Two-dimensional (2D) materials have potential applications in nanoscale sensors and spintronic devices. Herein, motivated by experimental synthesis of a CrI${}_3$ monolayer possessing intrinsic magnetism and a Janus MoSSe monolayer with piezoelectricity, we propose a 2D Janus Cr${}_2$I${}_3$F${}_3$ monolayer as a multifunctional material exhibiting both piezoelectricity and ferromagnetism. Using density functional theory calculations, we systematically investigated the structural stability and the electronic, magnetic, and piezoelectric properties of the Janus Cr${}_2$I${}_3$F${}_3$ monolayer. We predicted that a vertical polarization of up to −0.155 × 10${}^{-10}$ C/m is induced in the Cr${}_2$I${}_3$F${}_3$ monolayer due to the breaking of symmetry. The origination mechanism of polarization was demonstrated in terms of a local dipole moment calculated by maximally localized Wannier functions. Meanwhile, it was found that a remarkable piezoelectric response can be produced under a uniaxial strain in the basal plane. The calculated piezoelectric coefficients of the Cr${}_2$I${}_3$F${}_3$ monolayer compare favorably with those of the frequently used bulk piezoelectric materials such as $\alpha$–quartz and wurtzite AlN. Particularly, the e${}_{31}$ and d${}_{31}$ values of the Cr${}_2$I${}_3$F${}_3$ monolayer are nearly 10 times as large as that of Mo-based transition metal dichalcogenides. We also found that the magnitude of e${}_{31}$ mainly arises from the ionic contribution, while the electronic contribution can be nearly neglected. The considerable piezoelectric response combined with the intrinsic magnetism make the Janus Cr${}_2$I${}_3$F${}_3$ monolayer a potential candidate for novel multifunctional devices integrating both piezoelectric and spintronic applications.

Segregation of rare earth alloying elements are known to segregate to grain boundaries in Mg and suppress grain boundary sliding via strong chemical bonds. Segregation of Mn, however, has recently been found to enhance grain boundary sliding in Mg, thereby boosting its ductility. Taking the Mg ($\overline{2}$114) twin boundary as an example, we performed a first-principles comparative study on the segregation and chemical bonding of Y, Zn, and Mn at this boundary. We found that both Y-4d and Mn-3d states hybridized with the Mg-3sp states, while Zn–Mg bonding was characterized by charge transfer only. Strong spin-polarization of Mn pushed the up-spin 3d states down, leading to less anisotropic Mn–Mg bonds with more delocalized charge distribution at the twin boundary, and thus promotes grain boundary plasticity, e.g., grain boundary sliding.

Helium ions implanted into metals can form ordered bubbles that are isomorphic to the host lattice. While long-range elastic interactions are generally believed to drive bubble superlattice formation, the interactions between individual helium solutes are not yet fully understood. Our first-principles calculations reveal that in molybdenum, Friedel oscillations induced by individual helium atoms generate potential barriers and wells that influence helium pairing and clustering at short He-He distances. These repulsive and attractive interactions at high concentrations provide thermodynamic driving forces that align randomly distributed helium atoms into Mo-He superlattices. Friedel oscillations may have broad impacts on solute–solute interactions in alloys.