Infrared spectra in amorphous alumina: A combined <i>ab initio</i> and experimental study

We present a combined study based on experimental measurements of infrared (IR) dielectric function and first-principles calculations IR spectra vibrational density states (VDOS) amorphous alumina (am-Al$_2$O$_3$). In particular, we show that the main features imaginary part $\epsilon_2(\omega)$ at $\sim$380 630 cm$^{-1}$ are related to motions threefold coordinated oxygen atoms, which vast majority atoms in am-Al$_2$O$_3$. Our analysis provides an alternative point view with respect earlier suggested assignment modes, relates them stretching bending modes AlO$_{n}$ ($n=$ 4, 5, 6) polyhedra. is additive decomposition VDOS spectra, shows that: (i) band occurring direction normal plane defined by three nearest-neighbor aluminum i.e. out-of-plane atoms; (ii) Al--O vibrations (i.e. in-plane atoms) appear frequencies above $\sim$500 cm$^{-1}$, characterize underlying $\sim$630 cm$^{-1}$. Aluminum fourfold contribute uniformly frequency region $\sim$350--650 without causing specific features. numerical results good agreement previous presently obtained data am-Al$_2$O$_3$ films. Finally, spectrum can be modeled assuming isotropic Born charges for while requiring use parameters, local reference frame, anisotropic atoms.