Crystalline sodium chloride can be doped with a small concentration of ions that…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
Crystalline sodium chloride can be doped with a small concentration of Al3+ ions that occupy former Na+ lattice sites. To maintain overall charge neutrality, cation vacancies (□) are formed.
Consider a crystal with the formula Na1−yAlyCl1−3y□3y, where □ represents a vacant Na+ site.
(a) Verify that the given formula is electrically neutral for any value of y. Show all charge-balance reasoning.
(b) For a doping level of y=0.03, calculate the fraction of cation sites that are vacant.
(c) Discuss qualitatively how the presence of Al3+ and cation vacancies is expected to affect the lattice energy per ion pair compared with pure NaCl.
(d) Explain how the introduction of cation vacancies might influence the electrical conductivity of the crystal at elevated temperature.