I would like to know why some authors say that rock-salt structure consists of two interpenetrating FCC structures if sodium has a body-centred cubic and chlorine has an orthorhombic structure? Thank you in advance. I am reading about crystal and lattice structures. I will really appreciate your academic responses.
If you look at NaCl rocksalt crystal structure and ignore all the chlorine ions, the sodium ions are in an FCC structure. In the same way, if you ignore the all the sodium and only look at the chlorine, they are also FCC, just with the origin shifted halfway along a cell edge. This is just a handy way to visualize the crystal structure; neither rocksalt FCC sub-lattice actually is stable on its own.
The crystal structures of pure or elemental sodium and chlorine are irrelevant. The rocksalt crystal structure can be found in lots of materials that aren't NaCl, and the structure can still be viewed as two interpenetrating FCC lattices.
As Mrs. Kirkham writes, the crystal structure of a binary compound (AB) is not necessary the same of that of the pure elemental A and B elements. This happens only when there is a complete solid solubilty between A and B in the entire composition phase diagram. Where there is not complete solid solubility the x=1/2 composition can crystallize in another structure. See the attach figure taken from http://www.mx.iucr.org/iucr-top/comm/cteach/pamphlets/16/node9.html.
Best regards,
Pedro
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