Electronic structure of aluminium trihydride studied using soft x-ray emission and absorption spectroscopy

We have performed soft x-ray emission spectroscopy (SXES) and soft x-ray absorption spectroscopy (SXAS) experiments on aluminum hydride $\alpha$-AlH${}_3$. The occupied and unoccupied electronic states of the Al 3$p$ partial density of states are obtained experimentally. By comparing the data from Al metal and $\alpha$-AlH${}_3$, a band gap with a few eV is found for $\alpha$-AlH${}_3$. In addition, the occupied states of $\alpha$-AlH${}_3$ have a larger spectral intensity than that of Al metal, indicating an increase in the number of electrons with the Al 3$p$ character through Al-H bond formations. The results of a band-structure calculation account for the formation of the energy gap and the increase of the Al 3$p$ electrons qualitatively. This suggests that a covalent-like nature is important to the Al-H bond in $\alpha$-AlH${}_3$.

Figure 1

The SXES spectra of $\alpha$-AlH${}_3$ and Al metal excited at $h\nu$ $=$ 1650 eV. These spectra are normalized by the intensity of the $K_{\alpha }$ characteristic x ray (2$p$ $\to$ 1$s$) at 1486.6 eV. The $K_{\beta }$ emission (3$p$ $\to$ 1$s$), which reflects the Al 3$p$ occupied states is observed around 1550 eV.

Figure 2

Al 3$p$ PDOS of occupied (SXES) and unoccupied (SXAS) states for $\alpha$-AlH${}_3$ (red) and Al metal (blue). The closed circles and solid lines show SXES and SXAS spectra, respectively. The normalization method between these SXES and SXAS spectra is described in the text.

Figure 3

Calculated DOS within LDA. (a) Total DOS, (b) H PDOS, and (c) Al PDOS in $\alpha$-AlH${}_3$ are projected. The energy axis is scaled as the relative energy to valence-band maximum (VBM). (d) Total DOS and (e) Al PDOS in Al metal are also projected.