Grain boundary structural transformation induced by co-segregation of aliovalent dopants

Abstract

Impurity doping is a conventional but one of the most effective ways to control the functional properties of materials. In insulating materials, the dopant solubility limit is considerably low in general, and the dopants often segregate to grain boundaries (GBs) in polycrystals, which significantly alter their entire properties. However, detailed mechanisms on how dopant atoms form structures at GBs and change their properties remain a matter of conjecture. Here, we show GB structural transformation in α-Al2O3 induced by co-segregation of Ca and Si aliovalent dopants using atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations. To accommodate large-sized Ca ions at the GB core, the pristine GB atomic structure is transformed into a new GB structure with larger free volumes. Moreover, the Si and Ca dopants form a chemically ordered structure, and the charge compensation is achieved within the narrow GB core region rather than forming broader space charge layers. Our findings give an insight into GB engineering by utilizing aliovalent co-segregation.

An effective strategy to inhibit grain coarsening: Construction of multi-element co-segregated grain boundary complexion

“…The mismatch (ε) is calculated as: ε=(rh-rd)/rh×100%, where rh and rd are the ionic radius of the host ion and the dopant one, respectively. In addition, ZrO2 GBs are positively charged, and dopant ions with valence states lower than +4 can form a negative space charge cloud at the GBs, which could also enhance the GB segregation tendency of the dopant ions [24,30]. Taking the above two criteria into consideration, we selected five dopant ions, i.e., Cs + , Ba +2 , La +3 , Ca +2 , Al +3 ions (Fig.…”

“…The second important goal of this study is to obtain atomic-scale, 3D experimental identification and quantification of GB complexion, since these are fundamental to the manipulation and utilization of GB complexion. The advancements of transmission electron microscopy (TEM) techniques make obtaining atomic-scale microstructure and composition information of GB complexion a routine work [29,30]. However, one obvious limitation of TEM techniques is that they are projection-based techniques and can only provide 2D information, which makes GB complexions far from well understood because of their structural and composition complexity.…”

Atomistic grain boundary migration in Al2O3

“…On the other hand, with the remarkable progress of aberration-corrected scanning transmission electron microscopy (STEM) over the two decades, direct imaging of complex GB is achievable, and much has been clarified about the structural and chemical character of GBs in many ceramic materials. 4,20,[36][37][38][39][40][41][42][43][44][45][46][47] However, the temporal resolution of STEM is much lower than HRTEM, which strongly limits the dynamic observations of GB migration. In this way, it is still extremely challenging to observe the atomistic GB migration even with state-of-the-art electron microscopy techniques, and the underlying GB migration mechanism remains elusive.…”

“…Therefore, although there are several in‐situ HRTEM studies on GB migrations, most of the studies were focused on the coherent GB with a relatively simple structure in metals, and it is still difficult to fully clarify the intrinsic GB migration mechanism in ceramic materials. On the other hand, with the remarkable progress of aberration‐corrected scanning transmission electron microscopy (STEM) over the two decades, direct imaging of complex GB is achievable, and much has been clarified about the structural and chemical character of GBs in many ceramic materials 4,20,36–47 . However, the temporal resolution of STEM is much lower than HRTEM, which strongly limits the dynamic observations of GB migration.…”

Oxygen vacancy-engineered titanium-based perovskite for boosting H 2 O activation and lower-temperature hydrolysis of organic sulfur

“…Surface modification with defect engineering, such as the introduction of cation or anion vacancies, the creation of lattice distortion and so on, may tune the surface electronic properties to optimize the chemical adsorption and enrich the active sites, and thus enhance the catalytic activity ( 9 – 12 ). Among these strategies, oxygen vacancy (V O ) engineering with low formation energy attracts the most attention ( 13 – 16 ).…”