Robust templates for domain boundary engineering in ErMnO<inf>3</inf>

Emerging properties of domain boundaries define the emerging field of domain boundary engineering. For many applications, the domain boundary acts as template onto which the desired properties, such as (super-) conductivity, polarity, ferroelectricity, magnetism, are imposed. This requires for most applications that the domain structures remain unchanged under appropriate chemical doping. Hassanpour et al (2016 New J. Phys. 18 043015) have now shown, for the first time, that the magnetic and electric domain structures remain indeed robust against charge carrier doping (Ca2+ and Zr4+) of the workbench multi-ferroic ErMnO3. This opens the way into novel functionalities based on the nanostructure of ErMnO3.This is the final version of the article. It first appeared from IOP Publishing via http://dx.doi.org/10.1088/1367-2630/18/5/05100

Porosity in minerals

&lt;abstract&gt; &lt;p&gt;Minerals typically form porous assemblies with porosity extending from a few percent to ca. 35% in porous sandstones, and over 50% in tuff, clays, and tuff. While transport of gases and liquids are widely researched in these materials, much less is known about their mechanical behaviour under stress. With the development of artificial porous materials such questions become more pertinent, e.g., for applications as fillers in car bumpers and airplane wings, and nanoscale applications in memistors and neuromorphic computers. This article argues that elasticity and related dielectric and magnetic properties can be described‑to some extend-as universal in porous materials. The collapse of porous materials under stress triggers in many cases avalanches of collapsed regions which are scale invariant and follow irreversible power law energy emission. Emphasis is given to a recent simple collapse model by Casals and Salje which covers many of the observed phenomena.&lt;/p&gt; &lt;/abstract&gt;</jats:p

Tweed, twins, and holes

Tweed, twin, and porous microstructures are traditionally studied in mineralogy to understand the thermal history of minerals, and to identify their properties such as chemical transport and elastic behavior. Recently, the same research area has blossomed in material sciences and physics with the aim to design and build devices that are based on the properties of nano-structures. Only the very existence and the properties of tweed, twins, and holes matters in this quest while the crystalline matrix plays only a minor role in the current search for novel device materials. This development has largely bypassed mineralogists while physicists did not profit from the age-long experience of mineralogists in dealing with such materials. In this Invited Centennial article, I will first discuss some key findings and approaches to foster the transfer of ideas in both directions: mineralogists can potentially inspire material scientists while the physics of the fine structure of twin walls and tweed can help mineralogists understand mineral properties in much more detail than hereto possible. Besides the observation that novel physical properties can spring from microstructures, most recent work also includes the dynamics of microstructures under external stress or electric fields. The dynamics is virtually always non-smooth or "jerky." One of the best studied jerk distribution is that of collapsing porous minerals under stress, where the main focus of research is the identification of precursor effects as warning signs for larger events such as the collapse of mines, boreholes, or even regional earthquakes. The underlying physics is the same as in large earthquakes (which can be modeled but not observed in laboratory experiments). The agreement between laboratory experiments of porous collapse and large-scale earthquakes goes well beyond each quake's statistics and includes waiting-time distributions and the Omori law of after-shocks. The same approach is used to characterize high-tech materials in aircraft industry and functional materials such as used in electronic memory devices, ferroelectric sensors and non-volatile memories and ferromagnets.The author is grateful to the Leverhulme foundation (Grant No. RPG-2012-564) and EPSRC (Grant No. EP/K009702/1) for financial support

Elastic softening of leucite and the lack of polar domain boundaries

Elastic properties of leucite have been investigated using resonant ultrasound spectroscopy over a temperature range from 300 K to 1400 K. According to these measurements, elastic moduli soften by ~50% at the Ia3d-I41/acd ferroelastic transition temperature Tc1 = 940 K relative to the value at 1400 K. A second softening is observed at Tc2 = 920 K, corresponding to the structural change from the space group I41/acd to I41/a. These elastic anomalies are analyzed in a simple model of a pseudoproper ferroelastic transition under the assumption that the transitions observed at Tc1 and Tc2 can be approximated by a single pseudoproper ferroelastic transition. The two phase transitions are accompanied by a single peak in mechanical damping attributed to the high mobility of twin walls in the intermediate phase followed by pinning in the low temperature phase. To determine whether twin walls in tetragonal leucite are polar, resonant piezoelectric spectroscopy and second harmonic generation measurements were performed but no evidence of polarity was found.E.K.H.S. is grateful to EPSRC (EP/K009702/1) and the Leverhulme Foundation (RPG-2012-564) for support. M.A.C. acknowledges NERC grants (NER/A/S/2000/01055 and NE/F017081/1).This is the final version of the article. It first appeared from De Gruyter via http://dx.doi.org/10.2138/am-2015-5313ccb

Flicker vortex structures in multiferroic materials

Computer simulation of ferroelastic materials reveals dynamic polar vortex structures related to flexo-electricity between cation and anion lattices. At finite temperatures, the vortices are found to flicker in time and space. Widely spaced ferroelastic twin boundaries nucleate vortices while dense twin boundaries suppress them. The time averaged number of vortices at any site decays exponentially, indicating the highly mobile dynamics of the vortex lattice. Applied electric fields break the rotational symmetry of vortices and finally destroy them. The total number density of vortices follows a field and temperature dependence as N(E)=N0/[1+A exp(E/k(T−TVF))] with TVF &amp;lt; 0. The observed vortex structures are akin to those observed in magnetic and superconducting disordered vortex lattices.This is the author's accepted version. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/105/11/10.1063/1.4896143?showFTTab=true&containerItemId=content/aip/journal/ap

First-principles reinvestigation of bulk WO3_{3}

Using first-principles calculations, we analyze the structural properties of tungsten trioxide WO$_{3}$. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. We show that the hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations, and tilts of oxygen octahedra) structural instabilities. Although the ferroelectric instability is the largest, the instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a $\textit{P}$2$_{1}$/$\textit{c}$ ground state. This nonpolar $\textit{P}$2$_{1}$/$\textit{c}$ phase is only different from the experimentally reported $\textit{Pc}$ ground state by the absence of a very tiny additional ferroelectric distortion. The calculations performed on a stoichiometric compound so suggest that the low-temperature phase of WO$_{3}$ is not intrinsically ferroelectric and that the experimentally observed ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed $\textit{R3m}$ and $\textit{R3c}$ ferroelectric metastable phases with large polarizations and low energies close to the $\textit{P}$2$_{1}$/$\textit{c}$ ground state, which makes WO$_{3}$ $\textbf{a}$ potential antiferroelectric material. The relative stability of various phases is discussed in terms of the anharmonic couplings between different structural distortions, highlighting a very complex interplay.The work was supported by the ARC project AIMED and the F.R.S-FNRS PDR projects HiT4FiT and MaRePeThe. Calculations have been performed within the PRACE project TheDeNoMo and relied on the Céci facilities funded by F.R.S-FNRS (Grant No. 2.5020.1) and Tier-1 supercomputer of the Fédération Wallonie-Bruxelles funded by the Walloon Region (Grant No. 1117545). E.K.H.S. is grateful for support to EPSRC and the Leverhulme trust. H.H. thanks the AVERROES-ERASMUS Mundus project

Effect of pores and grain size on the elastic and piezoelectric properties of quartz-based materials

The role of grain size and porosity in the piezoelectric and elastic properties of SiO2-based materials was investigated using resonant piezoelectric spectroscopy, RPS, and resonant ultrasound spectroscopy, RUS. RPS performed on agate revealed a piezoelectric effect comparable in magnitude to that in single crystal quartz. The observed strong piezoelectricity in agate requires preferential orientation of SiO2 during crystal growth. Similarly, in novaculite and sandstone finite (but weak) RPS signals were evident, suggesting that the expected randomization of the piezoelectric quartz grains is incomplete. On the other hand, Vycor, a silica glass with a porosity of 40%, showed no evidence of the piezoelectric effect. According to temperature dependent RPS and RUS measurements, the α-β transition temperature in quartz does not change in polycrystalline samples. Finally, the temperature dependence under heating of the elastic constants is reversible in quartz and agate and irreversible in sandstone and vycor.RUS facilities in Cambridge were established through grant no. NE/B505738/1 to MAC from the Natural Environment Research Council. EKHS is grateful to the Leverhulme Foundation (RPG-2012-564) and EPSRC (EP/K009702/1) for financial support.This is the final version. It was first published by De Gruyter at http://www.degruyter.com/view/j/ammin.2015.100.issue-5-6/am-2015-5180ccby/am-2015-5180ccby.xm

Simulating acoustic emission: The noise of collapsing domains

EPSRCThis is the accepted version of an article which is published in 'Physical Review B' at https://journals.aps.org/prb/ - the link to the published version is http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.06410

Piezoelectricity and electrostriction in ferroelastic materials with polar twin boundaries and domain junctions

Weak piezoelectricity, compared with electrostriction, occurs in twinned ferroelastic materials even when the uniform bulk material is centro-symmetric. In a simple computer simulation, polarity is exclusively generated by the flexoelectric effect. Simple twinned structures (parallel twin walls) are electrostrictive and show no piezoelectricity. Complex twinned structures break inversion symmetry by the simultaneous appearance of junctions, kinks, needle domains, etc. Such structures show weak piezoelectricity (d ∼ 10−4 pm/V) under periodic boundary conditions together with significant electrostriction. The macroscopic piezoelectric response is stronger (d ∼ 10−3 pm/V) under free boundary conditions due to the effect of relaxing surfaces.EPSR