{\it Ab initio} $^{27}Al$ NMR chemical shifts and quadrupolar parameters for $Al_2O_3$ phases and their precursors

TL;DR

This study uses advanced DFT-based GIPAW calculations to accurately predict $^{27}Al$ NMR parameters in various aluminium oxide phases and their precursors, validating the method against experimental data and analyzing structural models.

Contribution

It applies GIPAW-DFT to compute NMR parameters for multiple Al$_2$O$_3$ phases, including $ ext{γ}$-alumina models, and links NMR shifts to local geometry and effective charges.

Findings

GIPAW-DFT results agree well with experimental data for crystalline phases.

Among models for $ ext{γ}$-alumina, the Paglia et al. structure shows the best match.

Chemical shifts correlate with Born effective charges, not just local geometry.

Abstract

The Gauge-Including Projector Augmented Wave (GIPAW) method, within the Density Functional Theory (DFT) Generalized Gradient Approximation (GGA) framework, is applied to compute solid state NMR parameters for $^{27}Al$ in the $\alpha$, $\theta$, and $\kappa$ aluminium oxide phases and their gibbsite and boehmite precursors. The results for well-established crystalline phases compare very well with available experimental data and provide confidence in the accuracy of the method. For $\gamma$-alumina, four structural models proposed in the literature are discussed in terms of their ability to reproduce the experimental spectra also reported in the literature. Among the considered models, the $Fd\bar{3}m$ structure proposed by Paglia {\it et al.} [Phys. Rev. B {\bf 71}, 224115 (2005)] shows the best agreement. We attempt to link the theoretical NMR parameters to the local geometry. Chemical shifts depend on coordination number but no further correlation is found with geometrical parameters. Instead our calculations reveal that, within a given coordination number, a linear correlation exists between chemical shifts and Born effective charges.