Editing Cubic crystal system (section)

===Caesium chloride structure===
{{Category see also|Caesium chloride crystal structure}}
[[File:CsCl crystal.svg|thumb|left|upright|A [[caesium chloride]] unit cell. The two colors of spheres represent the two types of atoms.]]

One structure is the "interpenetrating primitive cubic" structure, also called a "caesium chloride" or B2 structure. This structure is often confused for a body-centered cubic structure because the arrangement of atoms is the same. However, the caesium chloride structure has a basis composed of two different atomic species. In a body-centered cubic structure, there would be translational symmetry along the [111] direction. In the caesium chloride structure, translation along the [111] direction results in a change of species. The structure can also be thought of as two separate simple cubic structures, one of each species, that are superimposed within each other. The corner of the chloride cube is the center of the caesium cube, and vice versa.<ref name=":2">{{Cite web|title=Cubic Lattices and Close Packing|date=3 October 2013|url=https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_Chem1_(Lower)/07%3A_Solids_and_Liquids/7.08%3A_Cubic_Lattices_and_Close_Packing|url-status=live|archive-url=https://web.archive.org/web/20201101010927/https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_Chem1_(Lower)/07%3A_Solids_and_Liquids/7.08%3A_Cubic_Lattices_and_Close_Packing |archive-date=2020-11-01 }}</ref> 
[[File:Cesium_Chloride.jpg|thumb|This graphic shows the interlocking simple cubic lattices of cesium and chlorine.  You can see them separately and as they are interlocked in what looks like a body-centered cubic arrangement]]

It works the same way for the NaCl structure described in the next section.&nbsp; If you take out the Cl atoms, the leftover Na atoms still form an FCC structure, not a simple cubic structure.

In the unit cell of CsCl, each ion is at the center of a cube of ions of the opposite kind, so the [[coordination number]] is eight. The central cation is coordinated to 8 anions on the corners of a cube as shown, and similarly, the central anion is coordinated to 8 cations on the corners of a cube. Alternately, one could view this lattice as a simple cubic structure with a secondary atom in its [[interstitial site|cubic void]].

In addition to caesium chloride itself, the structure also appears in certain other [[Alkali metal halide|alkali halides]] when prepared at low temperatures or high pressures.<ref name=Seitz>Seitz, ''Modern Theory of Solids'' (1940), p.49</ref> Generally, this structure is more likely to be formed from two elements whose ions are of roughly the same size (for example, ionic radius of Cs<sup>+</sup> = 167 pm, and Cl<sup>−</sup> = 181 pm).

The [[space group]] of the [[caesium chloride]] (CsCl) structure is called Pm{{overline|3}}m (in [[Hermann–Mauguin notation]]), or "221" (in the International Tables for Crystallography). The [[Strukturbericht designation]] is "B2".<ref>[http://cst-www.nrl.navy.mil/lattice/struk/b2.html The CsCl (B2) Structure] {{webarchive|url=https://web.archive.org/web/20080915132850/http://cst-www.nrl.navy.mil/lattice/struk/b2.html|date=2008-09-15}}</ref>

There are nearly a hundred [[Rare-earth element|rare earth]] [[intermetallic compounds]] that crystallize in the CsCl structure, including many [[Binary phase|binary compounds]] of rare earths with [[magnesium]],<ref>{{cite journal | last1=Saccone | first1=A. | last2=Delfino | first2=S. | last3=Macció | first3=D. | last4=Ferro | first4=R. | title=Magnesium-rare earth phase diagrams: Experimental investigation of the Ho-Mg system | journal=Journal of Phase Equilibria 
 | volume=14 | issue=3 | year=1993 | doi=10.1007/bf02668225 | pages=280–287| s2cid=95011597 }}</ref> and with elements in groups [[Group 11 element|11]], [[Group 12 element|12]],<ref>{{cite journal | last1=Kanematu | first1=K | last2=T. Alfieri | first2=G. | last3=Banks | first3=E. | title=Magnetic Studies of Rare Earth Zinc Compounds with CsCl Structure | journal=Journal of the Physical Society of Japan 
| volume=26 | issue=2 | year=1969 | doi=10.1143/jpsj.26.244 | pages=244–248| bibcode=1969JPSJ...26..244K }}</ref><ref>{{cite journal | last=Buschow | first=K. H. J. | title=Magnetic properties of CsCl‐type rare‐earth cadmium compounds | journal=The Journal of Chemical Physics | 
volume=61 | issue=11 | year=1974| doi=10.1063/1.1681788 | pages=4666–4670| bibcode=1974JChPh..61.4666B }}</ref> and [[Group 13 element|13]]. Other compounds showing caesium chloride like structure are [[Cesium bromide|CsBr]], [[Cesium iodide|CsI]], high-temperature [[Rubidium chloride|RbCl]], AlCo, AgZn, BeCu, MgCe, RuAl and SrTl.{{citation needed|date=July 2015}}