Surface Atlas
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Low-index BCC · Square net

BCC(100)

BCC(100) exposes a square net, but successive layers alternate between corner and body-centred positions. The subsurface atom therefore changes the meaning of the square opening.

Plane
(100)
Layer character
Flat · alternating layers
ASE slab builder
bcc100

From bulk to facet

How BCC(100) is cut

(100) plane cutting through a Body-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 cut is perpendicular to one cube axis. Unlike FCC, translating it through BCC alternates between corner-atom and body-centred layers.

Bulk lattice
Body-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 BCC(100) with numbered adsorption sites
The outlined reference cell has |a1| = 2.8700 Å, |a2| = 2.8700 Å, and γ = 90.00° for the Fe slab used in the drawing. Fractional coordinates mean r∥ = ua1 + va2.
  1. ontop1-fold

    Directly above a surface atom.

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

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

    ASE keywordontop
  2. bridge2-fold

    Halfway along an edge of the square net.

    (u,v) = (1/2, 0)s = (s₁ + s₂) / 2 · support shell: mean Δz = +0.000 Å from L1

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

    ASE keywordbridge
  3. hollow4-fold

    At the square centre above a subsurface atom.

    (u,v) = (0, 0)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, -1/2), L1 (1/2, -1/2), L1 (-1/2, 1/2), L1 (1/2, 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

The second layer is shifted to the centre of the surface square, giving the hollow substantial subsurface coordination.

Side profile of BCC(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 BCC(100)
Layer registry viewed from above; opacity increases towards the surface.

Cell

Geometry at a glance

The outlined square cell follows the cubic axes with side \(a\). While \(d_{100}=a\), the body-centred basis produces alternating atom-bearing layers separated by \(a/2\).

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 bcc100

slab = bcc100("Fe", size=(3, 3, 6), a=2.87, vacuum=10)
Things that are easy to misread
  • Assuming its square hollow has the same subsurface registry as FCC(100).
  • Forgetting that successive BCC(100) layers are not laterally equivalent.