When rock salt crystal is pressurized, its FCC structure changes to BCC structure whereby coordination ratio changes from 6:6 to 8:8. At the same time packing efficiency of the crystal decreases from 74% of FCC to 68% of BCC. So will the crystal actually expand or contract during this process?


Your termination of BCC is incorrect about the $\ce{NaCl}$ crystal structure upon pressurization. It is actually simple cubic (SC as $\ce{CsCl}$) where eight $\ce{Cl-}$ ions ate the corners of unit cell and one $\ce{Na+}$ in the center or vise versa. Although either $\ce{Na+}$ or $\ce{Cl-}$ in the center, it is not BCC because ion possessing the center of unit cell is different from the ions at the corners of unit cell:

CsCl Crystal Structure

Thus, lattice constant $a$ can be expressed as:

$$a_\mathrm{SC} = \frac{2(r_\ce{Na+} + r_\ce{Cl-})}{\sqrt{3}} \tag1$$

For FCC $\ce{NaCl}$ crystal structure:

$$a_\mathrm{FCC} = 2(r_\ce{Na+} + r_\ce{Cl-}) \tag2$$

Since $a_\mathrm{SC} \lt a_\mathrm{FCC}$, we can conclude that upon pressurization, $\ce{NaCl}$ crystal structure would be contracted.

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  • $\begingroup$ Most information on the net says FCC changes here to BCC and not SC. This research data also points at this:jetp.ac.ru/cgi-bin/dn/e_016_04_0855.pdf $\endgroup$ – Nandakumar U K Sep 18 at 2:55
  • $\begingroup$ And most information on the net would be wrong. The referenced paper smartly avoids BCC and SC in favor of just calling it CsCl-type. $\endgroup$ – Oscar Lanzi Oct 13 at 13:35

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