6. Pure potassium bromide, KBr, which adopts the sodium chloride structure, has the fraction of empty cation sites due to Schottky defects, nq/N, equal to 9.159x10-21 at 20C (293 K). (a) Estimate the enthalpy of formation of a Schottky defect, Ahs. (b) Calculate the number of anion vacancies per cubic meter of KBr at 730C (1003 K), just below the melting point of KBr. The unit cell of KBr is cubic with ao = 0.660 nm and contains four formula units of KBr. (20 points total)