correlation loss due to twist, and highlighting the limitations of X-ray diffraction analysis.

## Contribution

It demonstrates the necessity of including anisotropic strain and image fields in nanocrystal diffraction analysis and clarifies that correlation loss is not caused by twist but by measurement limitations.

## Key findings

- Anisotropic effects significantly influence strain fields in <011> Pd nanowires.
- Image fields should be incorporated into elastic field calculations.
- Correlation loss is due to diffraction limitations, not twist.

## 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.