Surface Atlas
← Common surfaces

Low-index FCC · Square net

FCC(100)

The (100) cut exposes a square array of equivalent atoms. Its open centre is the characteristic fourfold hollow, while the layer below sits underneath each bridge direction.

Plane
(100)
Layer character
Flat · fourfold symmetry
ASE slab builder
fcc100

From bulk to facet

How FCC(100) is cut

(100) plane cutting through a Face-centred cubic bulk unit cell
The coloured sheet is one translated member of the (100) plane family; translating it along its normal gives an equivalent termination.

(100)

The (100) family is perpendicular to one cube axis and parallel to the other two. Sliding that plane through the FCC cell repeatedly exposes equivalent square layers.

Bulk lattice
Face-centred cubic
Plane normal
[100]
What remains
Square net

The plane is drawn through the centre so high-index cuts remain legible. Its orientation is what the indices specify, not its absolute position inside one cell.

Surface geometry

Read the surface from above

surface second layer third layer

L1 is the highest atom-bearing plane, followed by L2 and L3. Support coordinates outside 0–1 are periodic images. The listed Δz describes the supporting atoms, not the marker height. In the interactive model, every numbered site is placed on the same schematic guide plane above the slab; no adsorption distance or energetic ordering is implied.

Top view of FCC(100) with numbered adsorption sites
The outlined reference cell has |a1| = 3.6100 Å, |a2| = 3.6100 Å, and γ = 90.00° for the Cu slab used in the drawing. Fractional coordinates mean r∥ = ua1 + va2.
  1. ontop1-fold

    Directly above one outermost atom.

    (u,v) = (1/2, 0)s = s₁ · support shell: mean Δz = +0.000 Å from L1

    Projection of the L1 atom at (1/2, 0).

    ASE keywordontop
  2. bridge2-fold

    Halfway between adjacent surface atoms.

    (u,v) = (1/4, 1/4)s = (s₁ + s₂) / 2 · support shell: mean Δz = +0.000 Å from L1

    Periodic midpoint of L1 (1/2, 0) and L1 (0, 1/2).

    ASE keywordbridge
  3. hollow4-fold

    At the centre of a square of four surface atoms.

    (u,v) = (1/2, 1/2)s = (Σᵢ sᵢ) / 4 · support shell: mean Δz = +0.000 Å from L1

    Least-squares centroid of the 4-atom projected shell: L1 (1/2, 0), L1 (0, 1/2), L1 (1/2, 1), L1 (1, 1/2).

    ASE keywordhollow

Interactive model

Rotate the slab

Sites share a schematic display height · drag to rotate · hover for names

Building the model…

Below the top layer

Why the sites are different

Successive square layers alternate laterally between A and B positions. A second-layer atom sits below each fourfold hollow, adding subsurface coordination that a top-layer-only picture would miss.

Side profile of FCC(100) showing its first repeating atomic layers
Side profile showing one compact stacking repeat. The dashed line follows the macroscopic surface plane.
Layer registry of FCC(100)
Layer registry viewed from above; opacity increases towards the surface.

Cell

Geometry at a glance

For cubic crystals, \(d_{100}=a\). FCC basis atoms create atom-bearing sublayers every \(a/2\). The outlined \(a\times a\) cell contains two surface atoms; a primitive surface cell has side \(a/\sqrt{2}\).

A site name describes the ideal starting geometry. Relaxation can move an adsorbate away from it.

Practical model

Build it with ASE

The builder creates the slab. Sites marked ASE keyword can be passed directly as a named position; ASE source marks current but inconsistently documented support. Other sites require explicit Cartesian coordinates converted from the fractional construction above.

from ase.build import add_adsorbate, fcc100

slab = fcc100("Cu", size=(3, 3, 4), a=3.61, vacuum=10)
add_adsorbate(slab, "H", 1.5, position="hollow")
Things that are easy to misread
  • Calling the central hollow threefold; four top-layer atoms surround it.
  • Confusing the cubic lattice constant with the shorter surface nearest-neighbour spacing.