Graphite unit cell - incorrect c-parameters?

Materials Project
1

Hi,

I was doing some basic density calculations for some work, and was trying to use the crystal structure parameters of graphite provided here: mp-48: C (hexagonal, P6_3/mmc, 194)

The c-parameter of graphite is quoted as 8.685 Angstrom, which implies a (002) X-ray diffraction peak of d=4.343 (at a little over 20 2theta for Cu K alpha) but this is incorrect, graphite is well known to have a d-spacing of 3.35 Angstrom, corresponding to a c-lattice parameter of 6.69 Angstrom, and a (002) peak at roughly 26.6 2theta.

Am I missing something here? Are the values quoted on the materials project calculated computationally? Or does that entry need to be updated?

Just thought I would raise it as I was confused.

Gabe

2

Hi Gabe,

No worries, it’s a common point of confusion. The simple answer is that van der Waals interactions are not well-described by the simulation methods currently in use by the Materials Project: newer data released soon may improve on this! However, in the meantime, it means that many interlayer spacings are quite significantly wrong.

You can read more on our FAQ page:

Best,

Matt

3

Hi Matt,

Many thanks for the reply! Appreciate you taking the time.

That makes sense, will be great to see how it all progresses. Such a valuable resource!

Best,
Gabe :slight_smile:

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