General crystal structures and surfaces

Modules for creating crystal structures are found in the module ase.lattice. Most Bravais lattices are implemented, as are a few important lattices with a basis. The modules can create lattices with any orientation (see below). These modules can be used to create surfaces with any crystal structure and any orientation by later adding a vacuum layer with ase.build.add_vacuum().

Example

To set up a slab of FCC copper with the [1,-1,0] direction along the x-axis, [1,1,-2] along the y-axis and [1,1,1] along the z-axis, use:

>>> from ase.lattice.cubic import FaceCenteredCubic
>>> atoms = FaceCenteredCubic(directions=[[1,-1,0], [1,1,-2], [1,1,1]],
...                           size=(2,2,3), symbol='Cu', pbc=(1,1,0))

The minimal unit cell is repeated 2*2*3 times. The lattice constant is taken from the database of lattice constants in ase.data module. There are periodic boundary conditions along the x and y axis, but free boundary conditions along the z axis. Since the three directions are perpendicular, a (111) surface is created.

To set up a slab of BCC copper with [100] along the first axis, [010] along the second axis, and [111] along the third axis use:

>>> from ase.lattice.cubic import BodyCenteredCubic
>>> atoms = BodyCenteredCubic(directions=[[1,0,0], [0,1,0], [1,1,1]],
...                           size=(2,2,3), symbol='Cu', pbc=(1,1,0),
...                           latticeconstant=4.0)

Since BCC is not the natural crystal structure for Cu, a lattice constant has to be specified. Note that since the repeat directions of the unit cell are not orthogonal, the Miller indices of the surfaces will not be the same as the Miller indices of the axes. The indices of the surfaces in this example will be (1,0,-1), (0,1,-1) and (0,0,1).

Available crystal lattices

The following modules are currently available (the * mark lattices with a basis):

  • lattice.cubic

    • SimpleCubic
    • FaceCenteredCubic
    • BodyCenteredCubic
    • Diamond (*)
  • lattice.tetragonal

    • SimpleTetragonal
    • CenteredTetragonal
  • lattice.orthorhombic

    • SimpleOrthorhombic
    • BaseCenteredOrthorhombic
    • FaceCenteredOrthorhombic
    • BodyCenteredOrthorhombic
  • lattice.monoclinic

    • SimpleMonoclinic
    • BaseCenteredMonoclinic
  • lattice.triclinic

    • Triclinic
  • lattice.hexagonal

    • Hexagonal
    • HexagonalClosedPacked (*)
    • Graphite (*)
  • The rhombohedral (or trigonal) lattices are not implemented. They will be implemented when the need arises (and if somebody can tell us the precise definition of the 4-number Miller indices - we only know that they are “almost the same as in hexagonal lattices”).

  • lattice.compounds

    Lattices with more than one element. These are mainly intended as examples allowing you to define new such lattices. Currently, the following are defined

    • B1 = NaCl = Rocksalt
    • B2 = CsCl
    • B3 = ZnS = Zincblende
    • L1_2 = AuCu3
    • L1_0 = AuCu

Usage

The lattice objects are called with a number of arguments specifying e.g. the size and orientation of the lattice. All arguments should be given as named arguments. At a minimum the symbol argument must be specified.

symbol
The element, specified by the atomic number (an integer) or by the atomic symbol (i.e. ‘Au’). For compounds, a tuple or list of elements should be given. This argument is mandatory.
directions and/or miller:

Specifies the orientation of the lattice as the Miller indices of the three basis vectors of the supercell (directions=...) and/or as the Miller indices of the three surfaces (miller=...). Normally, one will specify either three directions or three surfaces, but any combination that is both complete and consistent is allowed, e.g. two directions and two surface miller indices (this example is slightly redundant, and consistency will be checked). If only some directions/miller indices are specified, the remaining should be given as None. If you intend to generate a specific surface, and prefer to specify the miller indices of the unit cell basis (directions=...), it is a good idea to give the desired Miller index of the surface as well to allow the module to test for consistency. Example:

>>> atoms = BodyCenteredCubic(directions=[[1,-1,0],[1,1,-1],[0,0,1]],
...                           miller=[None, None, [1,1,2]], ...)

If neither directions nor miller are specified, the default is directions=[[1,0,0], [0,1,0], [0,0,1]].

size:
A tuple of three numbers, defining how many times the fundamental repeat unit is repeated. Default: (1,1,1). Be aware that if high-index directions are specified, the fundamental repeat unit may be large.
latticeconstant:

The lattice constant. If no lattice constant is specified, one is extracted from ASE.ChemicalElements provided that the element actually has the crystal structure you are creating. Depending on the crystal structure, there will be more than one lattice constant, and they are specified by giving a dictionary or a tuple (a scalar for cubic lattices). Distances are given in Angstrom, angles in degrees.

Structure Lattice constants Dictionary-keys
Cubic a ‘a’
Tetragonal (a, c) ‘a’, ‘c’ or ‘c/a’
Orthorhombic (a, b, c) ‘a’, ‘b’ or ‘b/a’, ‘c’ or ‘c/a’
Triclinic (a, b, c, \(\alpha\), \(\beta\), \(\gamma\)) ‘a’, ‘b’ or ‘b/a’, ‘c’ or ‘c/a’, ‘alpha’, ‘beta’, ‘gamma’
Monoclinic (a, b, c, alpha) ‘a’, ‘b’ or ‘b/a’, ‘c’ or ‘c/a’, ‘alpha’
Hexagonal (a, c) ‘a’, ‘c’ or ‘c/a’

Example:

>>> atoms = Monoclinic( ... , latticeconstant={'a': 3.06,
...     'b/a': 0.95, 'c/a': 1.07, 'alpha': 74})
debug:
Controls the amount of information printed. 0: no info is printed. 1 (the default): The indices of surfaces and unit cell vectors are printed. 2: Debugging info is printed.

Defining new lattices

Often, there is a need for new lattices - either because an element crystallizes in a lattice that is not a simple Bravais lattice, or because you need to work with a compound or an ordered alloy.

All the lattice generating objects are instances of a class, you generate new lattices by deriving a new class and instantiating it. This is best explained by an example. The diamond lattice is two interlacing FCC lattices, so it can be seen as a face-centered cubic lattice with a two-atom basis. The Diamond object could be defined like this:

from ase.lattice.cubic import FaceCenteredCubicFactory
class DiamondFactory(FaceCenteredCubicFactory):
    """A factory for creating diamond lattices."""
    xtal_name = 'diamond'
    bravais_basis = [[0, 0, 0], [0.25, 0.25, 0.25]]

Diamond = DiamondFactory()

Lattices with more than one element

Lattices with more than one element is made in the same way. A new attribute, element_basis, is added, giving which atoms in the basis are which element. If there are four atoms in the basis, and element_basis is (0,0,1,0), then the first, second and fourth atoms are one element, and the third is the other element. As an example, the AuCu3 structure (also known as \(\mathrm{L}1_2\)) is defined as:

# The L1_2 structure is "based on FCC", but is really simple cubic
# with a basis.
class AuCu3Factory(SimpleCubicFactory):
    "A factory for creating AuCu3 (L1_2) lattices."
    bravais_basis = [[0, 0, 0], [0, 0.5, 0.5], [0.5, 0, 0.5], [0.5, 0.5, 0]]
    element_basis = (0, 1, 1, 1)

AuCu3 = L1_2 = AuCu3Factory()

Sometimes, more than one crystal structure can be used to define the crystal structure, for example the Rocksalt structure is two interpenetrating FCC lattices, one with one kind of atoms and one with another. It would be tempting to define it as

class NaClFactory(FaceCenteredCubicFactory):
    "A factory for creating NaCl (B1, Rocksalt) lattices."

    bravais_basis = [[0, 0, 0], [0.5, 0.5, 0.5]]
    element_basis = (0, 1)


B1 = NaCl = Rocksalt = NaClFactory()

but if this is used to define a finite system, one surface would be covered with one type of atoms, and the opposite surface with the other. To maintain the stochiometry of the surfaces, it is better to use the simple cubic lattice with a larger basis:

# To prevent a layer of element one on one side, and a layer of
# element two on the other side, NaCl is based on SimpleCubic instead
# of on FaceCenteredCubic
class NaClFactory(SimpleCubicFactory):
    "A factory for creating NaCl (B1, Rocksalt) lattices."

    bravais_basis = [[0, 0, 0], [0, 0, 0.5], [0, 0.5, 0], [0, 0.5, 0.5],
                     [0.5, 0, 0], [0.5, 0, 0.5], [0.5, 0.5, 0],
                     [0.5, 0.5, 0.5]]
    element_basis = (0, 1, 1, 0, 1, 0, 0, 1)


B1 = NaCl = Rocksalt = NaClFactory()

More examples can be found in the file ase/lattice/compounds.py.