# What does an 2H- in front of a chemical symbol mean? (i.e. 2H-MoX_2)

1. ### maverick_starstrider

Title says it all. I'm reading a lot about the transition metal dichalcogenides and I often see something like

2H-MoS_2

What does that 2H- mean? My best guess is that it has something to do with a hexagonal lattice or stacking or something. Anybody know?

2. ### Hypersphere

191
Yeah, it has to do with stacking. There are several notations, of which I'm most used the ABC one myself. However, I think this is Ramsdell notation, in which H=hexagonal and the number two would signify the number of layers. Hence 2H should correspond to AB stacking, which agrees with the figure here.

3. ### t!m

147
Indeed, 2H is the hexagonal stacking motif, with two layers per unit cell. Note that this isn't quite ABAB stacking, since one layer is not a simple translation of the other, but rather requires a rotation. 2H should be contrasted with the 3R (rhombohedral) structure, with three layers per unit cell, all simple translations. See here for example.