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

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SUMMARY

The notation "2H-" in front of a chemical symbol, such as in "2H-MoS2", indicates a hexagonal stacking arrangement of the material, specifically referring to the Ramsdell notation. The "2" signifies that there are two layers per unit cell, which corresponds to AB stacking, although it is not a simple translation due to the required rotation between layers. This contrasts with the "3R" notation, which denotes a rhombohedral structure with three layers per unit cell that are all simple translations.

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maverick_starstrider
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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?
 
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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.
 
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.
 

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