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CoSe2 is Marcasite structured and crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Co4+ is bonded to six equivalent Se2- atoms to form a mixture of corner and edge-sharing CoSe6 octahedra. The corner-sharing octahedral tilt angles are 57°. There are two shorter (2.40 Å) and four longer (2.41 Å) Co–Se bond lengths. Se2- is bonded in a 3-coordinate geometry to three equivalent Co4+ atoms.
CoSe2 is Pyrite structured and crystallizes in the cubic Pa-3 space group. The structure is three-dimensional. Co4+ is bonded to six equivalent Se2- atoms to form corner-sharing CoSe6 octahedra. The corner-sharing octahedral tilt angles are 63°. All Co–Se bond lengths are 2.42 Å. Se2- is bonded in a distorted trigonal planar geometry to three equivalent Co4+ atoms.
V(CoSe2)2 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. V4+ is bonded to six Se2- atoms to form VSe6 octahedra that share corners with twelve equivalent CoSe6 octahedra, edges with two equivalent VSe6 octahedra, and faces with two equivalent CoSe6 octahedra. The corner-sharing octahedra tilt angles range from 50–54°. There are two shorter (2.52 Å) and four longer (2.53 Å) V–Se bond lengths. Co2+ is bonded to six Se2- atoms to form CoSe6 octahedra that share corners with six equivalent VSe6 octahedra, edges with six equivalent CoSe6 octahedra, and a faceface with one VSe6 octahedra. The corner-sharing octahedra tilt angles range from 50–54°. There are a spread of Co–Se bond distances ranging from 2.35–2.49 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 5-coordinate geometry to two equivalent V4+ and three equivalent Co2+ atoms. In the second Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one V4+ and three equivalent Co2+ atoms.
Fe(CoSe2)2 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Fe3+ is bonded to six Se2- atoms to form FeSe6 octahedra that share corners with twelve equivalent CoSe6 octahedra, edges with two equivalent FeSe6 octahedra, and faces with two equivalent CoSe6 octahedra. The corner-sharing octahedra tilt angles range from 51–52°. There are two shorter (2.45 Å) and four longer (2.50 Å) Fe–Se bond lengths. Co+2.50+ is bonded to six Se2- atoms to form CoSe6 octahedra that share corners with six equivalent FeSe6 octahedra, edges with six equivalent CoSe6 octahedra, and a faceface with one FeSe6 octahedra. The corner-sharing octahedra tilt angles range from 51–52°. There are a spread of Co–Se bond distances ranging from 2.35–2.48 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 5-coordinate geometry to two equivalent Fe3+ and three equivalent Co+2.50+ atoms. In the second Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Fe3+ and three equivalent Co+2.50+ atoms.
Fe(CoSe2)2 crystallizes in the monoclinic Cm space group. The structure is three-dimensional. Fe3+ is bonded to six Se2- atoms to form FeSe6 octahedra that share corners with six equivalent CoSe6 octahedra, edges with two equivalent FeSe6 octahedra, edges with four equivalent CoSe6 octahedra, and a faceface with one CoSe6 octahedra. The corner-sharing octahedra tilt angles range from 51–54°. There are a spread of Fe–Se bond distances ranging from 2.35–2.51 Å. There are two inequivalent Co+2.50+ sites. In the first Co+2.50+ site, Co+2.50+ is bonded to six Se2- atoms to form CoSe6 octahedra that share corners with six equivalent FeSe6 octahedra, corners with six equivalent CoSe6 octahedra, edges with two equivalent CoSe6 octahedra, a faceface with one FeSe6 octahedra, and a faceface with one CoSe6 octahedra. The corner-sharing octahedra tilt angles range from 50–54°. There are a spread of Co–Se bond distances ranging from 2.42–2.45 Å. In the second Co+2.50+ site, Co+2.50+ is bonded to six Se2- atoms to form CoSe6 octahedra that share corners with six equivalent CoSe6 octahedra, edges with two equivalent CoSe6 octahedra, edges with four equivalent FeSe6 octahedra, and a faceface with one CoSe6 octahedra. The corner-sharing octahedra tilt angles range from 50–54°. There are a spread of Co–Se bond distances ranging from 2.34–2.52 Å. There are four inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to two equivalent Fe3+ and two Co+2.50+ atoms. In the second Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Fe3+ and three Co+2.50+ atoms. In the third Se2- site, Se2- is bonded in a 5-coordinate geometry to two equivalent Fe3+ and three Co+2.50+ atoms. In the fourth Se2- site, Se2- is bonded in a 5-coordinate geometry to one Fe3+ and four Co+2.50+ atoms.
Abstract Dimensional modifications play a crucial role in various applications, especially in the context of device miniaturization, giving rise to novel quantum phenomena. The many-body dynamics induced by dimensional modifications, including electron-electron, electron-phonon, electron-magnon and electron-plasmon coupling, are known to significantly affect the atomic and electronic properties of the materials. By reducing the dimensionality of orthorhombic CoSe 2 and forming heterostructure with bilayer graphene using molecular beam epitaxy, we unveil the emergence of two types of phase transitions through angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements. We disclose that the 2 × 1 superstructure is associated with charge density wave induced by Fermi surface nesting, characterized by a transition temperature of 340 K. Additionally, another phase transition at temperature of 160 K based on temperature dependent gap evolution are observed with renormalized electronic structure induced by electron-boson coupling. These discoveries of the electronic and atomic modifications, influenced by electron-electron and electron-boson interactions, underscore that many-body physics play significant roles in understanding low-dimensional properties of non-van der Waals Co-chalcogenides and related heterostructures. Graphical Abstract