I was going to start a project on Borophene, and hence I was going through a review journal, 2D Boron: Boraphene, Borophene, Boronene, where they have said many types of borophenes had been proposed. They have been given different greek letters like α, β, γ (β12), δ and χ(chi) to classify different types and they also classified those types into different sub-types by using subscripted numbers like α1, α2, α3 or β1, β2, β3, β12 etc.
At first, I thought it was coming from the hexagonal hole density (η) term which is defined by - η = (a number of hexagonal holes/a number of atoms in the original triangular sheet). But later I found that it's not the case (considering all the types).
My question is - how exactly these subscripted numbers are given to sub-classify the different types of borophene?
P.S. - All the numbers next to the greek letters will be in subscript.