# Change of ionic packing upon crystals are pressurized

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:

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.

• 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 Sep 18 '20 at 2:55
• 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. Oct 13 '20 at 13:35