High-entropy materials have attracted considerable interest due to the combination of useful properties and promising applications. Predicting their formation remains the major hindrance to the discovery of new systems. Here we propose a descriptor—entropy forming ability—for addressing synthesizability from first principles. The formalism, based on the energy distribution spectrum of randomized calculations, captures the accessibility of equally-sampled states near the ground state and quantifies configurational disorder capable of stabilizing high-entropy homogeneous phases. The methodology is applied to disordered refractory 5-metal carbides—promising candidates for high-hardness applications. The descriptor correctly predicts the ease with which compositions can be experimentally synthesized as rock-salt high-entropy homogeneous phases, validating the ansatz, and in some cases, going beyond intuition. Several of these materials exhibit hardness up to 50% higher than rule of mixtures estimations. The entropy descriptor method has the potential to accelerate the search for high-entropy systems by rationally combining first principles with experimental synthesis and characterization.
NASICON is one of the most promising sodium solid electrolytes that can enable the assembly of cheaper and safer sodium all-solid-state batteries. In this study, we perform a combined experimental and computational investigation into the effects of aliovalent doping in NASICON on both bulk and grain boundary (secondary phase) ionic conductivity. Our results show that the dopants with low solid solubility limits in NASICON lead to the formation of a conducting (less insulating) secondary phase, thereby improving the grain boundary conductivity measured by electrochemical impedance spectroscopy (including grain-boundary, secondary-phase, and other microstructural contributions) that is otherwise hindered by the poorly-conducting secondary phases in undoped NASICON. This is accompanied by a change in the Si/P ratio in the primary NASICON bulk phase, thereby transforming monoclinic NASICON to rhombohedral NASICON. Consequently, we have synthesized NASICON chemistries with significantly improved and optimized total ionic conductivity of 2.7 mS/cm. More importantly, this study has achieved a understanding of the underlying mechanisms of improved conductivities via doping (differing from the common wisdom) and further suggests a new general direction to improve the ionic conductivity of † M.S. and B.R. contributed equally to this work.
In this study we experimentally investigated the effects of two processing techniques on the sodium-rich anti-perovskite, Na 3 OBr; namely, conventional cold pressing (CP) and spark plasma sintering (SPS). We demonstrated that the electrolyte can be synthesized via a single-step solid state reaction. We compared the CP and SPS processed samples using XRD, SEM, and EIS. From these analyses it was found that SPS reduced Na 3 OBr's interfacial impedance by three orders of magnitude, which translated into an increase in the overall ionic conductivity and a reduction in the activation energy, from 1.142 eV to 0.837 eV. DFT was used to probe the mechanisms for ionic transport in Na-rich Na 3 OBr. The formation energies of ion diffusion-facilitating defects in Na 3 OBr were found to be much higher compared to the lithium-rich anti-perovskites (LiRAP), which can explain the difference in overall ionic conductivity between the two.
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