Effect of water adsorption on positron annihilation lifetimes in barium oxide and barium peroxide

Abstract

The effect of adsorption of water molecules on the electron density and defect spectrum was investigated in barium oxide and barium peroxide by the method of lifetime positron spectroscopy. The dispersed samples of BaO and BaO 2 were studied before and after exposure for various times in water vapor. At short exposure time ( t) the adsorption coefficient (α) of water molecules in BaO is 2–10 times greater than in BaO 2, but at t > 1600 min the α values of both compounds are approximately equal. Annihilation spectra of the initial compounds are characterized by two components τ 1 = 203 ps, τ 2 = 406 ps with intensities, I 1, I 2 respectively of 78.7% and 21.3% for BaO and τ 1 = 211 ps, τ 2 = 537 ps with I 1 =92.8%, I 2 = 7.2% for BaO 2. After exposure to water vapor the τ 1, τ 2 behavior of both systems is similar, whereas the I 1, I 2 behavior is different. At short exposure times τ 1, τ 2 are unchanged and with increasing exposure time they decrease, while I 1 is reduced and I 2 is increased. The annihilation velocity ( λ f ) of free positrons of both systems is increased with a rise in exposure time. The capture velocity (ν) is slightly decreased at t < 15 min for BaO and at t < 90 min for BaO 2, but increases at longer exposure times. The effect of water adsorption is ascribed to reduction of free annihilation volume after chemisorption and diffusion of water molecules in the crystal bulk. The ν behavior is caused by the decrease in the defect radius and increase in defect concentration. The transformation of the defect subsystem is explained by dissociation reactions of defects upon chemisorption and diffusion of water molecules in BaO and BaO 2 crystal lattices.