Cubic Close Packing (CCP)
Close packed structures consist of close packed layers which fit together so that each sphere of one layer rests in a hollow between three spheres on the layers above and below. In the cubic close packed arrangement, the first three layers are staggered relative to one another and the fourth layer is on top of the first.
74% of the total space is occupied and each atom is in contact with 12 other atoms.
Many pure metals, including copper, silver, gold, aluminium, nickel, palladium, platinum and lead, as well as many alloys crystallize with the CCP structure.
Although the structure is close packed, small holes ("interstitial sites") exist between the atoms. For every n atoms in the structure, there are:
n octahedral holes (surrounded by 6 atoms) and
2n smaller tetrahedral holes (surrounded by 4 atoms)
Many ionic structures can be described as close packed arrangements of anions with cations in these holes (or vice versa).
|
|
Rocksalt
This structural type consists of cubic close packed anions with cations in all of the octahedral sites. As there are n octahedral holes for every n close packed atoms, the ratio of cations : anions is n : n or 1 : 1 and so this structure is adopted by compounds with formula MX.
Examples include:
M+: LiF, LiCl, LiBr, LiI, LiH, NaF, NaCl, NaBr, NaI, NaH, KF, KCl, KBr, KI, RbF, RbCl, RbI, AgF, AgCl, ArBr,
M2+: MgO, CaO, SrO, BaO, TiO, MnO, FeO, CoO, NiO, CdO, MgS, CaS, SrS, BaS, MnS, MgSe, CaSe, SrSe, BaSe, CaTe, SrTe, BaTe, SnAs,
M3+: LaN, ScN, TiN, UN and
M4+: UC.
Cations are surrounded by 6 anions. Anions are surrounded by 6 cations.
|
|
Fluorite
This structural type consists of cubic close packed cations with anions in all of the tetrahedral sites. As there are 2n tetrahedral holes for every n close packed atoms, the ratio of cations : anions is n : 2n or 1 : 2 and so this structure is adopted by compounds with formula MX2.
Examples include:
M2+: CaF2, SrF2, SrCl2, BaF2, BaCl2, CdF2, HgF2, EuF2, PbF2 and
M4+: PbO2, CeO2, PrO2, ThO2, UO2, NpO2, CmO2, PuO2, AmO2.
Cations are surrounded by 8 anions. Anions are surrounded by 4 cations.
|
|
Anti-Fluorite
This structural type consists of cubic close packed anions with cations in all of the tetrahedral sites. It is thus the reverse of the Fluorite structure. As there are 2n tetrahedral holes for every n close packed atoms, the ratio of cations : anions is 2n : n or 2 : 2 and so this structure is adopted by compounds with formula M2X.
Examples include:
M+: Li2O, Li2S, Li2Se, Li2Te, Na2O, Na2S, Na2Se, Na2Te, K2O, K2S, K2Se, K2Te, Rb2O, Rb2S.
Cations are surrounded by 4 anions. Anions are surrounded by 8 cations.
|
|
Zinc Blende or Sphalerite
This structural type consists of cubic close packed anions with cations in half of the tetrahedral sites (either all of the T+ or all of the T- sites). As half of the 2n tetrahedral holes are occupied for every n close packed atoms, the ratio of cations : anions is n : n or 1 : 1 and so this structure is adopted by compounds with formula MX. Compounds which adopt this structure tend to be less ionic than those that adopt the Rocksalt structure and/or contain cations which are too big to fit into the octahedral holes.
Examples include:
M+: CuF, CuCl, CuBr, CuI, AgI,
M2+: MnS, MnSe, BeS, BeSe, BeTe, BeTe, ZnS, ZnSe, ZnTe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe,
M3+: BN, BP, BAs, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb and
M4+: SiC
Cations are surrounded by 4 anions. Anions are surrounded by 4 cations.
When the two elements in the Zinc Blende structure are the same, the Diamond structure, adopted by C, Si, Ge and Sn, is obtained.
|
|
Cadmium Chloride
This structural type consists of cubic close packed anions with cations in the octahedral sites in alternate layers so that half are occupied. With half of the n octahedral holes are occupied for every n close packed atoms, the ratio of cations : anions is n/2 : n or 1 : 2 and so this structure is adopted by compounds with formula MX2. As this structure leads to layers of anions without cations in between, compounds which adopt this structure tend to be much less ionic than those that adopt the Fluorite structure.
Examples include:
M2+: CdCl2, CdBr2, CoCl2, FeCl2, MgCl2, MnCl2, NiCl2, NiBr2, NiI2, ZnBr2, ZnI2.
Cations are surrounded by 6 anions. Anions are surrounded by 3 cations.
The inverse structure (Anti-CdCl2), with cubic close packed cations and anions in half of the octahedral sites is also known and is adopted by Cs2O. The relationship between the CdCl2 and Anti-CdCl2 structures is the same as that between Fluorite and Anti-Fluorite.
|
|
Rutile
This structural type consists of distorted layers of cubic close packed anions with cations in the half of the octahedral sites. With half of the n octahedral holes are occupied for every n close packed atoms, the ratio of cations : anions is n/2 : n or 1 : 2 and so this structure is adopted by compounds with formula MX2.
This structure is the CCP analogue of the much more common Rutile, which is also adopted by TiO2. It is also related to Cadmium Chloride as both consist of CCP anions with cations in half of the octahedral sites. In Anatase, these sites are occupied alternately whereas, in Cadmium Chloride, whole layers of sites are alternately occupied and vacant.
Cations are surrounded by 6 anions. Anions are surrounded by 3 cations.
|
|
CrCl3
This structural type consists of cubic close packed anions with cations in the 1/3 of the octahedral sites and between adjacent anion layers. With 1/3 of the n octahedral holes are occupied for every n close packed atoms, the ratio of cations : anions is n/3 : n or 1 : 3 and so this structure is adopted by compounds with formula MX3.
This structure is the CCP analogue of the BiI3. Both are quite common structures for transition metal trihalides. With no cations in alternate layers, there is only very weak bonding between these layers and they can be easily cleaved.
Cations are surrounded by 6 anions. Anions are surrounded by 2 cations.
|
|
CCP lattices
|
|
Cubic Close Packed Structure
Closed packed layers which repeat
in the sequence ABCABCABC...:
|
|
|
Sodium Chloride ("Rocksalt")
A cubic
closed packed array of anions with
the cations in all of the octahedral holes:
|
|
|
Sodium Oxide ("Anti-Fluorite")
A cubic closed packed array of anions with
the cations in all of the tetrahedral holes:
|
|
|
Calcium Fluoride ("Fluorite")
A cubic closed packed array of cations with
the anions in all of the
tetrahedral holes:
|
|
|
Zinc Sulfide ("Zinc Blende" or "Sphalerite")
A cubic closed packed array of anions with
the cations in
half of the tetrahedral holes:
|
|
|
Cadmium Chloride
A cubic closed packed array of anions
with the cations in octahedral
holes in alternate layers:
|
|
|
Titanium Dioxide ("Anatase")
A distorted cubic closed packed array of anions
with the cations in alternate octahedral
holes:
|
|
|
Chromuium(III) Chloride
A cubic closed packed array of anions
with the cations in 1/3 of octahedral holes
and between adjacent anion layers.
|
|
|
