0
$\begingroup$

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?

$\endgroup$
0
$\begingroup$

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.

| improve this answer | |
$\endgroup$
  • $\begingroup$ 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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.