Model chemlab1/2/2024 ![]() horizontal vertical The movie below is the same structure Click on the images below to view the lattice structure rotating. The resulting HCP structure is shown below. An alternative sequence would be A - C - A - C - A. Hexagonal Close Packed (hcp) The hexagonal close packed structure can be made by piling layers Go back and look at the lattice again and locate the tetrahedralĮxamples of fcc / ccp metals include nickel, silver, gold, copper,Īnother way of stacking these layers is to omit the "C" layers altogether This isĪ 6 - coordinate octahedral interstitial site. These cavities are surrounded by 6 atoms in octahedral geometry. The gold layer does not nestle into them as it does the left-pointing ones. If we look at the right-pointing holes in the blue layer, we see that This is a 4 - coordinate tetrahedral interstitial Under each gold atom is a small space surrounded by 4 atoms in a In the blue layer, and none of the right-pointing ones. Remember, the gold atoms cover all the left-pointing interstices Let us consider a pair of layers - blue and gold. horizontal vertical There are two types of interstitial sites in an fcc lattice. However, the internal structures and relationships can be examinedĬlick on the images below to view the open lattice rotating. ![]() Click on the images below to view the structure rotating. In a plane - it is the most efficient way to pack spheres. The resulting structure is a 3-D analog of the hexagonal packing An alternate sequence might be B - A - C - B - A - C. The cubic close packed structure can be constructed from the A - B - C - A - B - C. We can continue to stack these layers in any order, providing that (One left-pointing and one right-pointing). Remember there are twice as many interstitial If we put them over the right-pointing interstices we generate aĭifferent layer, labeled the green or "C" layer. Interstices or all the "right-pointing" interstices. We can put them over either all the "left-pointing" The animation below, we have arbitrarily chosen to cover the left-pointing Notice that there are 2 separate choices for the second layer in We then place a second close-packed layer (the gold "B" layer)Ītop the the first, so they nestle into the left-pointing holesĪll spheres are actually the same atom, the colors are to help Notice that this is a close-packed arrangement - there is no way to pack We start with a hexagonal array of spheres (the blue "A" layer). Since theseĬut across the other layers, each layer will contain all three colors.Ī different and better way of looking at this structure focuses on Note that diagonal layers also form along our line of sight. If we stack the cells into a lattice we notice that the atoms formĭiagonal layers - the reason for the colors is to make these stand out. Packing can exceed this efficiency (although there are others with the Name "close packed" refers to the packing efficiency of 74.05%. The unit cell is again shown expanded for visibility.Īctually, the corner atoms touch the one in the center of the face. The unit cell is: Click on the unit cell above to view it rotating. ![]() Remember that the atoms are all the same. The reason for the various colors is to help point out how the cells ![]() We can think of this cell as being made by inserting another atom intoĮach face of the simple cubic lattice - hence the "face centered cubic" Close Packed Structures CLOSE PACKED STRUCTURESįace Centered Cubic (fcc) or Cubic Close Packed (ccp) These are two different names for the same lattice.
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