Synthesis, crystal structure, and physical properties of a triangular-lattice magnet K2Co(HPO4)(C2O4)(OH2)⋅H2O

A previously unreported geometrically frustrating cobalt oxalatophosphate, K₂Co(HPO₄)(C₂O₄)(OH₂)⋅H₂O (KCoCP), is solvothermally synthesized and characterized. KCoCP shows a typical two-dimensional (2D) layered structure of [Co(HPO₄)(C₂O₄)(OH₂)]²⁻ separated by K⁺ and H₂O, which is isostructural to the previously reported Fe analogue. Specifically, the magnetic Co²⁺ ions sit on a distorted triangular lattice (TL) and introduce geometrical frustration into the system. The mainstay of the 2D layer [Co(HPO₄)(C₂O₄)(OH₂)]²⁻ is cobalt oxalate skeleton with HPO₄ and H₂O groups staggering on both sides. The Bravais–Friedel–Donnay–Harker (BFDH) theory, which is used to predict the polyhedral morphology and crystal growth faces, indicates a chamfer polyhedron morphology of KCoCP crystal. UV–vis diffuse reflection and electronic structure calculation suggest KCoCP being a semiconductor with an indirect band gap (0.214 ​eV). The absence of long-range magnetic ordering for KCoCP is proved via a combination of magnetic susceptibility and specific-heat measurements down to 1.8 ​K. Thus, KCoCP shows potential to investigate unconventional low temperature physical properties in TL system.