Identification of hydrogen defects inα−Al2O3by first-principles local vibration mode calculations

Abstract

The absorption peaks at ∼3200−3300 cm −1 in α-Al 2 O 3 crystal that were previously observed and assigned to hydrogen interstitial were reinvestigated by first-principles local vibration mode calculations. Based on our calculations, we found that the vibration of hydrogen interstitial is not consistent with the observed polarized IR absorption peaks at ∼3200−3300 cm −1 for two reasons: (1) the orientation of the O-H bond is not aligned with the direction of the oscillator observed and (2) the calculated vibrational frequency is much lower than the observed values. The calculated formation energies showed that interstitial hydrogen can strongly bind with aluminum vacancy, forming [H n-V Al ] complex defects. The complexes have the O-H bond in the direction aligned with the direction of the oscillator. The vibration frequencies of the complex defects are also close to the observed value. Therefore, we assign the experimentally observed ∼3200−3300 cm −1 modes to [H n-V Al ] complex defects.

Theoretical infrared spectra of OH defects in corundum (<i>α</i>-Al<sub>2</sub>O<sub>3</sub>)

“…The geometry (Fig. 3) of the model of the interstitial proton ((1H + ) i ) is consistent with that previously determined by Zhang et al (2014) and T-Thienprasert et al (2017). The theoretical stretching frequency 2994.5 cm −1 is very close to that reported by Thienprasert et al 2017(2989.25 cm −1 ).…”

“…Intrinsic defects in corundum involve vacant sites or atoms in interstitial positions (e.g., Lagerlöf and Grimes, 1998). Various schemes of proton association with Al vacancies have previously been investigated by T-Thienprasert et al (2017). In the present study, models displaying one (1H + ) Al or two protons (2H + ) Al associated with Al vacancies ( Fig.…”

“…Chemically complex Ti-bearing defects have previously been proposed as playing a role in the hydrogen speciation in olivine (Berry et al, 2005;Tollan et al, 2017) and diopside (Balan et al, 2020). While these defects could have a global role when observed in major phases of the Earth mantle (e.g., Faul et al, 2016;Demouchy and Bolfan Casanova, 2016), their presence and configurational equilibrium in natural samples could also provide information on the physicalchemical conditions and timescales of geological processes (Padrón-Navarta et al, 2014).…”

“…To this end, a theoretical approach linking the microscopic structure of the defects to their spectroscopic properties could be useful, as attested by theoretical investigations of OH groups in nominally anhydrous silicates (e.g., Wright, 2006;Balan et al, 2017Balan et al, , 2020Blanchard et al, 2017;Jollands et al, 2020). Previous theoretical studies have focused on the association of hydrogen with intrinsic defects in corundum (Zhang et al, 2014;T-Thienprasert et al, 2017). Based on theoretical vibrational frequencies, T-Thienprasert et al (2017) concluded that the OH-stretching bands observed at ∼ 3200-3300 cm −1 in corundum were consistent with the association of H and Al vacancies.…”

“…Previous theoretical studies have focused on the association of hydrogen with intrinsic defects in corundum (Zhang et al, 2014;T-Thienprasert et al, 2017). Based on theoretical vibrational frequencies, T-Thienprasert et al (2017) concluded that the OH-stretching bands observed at ∼ 3200-3300 cm −1 in corundum were consistent with the association of H and Al vacancies.…”