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Structural and magnetic transitions in minerals and functional materials: the pervasive roles of strain and elasticity

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Lattice distortions, formally described as “strain”, accompany almost all types of phase transitions in crystalline materials, either as the driving order parameter (acoustic mode instability) or by coupling with some other driving mechanism, which may be structural (soft mode, atomic ordering, hydrogen bonding, …), ferroelectric (displacive, order/disorder, relaxor, …), magnetic (ferro/antiferromagnetic, spin-glass …), or electronic (charge order, Jahn-Teller, spin state, superconducting, metal-insulator, …). The underlying physics is the same for minerals as for functional materials used in device applications: critical fluctuations are suppressed, coupling between multiple order parameters occurs via common strains, and microstructures such as twin walls, vortices and skyrmions interact with point defects or with each other. If there are changes in strain, it is inevitable that there will also be changes in elastic moduli and these provide clear insights into the dynamics and mechanisms of any phase transition of interest. Amongst minerals, transitions in quartz and stishovite give rise to large and characteristic patterns of elastic softening which should be detectable in seismic data. Recent focus, however, has been on materials which undergo more than one phase transition. Amongst minerals with such multiple instabilities are feldspars (displacive transitions, Al/Si ordering) and pyrrhotites (vacancy ordering, magnetic transitions). Landau theory provides a coherent and robust description of how these materials will evolve with temperature, while Resonant Ultrasound Spectroscopy provides a convenient experimental method for following the variations of elastic moduli through the phase transitions. Current focus, in particular, is on the strength of magnetoelastic coupling and examples provided by pyrrhotite, hematite and ilmenite show that this can be highly variable in minerals. As an example of a functional material, the interacting structural, magnetic and superconducting transition in the pnictide Ba(Fe1-xCox)2As2 will be described. Future directions for work on functional materials relate to the use of twin walls for device applications on a nanoscale.


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Evans, D.M., J.A.Schiemer, T.Wolf, P.Adelmann, A.E.Böhmer, C.Meingast, S.E.Dutton, Y.-T.Hsu, M.A.Carpenter (2019) Strain relaxation behaviour of vortices in a multiferroic superconductor. Journal of Physics: Condensed Matter 31, 135403.

Carpenter, M.A., D.L.Evans, J.A.Schiemer, T.Wolf, P. Adelmann, A.E.Böhmer, C.Meingast, S.E.Dutton, P.Mukherjee, C.J.Howard (2019) Ferroelasticity, anelasticity and magnetoelastic relaxation in Co-doped iron pnictide: Ba(Fe0.957Co0.043)2As2. Journal of Physics: Condensed Matter 31, 155401.

Haines, C.R.S., C.J.Howard, R.J.Harrison, M.A.Carpenter (2019) Group theoretical analysis of structural instability, vacancy ordering and magnetic transitions in the system troilite (FeS) – pyrrhotite (Fe1−xS). Acta Crystallographica B 75 1208 –1224.

Haines, C.R.S., S.E.Dutton, M.W.R.Volk, M.A.Carpenter (2020) Magnetoelastic properties and behaviour of 4C pyrrhotite, Fe7S8, through the Besnus transition. Journal of Physics: Condensed Matter 32, 405401.

Haines, C.R.S., G.I.Lampronti, M.A.Carpenter (2020) Magnetoelastic coupling associated with vacancy ordering and ferrimagnetism in natural pyrrhotite, Fe7S8. Journal of Physics: Condensed Matter 32 385401.

Zhang, Y., S.Fu, B.Wang, J.-F.Lin (2021) Elasticity of a pseudoproper ferroelastic transition from stishovite to post-stishovite at high pressure. Physical Review Letters 126, 025701

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