cmJCOMELJournal of Physics: Condensed MatterJPhysCMJ. Phys.: Condens. Matter0953-89841361-648XIOP Publishingcmabcb0f10.1088/1361-648X/abcb0fabcb0fJPCM-116921.R2PaperStructure, dynamics and phase transitionsOrder–disorder, ferroelasticity and mobility of domain walls in multiferroic Cu–Cl boracite0000-0003-3080-1637Fernandez-PosadaC M1*cmf61@cam.ac.uk0000-0001-9397-4944CochardC20000-0002-6451-7768GreggJ M20000-0001-9455-5848WhatmoreR W340000-0003-2855-0007CarpenterM A1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom Centre for Nanostructured Media, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, United Kingdom Department of Chemistry, University College Cork, Cork, Ireland Department of Materials, Faculty of Engineering, Imperial College London, London, SW7 2AZ, United Kingdom

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3320211112202011122020339095402472020259202017112020292020© 2020 The Author(s). Published by IOP Publishing Ltd2021 Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Abstract

Domain walls in Cu–Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F 4̄ 3cPca21 to be modelled simply as being close to the order–disorder limit. High acoustic loss in the cubic phase, revealed by resonant ultrasound spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ∼10−5–10−6 s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order–disorder process are immobile on this time scale. A Debye loss peak accompanied by ∼1% elastic stiffening at ∼40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ⩾∼0.01–0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

conductive domain wallsferroelastic twin wallsphase transitionsboracitemultiferroicUS-Ireland R&D PartnershipProgrammeUSI 120Engineering and Physical Sciences Research Councilhttps://doi.org/10.13039/501100000266 EP/I036079/1 EP/P024904/1Natural Environment Research Councilhttps://doi.org/10.13039/501100000270 NE/B505738/1 NE/F017081/1ccc1361-648X/20/095402+11$33.00printedPrinted in the UKcrossmarkyes