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

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 Ionics2004, 170, 25-32) 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.