Orthorhombic
Axes a ≠ b ≠ c, angles α = β = γ = 90° — 4 Bravais lattices and 3 crystallographic point groups.
Lattice geometry
The orthorhombic system is defined by the constraints a ≠ b ≠ c on the unit-cell edge lengths and α = β = γ = 90° on the angles between them. Every crystal whose translational symmetry satisfies those constraints belongs here, whatever its chemistry.
Within the system, 4 centerings are possible without producing a lattice already described by another system: Simple (P), Base-centered (C), Body-centered (I), Face-centered (F). That is what limits the 7 systems to 14 Bravais lattices in total rather than an arbitrary number.
Symmetry and point groups
3 of the 32 crystallographic point groups sit in this system. Point symmetry is the practical part: it decides whether stiffness, thermal expansion and conductivity are isotropic or direction-dependent, and only non-centrosymmetric groups can be piezoelectric at all.
Examples
α-S, cementite Fe₃C, aragonite CaCO₃, olivine
Elements that crystallise in this system
7 of the 118 elements adopt a orthorhombic structure in the solid state:
Definition
- Axes
- a ≠ b ≠ c
- Angles
- α = β = γ = 90°
- Bravais
- Simple (P), Base-centered (C), Body-centered (I), Face-centered (F)
- Point groups
- 3
- Elements
- 7
Structure search →
Look up computed structures and space groups in the open databases.
Crystallography in practice
14 Bravais lattices
Every crystalline material's translational symmetry is described by one of 14 Bravais lattices — the 7 crystal systems combined with permitted centerings (P, I, F, C, R). Adding point-symmetry operations yields 32 crystallographic point groups and, with translations (screw axes, glide planes), 230 space groups.
Common metallic structures
FCC (Cu, Al, Ni, austenite): 12 slip systems, ductile, close-packed (APF 0.74). BCC (Fe-α, W, Mo): stronger but with ductile-brittle transition. HCP (Mg, Ti-α, Zn): limited slip systems → anisotropy and lower formability.
Structure–property links
Slip systems govern ductility; packing density influences density and diffusion; symmetry controls anisotropy of stiffness, thermal expansion and piezoelectricity (only non-centrosymmetric groups can be piezoelectric). Polymorphism (e.g. Fe BCC↔FCC, ZrO₂ monoclinic↔tetragonal) underpins heat treatment and transformation toughening.
Defects
Point defects (vacancies, interstitials, dopants) drive diffusion and conductivity; dislocations carry plasticity and are obstructed by solutes, precipitates and grain boundaries (strengthening mechanisms); planar defects (grain boundaries, stacking faults, twins) and volume defects (pores, inclusions) control strength, toughness and failure.
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