Discovery of new compounds? is paramount in solid state and materials chemistry. Oftentimes, synthesis of such new compounds is contingent upon tedious optimizations. Application of theory and algorithms to predict possible crystal structures have aided in the discovery process however, complex crystal structures with large unit cells are still out of reach. Rapid compositional space scanning of K-Zn-Sb system performed using the hydride route resulted in the discovery of K8-xZn18+3xSb16. Subsequent reactions in the same system guided by machine learning led to the synthesis of the novel clathrate XI K58Zn122Sb207. The initial discovery of K8-xZn18+3xSb16 in the K-Zn-Sb was due to rapid screening of the ternary phase space using the hydride synthesis method. This allowed multiple reactions to be carried out in a short period of time and thus locate the region of existence of the target phase. This K8-xZn18+3xSb16 (P42/nmc) structure features a 3D Zn-Sb framework forming large channels inside which K cations reside. Throughout the optimization of synthesis of this initial phase, another unknown phase appeared. Using heats of formation calculated using machine learning algorithms and in-situ X-ray diffraction, the ideal reaction composition and synthetic conditions were derived allowing to establish the complex crystal structure of new clathrate XI: K58Zn122Sb207. This crystal structure features [Zn-Sb] frameworks with four types of cages, each containing K cations. The K cation is free to rattle within these cages which results in an extraordinarily low thermal conductivity. Two types of the cages have been observed in other clathrate structures however two new cage types were found in the structure of K58Zn122Sb207. The crystal structure solution was performed as a 6-component twin due to the pseudocubic symmetry and racemic twinning in the I-42m space group. High resolution synchrotron powder diffraction as well as state-of-the-art scanning transmission electron microscopy further confirmed this unique structure.

Clathrate XI and other compounds in K-Zn-Sb ternary system unlocked by hydride route

ACS Fall 2021

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