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

by maverick_starstrider
Tags: 2hmox2, chemical, symbol
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Apr25-14, 03:45 PM
maverick_starstrider's Avatar
P: 1,164
Title says it all. I'm reading a lot about the transition metal dichalcogenides and I often see something like


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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Apr26-14, 10:26 PM
P: 182
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.
Apr27-14, 10:48 PM
P: 142
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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