Combining Compositional Screening via Hydride Route, Machine Learning, and In Situ Diffraction to Discover Antimonides in the K–Cd–Sb System

Antimonides are attractive candidates for thermoelectric materials, but like other intermetallic compounds, their compositions and structures are not easy to predict, and once predicted, they may be difficult to synthesize. Three new ternary antimonides in the K–Cd–Sb system were discovered through a multifaceted approach that involves (i) use of a machine learning algorithm to pinpoint the compositional regions with low formation energy, (ii) rapid experimental compositional screening aided by the hydride route, and (iii) determination of optimal synthesis temperature from in situ high-temperature powder X-ray diffraction data. Various experimental compositions were screened efficiently through the use of KH instead of elemental K as the starting material, which allows greater compositional control and faster reactions through more rapid diffusion. Furthermore, a simple machine learning model was developed to classify ternary K-containing intermetallics according to denser network vs more open (clathrate, layer, and channel) structures and to identify compositional regions in which phases are likely to adopt open structures. This synergistic approach results in the synthesis of compositionally similar but structurally distinct antimonides: monoclinic K2Cd3Sb4 with a layered structure, tetragonal K3Cd11Sb8 with K+ filling channels in the CdSb framework, and hexagonal clathrate-like K3Cd17Sb14 with K+ in the center of 20-vertex polyhedral CdSb cages. The compositions of the three K–Cd–Sb compounds are nearly charge-balanced, and their chemical bonding can be rationalized by the Zintl concept. Low-temperature transport property measurements reveal that the electrical resistivity and thermopower change over several orders of magnitude from a semiconductor for hexagonal K3Cd17Sb14 to a heavily doped semiconductor for monoclinic K2Cd3Sb4. All three compounds exhibit low thermal conductivity, attributed to the disordered structures made of heavy Cd and Sb atoms. The strategy presented here can be expanded to other systems for the targeted discovery of new inorganic solids.