Tunable K vacancies in K_{1− x}Co₂Se₂ and their effects on structure and ferromagnetism

Deeply understanding the role of intermediate metal A on structure and related properties of ThCr₂Si₂-type transition metal compounds ATM₂X₂ is of great importance for designing novel layered functional materials. However, inducing A vacancies usually trends to destroy the original structure in reported systems so far, which hampers the further research. Here we report the controllable K vacancies in K_{1− x}Co₂Se₂ system (0 ≤ x ≤ 0.3), where both the ThCr₂Si₂-type structure and intact tetrahedral [CoSe] layers can be maintained with the varying occupancies of K. By inducing K vacancies in structure, tetragonality of the lattice for K_{1− x}Co₂Se₂ increases with the shortened a and elongated c. The (CoSe₄) tetrahedron is also compressed perpendicular to the c direction resulted from the K deficiency. X-ray absorption near-edge structure reveals that the valence state of Co is basically unaffected by K deficient with the absorption edge of Co K-edge unchanged. Concerning the physical properties, K vacancies increase the resistivity of metallic K_{1− x}Co₂Se₂ due to the decreased charge transfer from K⁺ to [CoSe] layers. More importantly, the ferromagnetic interaction of K_{1− x}Co₂Se₂ is unexpectedly weakened by raising K vacancies with the Curie temperature shifted from 80 to 52 K, despite the shortened Co-Co distance. First-principles calculation reveals that the spin polarization is weakened resulted from the K vacancies, mainly attributed to the reduced charge transfer from K⁺ to [CoSe] host. Our results clearly indicate the domination of transferred electrons from intermediate metal A on the magnetic interaction of ATM₂X₂, and also show the feasibility to regulate the structure and related properties of ATM₂X₂ by controlling the A content.