The interaction between vacancies and twin walls, junctions, and kinks, and their mechanical properties in ferroelastic materials

Abstract

Vacancies strongly interact with twin boundaries and often change dramatically the properties of a ferroelastic material. However, the understanding of this behavior at an atomic-level is still deficient. Here we study vacancy diffusion processes across very large length-and time-scales using a combination of molecular dynamics and Monte Carlo simulations. We find that vacancies reduce their energy by residing at twin boundaries, kinks inside domain boundaries, and junctions between domain boundaries. Vacancies have the largest binding energy inside junctions and co-migrate with the motion of the junctions. For the weaker trapping inside twin boundaries, a "ghost line" may be generated because vacancies do not necessarily diffuse with moving boundaries and are left behind, leaving a trace of a previous position of the domain boundary. Needle twins act as channels for fast diffusion with almost one order of magnitude higher vacancy diffusivity than in bulk. The relative concentration of vacancies at twin boundaries (ρVa) is a function of the average vacancy concentration (CVa) with ρVa ~ CVa α and α = 0.61, in contrast to that of immobile vacancies case with α = 0.4. The concentration of vacancies at twin boundaries is enriched ca. 5 times at low temperatures. With increasing temperature, the enrichment drops as the trapping potential at the twin boundaries decreases (thermal release). The distribution of energydrop upon twin pattern evolution follows a power law. The exponent ε increases from ~1.44 to 2.0 when the vacancy concentration increases. The power law exponent is the same in the athermal region, while Vogel-Fulcher behavior is found at high temperatures.

Current vortices and magnetic fields driven by moving polar twin boundaries in ferroelastic materials

“…In a similar approach, we expand this idea to the moving kinks in twin walls and needle tips in ferroelastic materials. We argue, using atomistic simulations, that topological nano-structures (stripe twins or needles domains [28][29][30][31][32][33][34][35][36] etc.) can, thus, generate noticeable magnetism.…”

Avalanche criticality during ferroelectric/ferroelastic switching

“…Mean-field (MF) theory applied to ferroelectrics predicts the power-law distribution of jerks 29 , which is one of the signatures of scale-free processes, and atomistic simulations show that avalanches are induced by kinks and domain walls junctions [30][31][32] , as well as defects acting as pinning centres 33 . Ultraslow processes have been modelled 34 and reveal that many of the relevant ferroelectric parameters, e.g., the size of the switching events, follow power-law dependences under switching conditions.…”

Crackling noise and avalanches in minerals

“…We know today that a transistor, as an example, does not need bulk materials to operate but is often localized in tiny areas inside twin boundaries or near junctions between boundaries. The same holds for ferroic memories and memristive conductors (Salje et al 2017a;He et al 2019;Bak et al 2020;Lu et al 2020a;Zhang et al 2020;Salje 2021) where only a few atoms near domain boundaries move. The diameters or thicknesses of these functional regions are a few inter-atomic distances (Lu et al 2019(Lu et al , 2020bMcCartan et al 2020).…”