PNNL

Abstract

We analyze the hydroxyl librational signatures of five structurally related aluminum (oxy)hydroxides, using inelastic neutron scattering (INS) and plane-wave lattice dynamics simulations. A clear trend across these aluminum-containing phases illustrates the relationship between hydrogen bonding, local atomic structure, and the spectral location and profile of the librational bands. The INS spectra have been compared to previous optical spectroscopy and computational studies, highlighting the complementary nature of the INS technique. Taking into account other structurally or chemically related material analogs, we then identify a blueshift behavior (to higher energy) between the upper librational band edge and the strength and geometry of hydrogen bond interactions, mirroring (with opposite correlation) the well-known redshift of intramolecular O-H stretching energy with increasing hydrogen bond strength. For hydroxyl groups that effectively do not participate in hydrogen bonding, the bending librations occur at lower energy and are found to hybridize with metal-oxygen lattice modes. We find that standard density functional theory approximations, including dispersion corrections, struggle to correctly predict vibrational frequencies of motions dominated by H but perform well for metal-oxygen modes. Using this method, we made detailed mode assignments in several cases, including a demonstration of how layer-to-layer disorder in the hydrogen bond orientations of boehmite is reflected in the sharp but minor low energy peaks of the INS spectrum.