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Both arrangements produce a face-centered cubic lattice – with different orientation to the ground. Hexagonal close-packing would result in a six-sided pyramid with a hexagonal base. Collections of snowballs arranged in pyramid shape. The front pyramid is hexagonal close-packed and rear is face-centered cubic.
Hexagonal close packed (hcp) unit cell. Hexagonal close packed (hcp) is one of the two simple types of atomic packing with the highest density, the other being the face-centered cubic (fcc). However, unlike the fcc, it is not a Bravais lattice, as there are two nonequivalent sets of lattice points.
Hexaferrum and epsilon iron (ε-Fe) are synonyms for the hexagonal close-packed (HCP) phase of iron that is stable only at extremely high pressure.. A 1964 study at the University of Rochester mixed 99.8% pure α-iron powder with sodium chloride, and pressed a 0.5-mm diameter pellet between the flat faces of two diamond anvils.
A close packed unit cell, both face-centered cubic and hexagonal close packed, can form two different shaped holes. Looking at the three green spheres in the hexagonal packing illustration at the top of the page, they form a triangle-shaped hole. If an atom is arranged on top of this triangular hole it forms a tetrahedral interstitial hole.
This arrangement of atoms in a crystal structure is known as hexagonal close packing (hcp). If, however, all three planes are staggered relative to each other and it is not until the fourth layer is positioned directly over plane A that the sequence is repeated, then the following sequence arises:
Here there is a choice between separating the spheres into regions of close-packed equal spheres, or combining the multiple sizes of spheres into a compound or interstitial packing. When many sizes of spheres (or a distribution ) are available, the problem quickly becomes intractable, but some studies of binary hard spheres (two sizes) are ...
In closest packing, every atom has 12 equidistant nearest neighbours, and therefore a coordination number of 12. If the close packed structures are considered as being built of layers of spheres, then the difference between hexagonal close packing and face-centred cubic is how each layer is positioned relative to others.
For the hexagonal close-packed structure the derivation is similar. Here the unit cell (equivalent to 3 primitive unit cells) is a hexagonal prism containing six atoms (if the particles in the crystal are atoms).