Bravais lattice

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In geometry and crystallography, a Bravais lattice, studied by Auguste Bravais (1850),[1] is an infinite array of discrete points in three dimensional space generated by a set of discrete translation operations described by:

where ni are any integers and ai are known as the primitive vectors which lie in different directions and span the lattice. This discrete set of vectors must be closed under vector addition and subtraction. For any choice of position vector R, the lattice looks exactly the same.

When the discrete points are atoms, ions, or polymer strings of solid matter, the Bravais lattice concept is used to formally define a crystalline arrangement and its (finite) frontiers. A crystal is made up of a periodic arrangement of one or more atoms (the basis) repeated at each lattice point. Consequently, the crystal looks the same when viewed from any equivalent lattice point, namely those separated by the translation of one unit cell (the motive).

Two Bravais lattices are often considered equivalent if they have isomorphic symmetry groups. In this sense, there are 14 possible Bravais lattices in three-dimensional space. The 14 possible symmetry groups of Bravais lattices are 14 of the 230 space groups.

Bravais lattices in 2 dimensions[edit]

Further information: Lattice (group)
1 – oblique, 2 – rectangular, 3 – rhombic, 4 – hexagonal, and 5 – square.

In two-dimensional space, there are 5 Bravais lattices,[2] grouped into four lattice systems:

2D lattices
Lattice systems Monoclinic Oblique
Orthorhombic Rectangular
(centered rhombic)
Rhombic
(centered rectangular)
Tetragonal Square
Hexagonal Hexagonal

The area of the unit cell can be calculated by evaluating ||a × b||, where a and b are the lattice vectors. The properties of the Bravais lattices are given below:

Lattice system Area Axial Distances (edge lengths) Axial Angle
Monoclinic a ≠ b θ ≠ 90°
Orthorhombic Rhombic axes a = b θ ≠ 90°
Rectangular axes a ≠ b θ = 90°
Tetragonal a = b θ = 90°
Hexagonal a = b θ = 120°

Bravais lattices in 3 dimensions[edit]

In three-dimensional space, there are 14 Bravais lattices. These are obtained by combining one of the seven lattice systems (or axial systems) with one of the seven lattice types (or lattice centerings). In general, the lattice systems can be characterized by their shapes according to the relative lengths of the cell edges (a, b, c) and the angles between them (α, β, γ). The lattice types identify the locations of the lattice points in the unit cell as follows:

  • Primitive (P): lattice points on the cell corners only (sometimes called simple)
  • Body-Centered (I): lattice points on the cell corners with one additional point at the center of the cell
  • Face-Centered (F): lattice points on the cell corners with one additional point at the center of each of the faces of the cell
  • Base-Centered (A, B, or C): lattice points on the cell corners with one additional point at the center of each face of one pair of parallel faces of the cell (sometimes called end-centered)
  • Rhombohedral-Centered (R) or Hexagonal-Centered (D): lattice points on the cell corners with two additional points along the longest body diagonal (only applies for the hexagonal crystal family)

Not all combinations of lattice systems and lattice types are needed to describe all of the possible lattices. There are in total (5 × 6 − 5) + 2 = 27 combinations, but it can be shown that several of these are in fact equivalent to each other. For example, the monoclinic I lattice can be described by a monoclinic C lattice by different choice of crystal axes. Similarly, all A- or B-centred lattices can be described either by a C- or P-centering. This reduces the number of combinations to 14 conventional Bravais lattices, shown in the table below (the notation used are Pearson symbols). The rhombohedral and hexagonal lattice systems can each be described by the other's crystal axes, resulting in R- and D-centering respectively.

Other crystal families
Lattice types
P Base Base I F
Lattice systems Triclinic aP
Triclinic
Monoclinic mP mC
Monoclinic, simple Monoclinic, centred
Orthorhombic oP oC oI oF
Orthohombic, simple Orthohombic, base-centred Orthohombic, body-centred Orthohombic, face-centred
Tetragonal tP tI
Tetragonal, simple Tetragonal, body-centred
Cubic cP (pcc) cI (bcc) cF (fcc)
Cubic, simple Cubic, body-centred Cubic, face-centred
Hexagonal crystal family
Lattice types
P R or D
Lattice systems Rhombohedral hR
Rhombohedral Rhombohedral, hexagonal axes
Hexagonal hP
Hexagonal Hexagonal, rhombohedral axes
Note: There is only one rhombohedral lattice and one hexagonal lattice respectively

The volume of the unit cell can be calculated by evaluating ||a · b × c||, where a, b, and c are the lattice vectors. The properties of the Bravais lattices are given below:

Lattice system Volume Axial Distances (edge lengths) Axial Angles Corresponding Examples
Triclinic a ≠ b ≠ c α ≠ β ≠ γ ≠ 90° K2Cr2O7, CuSO4.5H2O, H3BO3
Monoclinic a ≠ b ≠ c α = γ = 90°, β ≠ 90° Monoclinic sulfur, Na2SO4.10H2O
Orthorhombic a ≠ b ≠ c α = β = γ = 90° Rhombic sulfur, KNO3, BaSO4
Tetragonal a = b ≠ c α = β = γ = 90° White tin, SnO2, TiO2, CaSO4
Rhombohedral a = b = c α = β = γ ≠ 90° Calcite (CaCO3), Cinnabar (HgS)
Hexagonal a = b ≠ c α = β = 90°, γ = 120° Graphite, ZnO, CdS
Cubic a = b = c α = β = γ = 90° NaCl, Zinc Blende, Cu

Bravais lattices in 4 dimensions[edit]

In four dimensions, there are 64 Bravais lattices. Of these, 23 are primitive and 41 are centered. Ten Bravais lattices split into enantiomorphic pairs.[3]

See also[edit]

References[edit]

  1. ^ Aroyo, Mois I.; Ulrich Müller; Hans Wondratschek (2006). "Historical Introduction". International Tables for Crystallography (Springer) A1 (1.1): 2–5. doi:10.1107/97809553602060000537. Retrieved 2008-04-21. 
  2. ^ Kittel, Charles (1996) [1953]. "Chapter 1". Introduction to Solid State Physics (Seventh ed.). New York: John Wiley & Sons. p. 10. ISBN 0-471-11181-3. Retrieved 2008-04-21. 
  3. ^ Brown, Harold; Bülow, Rolf; Neubüser, Joachim; Wondratschek, Hans; Zassenhaus, Hans (1978), Crystallographic groups of four-dimensional space, New York: Wiley-Interscience [John Wiley & Sons], ISBN 978-0-471-03095-9, MR 0484179 

Further reading[edit]

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