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| Format: | Preprint |
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2024
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| Online Access: | https://arxiv.org/abs/2410.07731 |
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| _version_ | 1866929535543410688 |
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| author | Zibrowius, Bodo Fischer, Michael |
| author_facet | Zibrowius, Bodo Fischer, Michael |
| contents | The quadrupole coupling constant $C_{\text{Q}}$ and the asymmetry parameter $η$ of the aluminium nuclei in two polymorphs of the complex aluminium hydride CsAlH4 are determined from both $^{27}$Al MAS NMR spectra and $^{27}$Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The accuracy with which these parameters can be determined from the static spectra (CsAlH4(o): $C_{\text{Q}}=(1.42\pm0.01)$ MHz, $η=(0.62\pm0.01)$ and CsAlH4(t): $C_{\text{Q}}=(1.43\pm0.02)$ MHz, $η<0.03$) seems to be slightly higher than via the MAS approach. The experimentally determined parameters ($δ_{\text{iso}}$, $C_{\text{Q}}$ and $η$) are compared with those obtained from DFT-GIPAW (density functional theory - gauge-including projected augmented wave) calculations. When using DFT-optimized structures, the magnitude of the quadrupole coupling constant is overestimated by about 45% for both polymorphs. Further calculations in which the geometry of the AlH4 tetrahedra was varied show a high sensitivity of $C_{\text{Q}}$ on the H--Al--H angles in particular. Modest changes in the angles on the order of one to three degrees are sufficient to achieve near-perfect agreement between GIPAW calculations and experiment. The deviations found for the DFT-optimized structures are explained with the neglect of thermal motion, which typically leads to a reduction of distortions of the AlH4 tetrahedra. From a broader perspective, the uncertainty in the positions of the hydrogen atoms renders the accurate reproduction or prediction of quadrupole coupling constants for aluminium hydrides challenging. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2410_07731 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | $^{27}$Al NMR spectroscopic and DFT computational study of the quadrupole coupling of aluminium in two polymorphs of the complex aluminium hydride CsAlH4 Zibrowius, Bodo Fischer, Michael Materials Science The quadrupole coupling constant $C_{\text{Q}}$ and the asymmetry parameter $η$ of the aluminium nuclei in two polymorphs of the complex aluminium hydride CsAlH4 are determined from both $^{27}$Al MAS NMR spectra and $^{27}$Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The accuracy with which these parameters can be determined from the static spectra (CsAlH4(o): $C_{\text{Q}}=(1.42\pm0.01)$ MHz, $η=(0.62\pm0.01)$ and CsAlH4(t): $C_{\text{Q}}=(1.43\pm0.02)$ MHz, $η<0.03$) seems to be slightly higher than via the MAS approach. The experimentally determined parameters ($δ_{\text{iso}}$, $C_{\text{Q}}$ and $η$) are compared with those obtained from DFT-GIPAW (density functional theory - gauge-including projected augmented wave) calculations. When using DFT-optimized structures, the magnitude of the quadrupole coupling constant is overestimated by about 45% for both polymorphs. Further calculations in which the geometry of the AlH4 tetrahedra was varied show a high sensitivity of $C_{\text{Q}}$ on the H--Al--H angles in particular. Modest changes in the angles on the order of one to three degrees are sufficient to achieve near-perfect agreement between GIPAW calculations and experiment. The deviations found for the DFT-optimized structures are explained with the neglect of thermal motion, which typically leads to a reduction of distortions of the AlH4 tetrahedra. From a broader perspective, the uncertainty in the positions of the hydrogen atoms renders the accurate reproduction or prediction of quadrupole coupling constants for aluminium hydrides challenging. |
| title | $^{27}$Al NMR spectroscopic and DFT computational study of the quadrupole coupling of aluminium in two polymorphs of the complex aluminium hydride CsAlH4 |
| topic | Materials Science |
| url | https://arxiv.org/abs/2410.07731 |