Localized Soft Vibrational Modes and Coherent Structural Phase Transformations in Rutile TiO2 Nanoparticles under Negative Pressure

Kang Wang; Carla Molteni; Peter D Haynes

doi:10.1021/acs.nanolett.2c01939

Localized Soft Vibrational Modes and Coherent Structural Phase Transformations in Rutile TiO₂ Nanoparticles under Negative Pressure

Nano Lett. 2022 Jul 27;22(14):5922-5928. doi: 10.1021/acs.nanolett.2c01939. Epub 2022 Jul 7.

Authors

Kang Wang¹, Carla Molteni², Peter D Haynes¹

Affiliations

¹ Imperial College London, Department of Materials, Exhibition Road, London SW7 2AZ, U.K.
² King's College London, Department of Physics, Strand, London WC2R 2LS, U.K.

Abstract

We study the effect of size on the vibrational modes and frequencies of nanoparticles, by applying a newly developed, robust, and efficient first-principles-based method that we present in outline. We focus on rutile TiO₂, a technologically important material whose bulk exhibits a softening of a transverse acoustic mode close to $q = (\frac{1}{2}, \frac{1}{2}, \frac{1}{4})$ , which becomes unstable with the application of negative pressure. We demonstrate that, under these conditions, nanoparticles above a critical size exhibit unstable localized modes and we calculate their characteristic localization length and decomposition with respect to bulk phonons. We propose that such localized soft modes could initiate coherent structural phase transformations in small nanoparticles above a critical size.

Keywords: TiO2 nanoparticles; coherent structural phase transformations; density functional theory; localized distortion; size effect; soft vibrational modes.