Close-packed layer spacing as a practical guideline for structure symmetry manipulation of IV-VI/I-V-VI_(2) thermoelectrics

The crystal-structure symmetry in real space can be inherited in the reciprocal space,making high-symmetry materials the top candidates for thermoelectrics due to their potential for significant electronic band degeneracy.A practical indicator that can quantitatively describe structural changes would help facilitate the advanced thermoelectric material design.In face-centered cubic structures,the spatial environment of the same crystallographic plane family is isotropic,such that the distances between the close-packed layers can be derived from the atomic distances within the layers.Inspired by this,the relationship between inter-and intra-layer geometric information can be used to compare crystal structures with their desired cubic symmetry.The close-packed layer spacing was found to be a practical guideline of crystal structure symmetry in IV-VI chalcogenides and I-V-VI_(2) ternary semiconductors,both of which are historically important thermoelectrics.The continuous structural evolution toward high symmetry can be described by the layer spacing when temperature or/and composition change,which is demonstrated by a series of pristine and alloyed thermoelectric materials in this work.The layerspacing-based guideline provides a quantitative pathway for manipulating crystal structures to improve the electrical and thermal properties of thermoelectric materials.