DOPING CAN MAKE YOU HARDER (OR SOFTER)

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A group of researchers from the School of Materials Science and Engineering and the Integrated Materials Design Center in UNSW Sydney, have recently investigated the effects of chemical doping on the mechanical properties of BiFeO3 (BFO), an archetypal multiferroic material. The study has been performed on thin-film samples and has involved scanning probe-basedspectroscopic and atomic-force microscopy measurements, as well as first-principles simulations based on density functional theory.

The investigated BFO thin films have been synthesised in a strained state in which two different crystal structures with monoclinic crystal symmetry are energetically very competitive. In such a state, the system is particularly reactive to any external stimulus. The group of researchers have demonstrated that by introducing small concentrations (~1%) of isovalent (i.e., La) or aliovalent (i.e., Ca) Fe-substitutional doping, the mechanical properties of the resulting samples can be tuned with unprecedented control. In particular, La-doping induces a sizeable mechanical softening along the direction perpendicular to the substrate plane, whereas Ca-doping induces a sizeable mechanical hardening. The physical origins of these phenomena have been traced back to the appearance of oxygen-octahedral distortions and crystalline defects (e.g., vacancies) in the samples.

The results of this investigation provide new insights into the nature of BFO at the nanoscale and point to promising new avenues for applications in the fields of sensor and multifunctional materials. This study has been published in ACS Nano 11, 2805-2813 (2017).