Hexagonal Close Packing (HCP)
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 hexagonal close packed arrangement, the first two layers are staggered relative to one another and the third 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 beryllium, magnesium, zinc, cadmium, titanium, zirconium, ruthenium, osmium and rhenium, as well as many alloys crystallize with the HCP 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). The separation between the holes in the HCP lattice is smaller than in the CCP lattice. As the holes are occupied by like-charged ions, structures based on HCP packing tend to occur for less ionic compounds than for CCP packing.
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Niccolite
This structural type consists of hexagonal 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.
This structure is the HCP analogue of Rocksalt. The separation between the octahedral sites is smaller than in the CCP structure and so the Niccolite structure is adopted by compounds containing less electropositive metals.
Examples include:
M2+: NiS, NiAs, NiSb, NiSe, NiSn, NiTe, FeS, FeSe, FeTe, FeSb, CoS, CoSe, CoTe, CrSe, CrTe, CrSb, MnTe, PtSn and
M3+: MnAs, MnSb, MnBi, CoSb, PtSb and PtBi.
Cations are surrounded by 6 anions. Anions are surrounded by 6 cations.
The inverse structure (Anti-NiAs), with hexagonal close packed cations and anions in all of the octahedral sites is also known and is adopted by NbN.
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HCP Fluorite
This structural type consists of hexagonal 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 would be adopted by compounds with formula MX2.
Cations are surrounded by 8 anions. Anions are surrounded by 4 cations.
However, there are no known examples of this structural type. The separation between the T+ and T- sites is very small and presumably occupation of both by anions leads to too much electrostatic repulsion for this structure to be stable, compared to the CCP analogue (Fluorite).
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HCP Anti-Fluorite
This structural type consists of hexagonal 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 would be adopted by compounds with formula M2X.
Cations are surrounded by 4 anions. Anions are surrounded by 8 cations.
However, there are no known examples of this structural type. The separation between the T+ and T- sites is very small and presumably occupation of both by cations leads to too much electrostatic repulsion for this structure to be stable, compared to the CCP analogue (Anti-Fluorite).
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Wurtzite
This structural type consists of hexagonal 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.
This structure is the HCP analogue of Zinc Blende and is adopted by less ionic compounds and/or contain cations which are too big to fit into the octahedral holes.
Examples include:
M+: AgI, NH4F,
M2+: ZnO, ZnS, ZnSe, ZnTe, BeO, CdS, MnS, MnSe,
M3+: AlN, GaN, InN, TaN and
M4+: SiC
Cations are surrounded by 4 anions. Anions are surrounded by 4 cations.
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Cadmium Iodide
This structural type consists of hexagonal 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.
This structure is the HCP analogue of Cadmium Chloride and as tends to be adopted by less ionic compounds.
Examples include:
M2+: CdI2, CaI2, CoI2, FeI2, MgI2, MnI2, PbI2, ThI2, TiI2, TmI2, VI2, YbI2, ZnI2, VBr2, TiBr2, MnBr2, FeBr2, CoBr2, TiCl2, VCl2, Mg(OH)2, Ca(OH)2, Fe(OH)2, Co(OH)2, Ni(OH)2, Cd(OH)2.
Cations are surrounded by 6 anions. Anions are surrounded by 3 cations.
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Rutile
This structural type consists of distorted layers of hexagonal 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 HCP analogue of Anatase. It is also related to Cadmium Iodide as both consist of HCP anions with cations in half of the octahedral sites. In Rutile, these sites are occupied alternately whereas, in Cadmium Iodide, whole layers of sites are alternately occupied and vacant.
Examples include:
M2+: CoF2, FeF2, MgF2, MnF2, NiF2, PdF2, ZnF2,
M4+: TiO2, CrO2, GeO2, IrO2, MnO2, MoO2, NbO2, OsO2, PbO2, RuO2, SnO2, TaO2, WO2.
Cations are surrounded by 6 anions. Anions are surrounded by 3 cations.
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HCP lattices
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Hexagonal Close Packed Structure
Closed packed layers which repeat in the sequence ABABAB...:
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Nickel Arsenide (Niccolite)
A hexagonal closed packed array of anions with cations in all of the
octahedral holes:
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HCP Anti-Fluorite
A hexagonal closed packed array of anions and cations in all of the
tetrahedral holes. There are no known compounds with this structural type..
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HCP Fluorite
A hexagonal closed packed array of cations and anions in all of the
tetrahedral holes. There are no known compounds with this structural type.
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Zinc Sulfide (Wurtzite)
A hexagonal closed packed array of anions with the cations in half of the
tetrahedral holes:
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Cadmium iodide
A hexagonal closed packed array of anions with the cations in octahedral
holes in alternate layers:
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Titanium Dioxide ("Rutile")
A distorted hexagonal closed packed array of anions with the cations in alternate octahedral
holes:
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