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A new series of 20-component fluorite-based compositionally complex oxides (20CCFBOxNb/Ta) with the general chemical formula (15RE1/15)2x+1(Ce1/3Zr1/3Hf1/3)3-3x(Nb1/2Ta1/2)xO8-delta (0 <= x <= 1, where 15RE1/15 = La1/15Pr1/15Nd1/15Sm1/15Eu1/15Gd1/15Tb1/15Dy1/15Y1/15Ho1/15Er1/15Tm1/15Yb1/15Lu1/15Sc1/15) are synthesized. Despite that the Gibbs phase rule allows for the existence of up to 20 phases at the thermodynamic equilibrium, 17 of the 20CCFBOxNb/Ta compositions synthesized in this study all possess single ultrahigh-entropy phases in fluorite, pyrochlore, or weberite structure, as shown by X-ray diffraction (XRD). Only < 1 vol.% of secondary phases are observed in two compositions near the phase-transition points. With changing compositional variable x, this series of 20CCFBOxNb/Ta undergoes an abrupt fluorite-pyrochlore transition at x = ~0.27 and an abrupt pyrochlore-weberite transition at x = ~0.87. Careful characterization reveals abrupt changes of order parameters at both phase transitions. In addition, weberite short-range ordering can persist into the long-range pyrochlore phase, which leads to the lowest thermal conductivities. 

Compositionally complex oxides (CCOs) are an emerging class of materials encompassing high entropy and entropy stabilized oxides. These promising advanced materials leverage tunable chemical bond structure, lattice distortion, and chemical disorder for unprecedented properties. Grain boundary (GB) and point defect segregation to GBs are relatively understudied in CCOs even though they can govern macroscopic material properties. For example, GB segregation can govern local chemical (dis)order and point defect distribution, playing a critical role in electrochemical reaction kinetics, and charge and mass transport in solid electrolytes. However, compared with conventional oxides, GBs in multi-cation CCO systems are expected to exhibit more complex segregation phenomena and, thus, prove more difficult to tune through GB design strategies. Here, GB segregation was studied in a model perovskite CCO LaFe0.7Ni0.1Co0.1Cu0.05Pd0.05O3−x textured thin film by (sub-)atomic-resolution scanning transmission electron microscopy imaging and spectroscopy. It is found that GB segregation is correlated with cation reducibility—predicted by an Ellingham diagram—as Pd and Cu segregate to GBs rich in oxygen vacancies (VO··). Furthermore, Pd and Cu segregation is highly sensitive to the concentration and spatial distribution of VO·· along the GB plane, as well as fluctuations in atomic structure and elastic strain induced by GB local disorder, such as dislocations. This work offers a perspective of controlling segregation concentration of CCO cations to GBs by tuning reducibility of CCO cations and oxygen deficiency, which is expected to guide GB design in CCOs. 

Understanding the dynamics of shear band propagation in metallic glasses remains elusive due to the limited temporal and spatial scales accessible in experiments. In micron-scale molecular dynamics simulations on two model metallic glasses, we studied the propagation of a dominant shear band under uniaxial tension with a macroscopic strain of 3-5%. For both materials, the shear band can be intersonic with a propagation speed exceeding their respective shear wave speeds. The propagation exhibits intrinsic instability that manifests itself as microbranching and considerable fluctuations in velocity. The shear strain singularity ahead of propagating shear band tip scales as 1/r (r is the distance away from the tip), independent of the macroscopic tensile strain. In addition, we studied the intersection of two shear bands under uniaxial tension, during which path deflection, speed slowing-down, and temperature rise at the junction region were observed. The dynamics of propagating shear band shown here indicate that shear band in metallic glasses can be viewed as shear crack under the framework of weakly nonlinear fracture mechanics theory.