Dramatic impact of crystallographic conflict on material properties

Many material properties are associated with structural disorder that exhibits local periodicity or correlations. A new form of this phenomenon exhibiting strong disorder-phonon coupling has been shown to arise in response to crystallographic conflict, with dramatic phonon lifetime suppression.

In recent years there has been a rapidly growing understanding that, hidden within the globally periodic structures of many crystals, various forms of disorder may exist that could form ‘locally periodic’ states, which the language of classic crystallography fails to describe. Such phenomena are commonly referred to as ‘correlated disorder’ and in many functional materials, from leading ferroelectric and thermoelectric candidates to photovoltaic perovskites and ionic conductors, this correlated deviation from perfect periodicity plays a pivotal role in governing functionality. As such, understanding the role of disorder, and the correlations that exist within it, is one of the defining challenges for the development of future functional materials.

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Image: Fig. 1: a) Reciprocal space reconstructions of the (hk2)s plane. All three samples investigated are shown with relevant at. % Mo indicated. Reflections are categorised and indexed in the bottom right quadrant, parent Bragg peaks (black) and diffuse superstructure reflections from two different domains (blue/green). b) Orientational relationship between parent (blue) and superstructure (red) unit cells for one of six possible domains. All atoms in the “shear plane” (highlighted red) move collinearly with the direction of motion indicated by arrows on the plane edge. Alternate planes, demarcated by I, I, III, … , move in antiphase. c) Top-down view showing the 45 relationship between the parent and superstructure. d) Schematic of the atomic motions in a “phonon plane.” Blue dashed and red dotted lines refer to interatomic bonding in the parent and superstructure unit cells, respectively.