# What is the "formula unit surface area"?

Firstly, sorry if this question is too basic for this community but seriously I couldn't find any answer. Does it mean the multiplication of the in-plane lattice constants? For example, lets say following geometry file belongs the graphene.

C: Graphite Lattice
1.0
+2.4410462393  +0.0000000000  +0.0000000000
-1.2205231197  +2.1140080551  +0.0000000000
+0.0000000000  +0.0000000000 +10.0000000000
2
Cartesian
+0.0000000000  +0.0000000000  +0.0000000000
+0.0000000000  +1.4093387034  +0.0000000000


So its formula unit surface area is: 2.4410462393 x 2.4410462393 = 5.958706742400672. Am I right? If I'm wrong I'd be very appreciated for the correct logic.

• Can you give an example of where this term is used?
– wcw
Feb 4, 2021 at 19:20
• Sure. Here it is: pubs.acs.org/doi/abs/10.1021/… Feb 4, 2021 at 19:23

In the paper you linked, the relevant phrase is "$$A_{\mathrm{fu}}$$ is the per formula unit total surface area of $$\mathrm{TiS_2}$$."
For the graphene example, it would be half the in-plane surface area of the cell, $$\frac{1}{2} \, \mathbf{a}_1 \times \mathbf{a}_2 \approx \frac{1}{2} (2.441)^2 \sin(60\deg) \approx 2.58 ,$$ because the cell contains two formula units.