Stepped BCC · Narrow ledges
BCC(211)
BCC(211) combines straight step rows with narrow terraces. The body-centred basis introduces a stagger between neighbouring ledges.
- Plane
- (211)
- Layer character
- Stepped · anisotropic
- ASE slab builder
surface
From bulk to facet
How BCC(211) is cut
(211)
The nonzero l component rotates the [210]-type cut out of its channel direction, generating staggered ledges rather than parallel coplanar ridges.
- Bulk lattice
- Body-centred cubic
- Plane normal
- [211]
- What remains
- Narrow ledges
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
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.
- terrace ontop1-fold
Above an atom within the terrace.
(u,v) = (1/6, 1/2)s = s₁ · support shell: mean Δz = -2.343 Å from L1Projection of the L3 atom at (1/6, 1/2).
Not named in ASE - step ontop1-fold
Above a low-coordinate atom at the step.
(u,v) = (0, 1/2)s = s₁ · support shell: mean Δz = +0.000 Å from L1Projection of the L1 atom at (0, 1/2).
Not named in ASE - lower-row ontop1-fold
Above the exposed row below the step.
(u,v) = (1/3, 0)s = s₁ · support shell: mean Δz = -1.172 Å from L1Projection of the L2 atom at (1/3, 0).
Not named in ASE - terrace bridge2-fold
Between neighbours within the terrace.
(u,v) = (1/12, 0)s = (s₁ + s₂) / 2 · support shell: mean Δz = -1.172 Å from L1Periodic midpoint of L2 (5/6, 0) and L2 (4/3, 0).
Not named in ASE - step bridge2-fold
Between adjacent atoms along the step.
(u,v) = (1/4, 1/2)s = (s₁ + s₂) / 2 · support shell: mean Δz = +0.000 Å from L1Periodic midpoint of L1 (-1/2, 1/2) and L1 (-1, 1/2).
Not named in ASE - cross-step bridge2-fold
Between atoms on different step levels.
(u,v) = (5/12, 1/4)s = (s₁ + s₂) / 2 · support shell: mean Δz = -0.586 Å from L1Periodic midpoint of L1 (-1/2, 1/2) and L2 (-2/3, 0).
Not named in ASE - mixed-height step pocket3-fold
A pocket coordinated across the step.
(u,v) = (5/6, 1/3)s = (Σᵢ sᵢ) / 3 · support shell: mean Δz = -1.172 Å from L1Least-squares centroid of the 3-atom projected shell: L1 (0, 1/2), L2 (-1/6, 0), L3 (-1/3, 1/2).
Not named in ASE
Interactive model
Rotate the slab
Sites share a schematic display height · drag to rotate · hover for names
Below the top layer
Why the sites are different
Adjacent rows alternate their height and lateral offset, which is easiest to see by rotating the 3D model towards a side view.
Cell
Geometry at a glance
The cubic reciprocal normal is [211], giving \(d_{211}=a/\sqrt{6}\); the surface repeat contains several inequivalent projected rows.
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 bulk, surface
fe = bulk("Fe", "bcc", a=2.87, cubic=True)
slab = surface(fe, (2, 1, 1), 10, vacuum=10)
Things that are easy to misread
- Assuming neighbouring step rows have identical registry.
- Counting projected rather than three-dimensional nearest neighbours.