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The Cubic Crystal System

A Foundation in Geometric Crystallography. Exploring the fundamental symmetries and structures that define crystalline matter.

Discover Cubics ๐Ÿ‘‡ Explore Lattices ๐Ÿ“Š

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Introduction

The Cubic Form

In the field of crystallography, the cubic, or isometric, crystal system is defined by a unit cell that assumes the shape of a cube. This geometric configuration represents one of the most prevalent and straightforward forms observed in crystals and minerals, serving as a fundamental building block in understanding solid-state structures.

Simplicity and Symmetry

The cubic system is characterized by its high degree of symmetry, possessing three equivalent crystallographic axes that are mutually perpendicular and of equal length. This inherent symmetry simplifies the description and analysis of crystal structures, making it a cornerstone in materials science and solid-state physics.

Ubiquity in Nature

Many common minerals and elements exhibit cubic structures due to the energetic favorability of close-packed arrangements. Understanding these structures is crucial for predicting and explaining the physical properties of materials, from metals to ionic compounds.

Bravais Lattices

Lattice Points

The cubic crystal system encompasses three distinct Bravais lattices, which are unique arrangements of points in space that describe the translational symmetry of a crystal. These are:

  • Primitive Cubic (cP): Features lattice points only at the corners of the cube.
  • Body-Centered Cubic (cI): Includes lattice points at the corners and one in the exact center of the cube.
  • Face-Centered Cubic (cF): Possesses lattice points at the corners and in the center of each face of the cube.

While the unit cells are cubic, the primitive unit cells for these lattices may not always be cubic themselves.

Lattice Characteristics

These lattices differ in their atomic packing efficiency and the number of lattice points per unit cell:

  • cP: Coordination Number (CN) = 6, Atomic Packing Factor (APF) โ‰ˆ 0.524.
  • cI: Coordination Number (CN) = 8, Atomic Packing Factor (APF) โ‰ˆ 0.680.
  • cF: Coordination Number (CN) = 12, Atomic Packing Factor (APF) โ‰ˆ 0.740.

The face-centered cubic (cF) structure represents the densest possible packing of spheres.

Bravais Lattice Table

The following table summarizes the key characteristics of the cubic Bravais lattices:

Bravais lattice Primitive
cubic
Body-centered
cubic
Face-centered
cubic
Pearson symbol cP cI cF
Unit Cell
(Conceptual)
โฌข โฌฃ โฌก
Lattice Points per Unit Cell 1 2 4
Coordination Number (CN) 6 8 12
Atomic Packing Factor (APF) ~0.524 ~0.680 ~0.740

Crystal Classes

Symmetry Groups

The cubic crystal system is further classified into 36 distinct space groups, which describe the full symmetry of the crystal structure, including translational symmetry. These groups are organized into several crystal classes based on their point group symmetry. The highest symmetry class, the hexoctahedral class, possesses the full symmetry of a cube.

Classification Framework

Crystallographic point groups define the rotational and reflectional symmetries of a crystal's lattice. In the cubic system, these range from lower symmetry groups like tetrahedral (T) to the highest octahedral (Oh) symmetry. Each point group corresponds to specific crystal classes and is associated with a set of space groups.

Crystal Classes Overview

The following table outlines the major crystal classes within the cubic system, their associated point group notations, and examples:

No. Point group Type Example Space groups
Name Schรถn. Intl Orb. Cox. Primitive Face-centered Body-centered
195โ€“197 Tetartoidal T 23 332 [3,3]+ enantiomorphic Ullmannite, Sodium chlorate P23 F23 I23
198โ€“199 P213 I213
200โ€“204 Diploidal Th 2/m3
(m3)
3*2 [3+,4] centrosymmetric Pyrite Pm3, Pn3 Fm3, Fd3 I3
205โ€“206 P3m Ia3
207โ€“211 Gyroidal O 432 432 [3,4]+ enantiomorphic Petzite P432, P4232 F432, F4132 I432
212โ€“214 P4332, P4132 I4132
215โ€“217 Hextetrahedral Td 43m *332 [3,3] F43m I43m
218โ€“220 P43m I43d
221โ€“230 Hexoctahedral Oh 4/m32/m
(m3m)
*432 [3,4] centrosymmetric Galena, Halite Pm3m, Pn3n, Pm3n Fm3m, Fm3c, Fd3m, Fd3c Im3m, Ia3d

Single Element Structures

Packing Efficiency

The arrangement of atoms in elemental solids is often dictated by the principle of maximizing packing efficiency. Structures with higher atomic packing factors (APFs) are generally more stable.

  • Primitive Cubic (cP): Rare due to its low APF (~0.524). Found in Polonium.
  • Body-Centered Cubic (cI): Common, with an APF of ~0.680. Examples include Iron, Chromium, Tungsten, and Niobium.
  • Face-Centered Cubic (cF): Also very common and represents the densest packing (APF ~0.740). Examples include Aluminum, Copper, Gold, and Silver.

Diamond Cubic Structure

A notable cubic structure is the diamond cubic arrangement, which is not a simple Bravais lattice but rather a lattice with a basis. This structure is found in elements like Carbon (diamond allotrope), Silicon, Germanium, and Tin. It is characterized by tetrahedral bonding, resulting from specific orbital hybridization.

Multi-Element Structures

Interpenetrating Lattices

Many compounds, particularly binary and ternary compounds, adopt crystal structures based on the cubic system. These structures can often be visualized as two or more interpenetrating Bravais lattices, where atoms occupy the interstitial sites of each other.

Common Cubic Structures

Several fundamental structures are prevalent in multi-element compounds:

  • Caesium Chloride (CsCl) Structure: A simple cubic arrangement of one ion type with the other ion type occupying the cubic interstitial site. It exhibits 8-fold coordination.
  • Rock-Salt (NaCl) Structure: Two interpenetrating face-centered cubic lattices, resulting in octahedral coordination (6-fold) for each atom.
  • Zincblende (ZnS) Structure: Similar to the rock-salt structure but with tetrahedral coordination (4-fold). It is derived from the diamond cubic structure by alternating atom types.

Structure Examples

The following tables list examples of compounds exhibiting these and other cubic structures:

Caesium Chloride Structure (B2)

Characterized by 8-fold coordination. Space group: Pm3m (No. 221).

Alkali Metal Halides & Hydrides
Element Hydrides Fluorides Chlorides Bromides Iodides
Lithium LiH LiF LiCl LiBr LiI
Sodium NaH NaF NaCl NaBr NaI
Potassium KH KF KCl KBr KI
Rubidium RbH RbF RbCl RbBr RbI
Caesium CsH CsF (CsCl structure)
Alkaline Earth Metal Chalcogenides
Element Oxides Sulfides Selenides Tellurides Polonides
Magnesium MgO MgS MgSe MgTe MgPo
Calcium CaO CaS CaSe CaTe CaPo
Strontium SrO SrS SrSe SrTe SrPo
Barium BaO BaS BaSe BaTe BaPo

Rock-Salt Structure (B1)

Features 6-fold octahedral coordination. Space group: Fm3m (No. 225).

Alkali Metal Halides & Hydrides (Common)
Element Hydrides Fluorides Chlorides Bromides Iodides
Lithium LiH LiF LiCl LiBr LiI
Sodium NaH NaF NaCl NaBr NaI
Potassium KH KF KCl KBr KI
Rubidium RbH RbF RbCl RbBr RbI
Caesium CsH CsF (CsCl structure)
Transition Metal Monoxides
Element Oxides
Titanium TiO
Vanadium VO
Chromium CrO
Manganese MnO
Iron FeO
Cobalt CoO
Nickel NiO
Cadmium CdO

Zincblende Structure (B3)

Characterized by 4-fold tetrahedral coordination. Space group: F43m (No. 216).

II-VI Compounds
Element Sulfides Selenides Tellurides Polonides
Beryllium BeS BeSe BeTe BePo
Zinc ZnS ZnSe ZnTe ZnPo
Cadmium CdS CdSe CdTe CdPo
Mercury HgS HgSe HgTe -
III-V Compounds
Element Nitrides Phosphides Arsenides Antimonides
Boron BN* BP BAs BSb
Aluminium AlN* AlP AlAs AlSb
Gallium GaN* GaP GaAs GaSb
Indium InN* InP InAs InSb

*Some nitrides also exist in the hexagonal wurtzite structure.

Heusler Structure

Common for ternary compounds, often involving transition metals. Space group: Fm3m (No. 225), Strukturbericht designation L21.

Heusler Compounds (e.g., Co2MnSi)
Example Compound Structure Type
Cu2MnAl Heusler
Co2FeSi Heusler
Ni2MnGa Heusler

Iron Monosilicide Structure (B20)

A chiral structure found in transition metal silicides and germanides. Space group: P213 (No. 198).

Transition Metal Silicides/Germanides
Element Silicides Germanides
Manganese MnSi MnGe
Iron FeSi FeGe
Cobalt CoSi CoGe

Weaireโ€“Phelan Structure

A complex structure related to clathrates. Space group: Pm3n (No. 223).

Clathrate Hydrate Structures
Molecule Structure Type
Water (with Methane) Type I Clathrate (Weaireโ€“Phelan)
Water (with Propane) Type I Clathrate (Weaireโ€“Phelan)

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References

References

  1.  Crystallography and Minerals Arranged by Crystal Form, Webmineral
  2.  Birkbeck College, University of London
  3.  The Zincblende (B3) Structure. Naval Research Laboratory, U.S.
A full list of references for this article are available at the Cubic crystal system Wikipedia page

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Important Notice

This content has been generated by an Artificial Intelligence model and is intended for educational and informational purposes only. The information presented is derived from publicly available data, primarily Wikipedia, and may not represent the most current or complete understanding of the subject matter.

This is not a substitute for expert consultation. The details provided herein should not be considered professional advice in crystallography, materials science, or any related field. Always consult with qualified experts and refer to authoritative sources for critical applications or decision-making.

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