CdCl2type

CdCl2-type is a crystal structure type used to describe a family of metal dihalides with the formula MX2. Named after cadmium chloride, which crystallizes in this motif, the structure is characterized by a layered arrangement in which halide ions form a hexagonally close-packed lattice and metal cations occupy octahedral holes in alternating halide layers.

In the CdCl2-type, each metal ion is coordinated by six halide ligands in an octahedral geometry. Within a layer, MX6 octahedra share edges, forming two-dimensional sheets of M–X connectivity. Each halide bridges three metal centers, creating a network that is strongly two-dimensional, while the layers are held together by weaker interlayer interactions, giving rise to notable anisotropy and van der Waals gaps between sheets.

The structure is common for MX2 dihalides, where M is a divalent metal and X is a halogen. The archetype is CdCl2, but the same motif occurs for other dihalides under appropriate conditions. In the literature, the CdCl2-type is often discussed alongside the CdI2-type; some sources treat the two as closely related or polytypes of a common MX2 layered motif, differing mainly in stacking of the halide layers.

Properties associated with this structure include the potential for intercalation and ion transport between layers, as well as distinctive electronic and magnetic behaviors in several transition-metal dihalides. The CdCl2-type motif remains a useful descriptor for understanding a wide class of layered MX2 compounds. See also CdI2-type and layered MX2 structures.