On the use of Solomon echoes in 27Al NMR studies of complex aluminium hydrides

TL;DR

This study demonstrates that Solomon echo sequences in static 27Al NMR provide accurate quadrupole parameters for complex aluminium hydrides, comparable to MAS NMR, and compares experimental results with DFT-GIPAW calculations.

Contribution

It introduces the use of Solomon echoes in static 27Al NMR to determine quadrupole parameters with high accuracy, offering an alternative to MAS NMR for complex aluminium hydrides.

Findings

Solomon echoes yield accurate $C_Q$ and $ au$ parameters.

Static spectra accuracy rivals MAS NMR.

DFT-GIPAW calculations mostly agree with experimental data.

Abstract

The quadrupole coupling constant $C_Q$ and the asymmetry parameter $\eta$ have been determined for two complex aluminium hydrides from $^{27}$Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The thus obtained data for KAlH4 ($C_Q=(1.30\pm0.02)$ MHz, $\eta=(0.64\pm0.02)$) and NaAlH4 ($C_Q=(3.11\pm0.02)$ MHz, $\eta<0.01$) agree very well with data previously determined from MAS NMR spectra. The accuracy with which these parameters can be determined from static spectra turned out to be at least as good as via the MAS approach. The experimentally determined parameters ($\delta_{iso}$, $C_Q$ and $\eta$) are compared with those obtained from DFT-GIPAW (density functional theory - gauge-including projected augmented wave) calculations. Except for the quadrupole coupling constant for KAlH4, which is overestimated in the GIPAW calculations by about 30%, the agreement is excellent. Advantages of the application of the Solomon echo sequence for the measurement of less stable materials or for in-situ studies are discussed.