Joint Experimental and Computational ¹⁷O and ¹H Solid State NMR Study of Ba₂In₂O₄(OH)₂ Structure and Dynamics

Abstract

A structural characterization of the hydrated form of the brownmillerite-type phase Ba2In2O5, Ba2In2O4(OH)2, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H2O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. (Solid State Ionics20041702532) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba2In2O4(OH)2 yield a set of low energy structures; GIPAW NMR calculations on these configurations yield 1H and 17O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct 1H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. 1H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 °C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The 17O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by 1H–17O double resonance experiments.

Local structure and vibrational dynamics of proton conducting Ba2In2O5(H2O)x

“…6a) can be interpreted as the coexistence of two types of proton environments over the H(1) site, characterised by hydrogen bond lengths of 1.90(4) and 2.13(4)Å, respectively, and associated to hydrogen bonds accepted by O(1) and O(3) oxygen atoms, respectively. This result is in agreement with the ndings of Dervişoglu et al, 14 who observed, using multinuclear solid state NMR spectroscopy, the presence of two types of H(1) protons, with shorter and longer hydrogen bonds, respectively. In particular, they calculated four low-energy congurations corresponding to distinct hydrogen bonding patterns in the O(2)-H(1) layer, and hypothesised that these four congurations may coexist as no clear groundstate was predicted.…”

“…In particular, they calculated four low-energy congurations corresponding to distinct hydrogen bonding patterns in the O(2)-H(1) layer, and hypothesised that these four congurations may coexist as no clear groundstate was predicted. 14 The coexistence of two types of proton environments over the H(1) site can be rationalised when considering the displacement amplitude of the O(1) and O(3) acceptor oxygen atoms, away from their respective planes, under the pull of the hydrogen bonds formed with H(1), as illustrated in Fig. 11a.…”

“…Recent results based on density functional theory (DFT) calculations and solid state nuclear magnetic resonance (NMR) however contradict this hypothesis, and instead suggest the coexistence of two congurations of the O(2)-H(1) units, with shorter and longer hydrogen bond lengths and a difference in hydrogen bond angles. 14 Recently, we investigated the dehydration from the pseudo-cubic phase, BaInO 3 H, to the brownmillerite phase, Ba 2 In 2 O 5 , using in situ Raman spectroscopy. 10 Crucially, we showed the emergence of an intermediate phase (…”

Combining solid-state NMR spectroscopy with first-principles calculations – a guide to NMR crystallography

“…For example, this approach has been used successfully in recent work on AlPO frameworks, 135 and perovskite-based materials. 136 Seymour et al 135 were able to assign the 27 Al and 31 P NMR spectra of as-made STA-2, where the number and position of OH groups coordinated to the framework was unknown. A combination of solid-state NMR experiments and powder XRD measurements determined that the OH groups bridged between Al1 and Al2, and identified three potential sites within the framework cancrinite cages.…”

Recent Advances in Theoretical and Physical Aspects of NMR Chemical Shifts

“…With these constructed shielding surfaces, successful predictions of 31 P chemical shifts in calcined aluminophosphates have been attained (Dawson and Ashbrook, 2014 when coordination number is carefully taken into account (Gambuzzi et al, 2014). A Boltzmann weighing of 17 O shielding from four low-energy structures of brownmilleritetype phase Ba2In2O5, Ba2In2O4(OH)2, is shown to reproduce the observed solid state 17 O NMR spectra obtained from these perovskites (Dervisoglu et al, 2015). In this hydrated form of brownmillerite, the oxygen nuclei of hydrogen bond donors are shown to be shielded while both nonhydroxyl and hydrogen bond acceptor oxygen sites are found to be deshielded.…”