Understanding HgS Crystal Structure and Packing

The question asks about the packing scheme of HgS. HgS, or Mercury(II) sulfide, exists in different crystalline forms. The two most common forms are cinnabar ($\alpha$-HgS) and metacinnabar ($\beta$-HgS).

These different forms have different crystal structures and therefore different packing schemes.

Metacinnabar ($\beta$-HgS) Structure

• Metacinnabar is the cubic form of HgS.
• It adopts the zincblende structure.
• The zincblende structure is based on a face-centered cubic (FCC) lattice.
• In the zincblende structure, one type of atom (e.g., S) forms an FCC lattice, and the other type of atom (e.g., Hg) occupies specific tetrahedral holes within that lattice.
• Therefore, the underlying lattice or packing scheme of metacinnabar HgS is FCC.

Cinnabar ($\alpha$-HgS) Structure

• Cinnabar is the stable form of HgS at room temperature and pressure.
• It has a hexagonal crystal structure.
• While hexagonal structures can sometimes be described using rhombohedral unit cells, the primary packing for cinnabar is not FCC or BCC.

Analyzing the Options

Let's look at the provided options in the context of HgS:

• BCC (Body-Centered Cubic): This is a standard cubic packing scheme, but it does not describe the structure of metacinnabar HgS (which is FCC-based) or cinnabar HgS (which is hexagonal).
• Cubic centre: This is not a recognized term for a crystal packing scheme. Packing schemes describe how particles are arranged to fill space, like FCC, BCC, or hexagonal close-packed (HCP).
• FCC (Face-Centered Cubic): This is the underlying lattice for the metacinnabar ($\beta$-HgS) structure, which is the zincblende structure.
• Rhombohedral: This relates to the hexagonal crystal system that describes cinnabar ($\alpha$-HgS). However, the question asks for a packing scheme, and FCC is a more common term for describing the packing basis in the options provided, particularly when considering the cubic form of HgS. Given FCC is an option, it points towards the metacinnabar structure.

Considering the standard crystalline forms and the options provided, the FCC packing scheme is associated with the cubic form of HgS (metacinnabar), which has a zincblende structure built upon an FCC lattice.

Therefore, HgS possesses an FCC packing scheme, specifically in its metacinnabar form.