Comment on "Colossal Pressure-Induced Softening in Scandium Fluoride".

I A Zaliznyak,A V Tkachenko,E Bozin

doi:10.1103/physrevlett.126.179601

Abstract

The results reported by Wei et al. [Phys. Rev. Lett. 124, 255502 (2020)] can be confronted with predictive, quantitative theories of negative thermal expansion (NTE) and pressure-induced softening, allowing to corroborate, or invalidate certain approaches. Motivated to corroborate the quantitative predictions of the recent Coulomb Floppy Network (CFN) microscopic theory of vibrational and thermomechanical properties of empty perovskite crystals [Tkachenko and Zaliznyak, arXiv:1908.11643 (2019)], we compared theory prediction for the mean-squared transverse displacement of the F atoms, U$_{perp}$, with that reported in Fig. 5 of Wei et al. and observed a marked discrepancy (an order-of-magnitude larger than the error bar). We then compared these results with the previously published Xray diffraction data of Greve, et al. [JACS 132, 15496 (2010)] and the neutron diffraction data of Wendt, et al. [Science Advances 5 (2019), 10.1126/sciadv.aay2748]. We found the latter two data sets to be in a good agreement with each other, as well as with the prediction of CFN theory. We thus conclude that U$_{perp}$ values reported in Fig. 5 of Wei et al. are substantially incorrect. The purpose of this Comment is twofold: (i) to caution the researchers against using the U$_{perp}$ data of Wei et al. for quantitative comparisons with theory, and (ii) to encourage Wei et al. to reconsider their analysis and obtain a reliable U$_{perp}$ data by better accounting for the beam transmission and attenuation effects.

Highlights

In a recent Letter [1], Wei et al report neutron powder diffraction measurements at variable temperature and pressure of negative thermal expansion material, scandium fluoride ScF3 [2]
The results reported by Wei et al [1] are very interesting and important because these results can be confronted with predictive, quantitative theories of negative thermal expansion (NTE) and pressure-induced softening and allow one to corroborate or invalidate certain approaches
Wei et al discuss their observations in the context of model molecular dynamics simulations and simple one-dimensional models, which capture qualitative features of the observed phenomena, but do not provide a quantitative theory

Summary

Introduction

In a recent Letter [1], Wei et al report neutron powder diffraction measurements at variable temperature and pressure of negative thermal expansion material, scandium fluoride ScF3 [2]. The diffraction patterns were fitted using the Rietveld method to refine the lattice and the atomic displacement parameters, with other aspects of the crystal structure fixed by Pm3 ̄m cubic symmetry.

Results

Conclusion