Vacancy formation energies and mechanisms of ion-molecule reactions from ionic sublimation measurements on potassium bromide single crystals

Using the mass spectrometry technique to study the ionic sublimation of a potassium bromide single crystal, a novel method of determination of the sign of surface charge and of the vacancy formation energies is proposed. It is found that a potassium bromide single crystals, when heated above 600 K, emit positive ionst K+, K-2(+), K2Br+ and K3Br2+. The temperature dependence of thermal ion currents, In I-i - 1/T, measured over the interval 600-920 K exhibited a successive maximum and a minimum with increasing temperature. The extrema are identified with the change in the electrical surface properties. It is inferred that in the intrinsic temperature range the sign of the surface charge for KBr is negative. Locating the temperature at which the surface potential has its maximum positive magnitude enables the free energy g of formation of cation vacancies and the free energy g(+) of formation of anion vacancies to be evaluated at T = 800 K, g(-) = 0.96 +/- 0.03 eV and g(+) = 0.90 +/- 0.03 eV. The relative current intensities are observed to change drastically upon reversal of the surface charge polarity. The mechanisms of formation of K-2(+) and K2Br+ ions are discussed. The influence of the surface charge on the rates of ion-molecule reactions is analysed.