High pressure infrared and X-ray Raman studies of aluminum nitride



We performed the first reported static high pressure studies of the wide bandgap material aluminum nitride (AlN) using mid-infrared (IR) and X-ray Raman spectroscopy (XRS) up to 35 and 33 GPa, respectively, in a diamond anvil cell (DAC) at ambient temperature. For the first (IR) experiment, we employed a synchrotron IR source. Below the wurtzite (WZ) → rock salt (RS) phase transition, the IR spectra shift monotonically toward higher energy with pressure. Above this phase transition, the spectral multiplet stabilizes and then shifts toward lower energies suggesting a weakening of the bonding with pressure. To better examine the bonding changes we utilized the 16 ID-D undulator beamline at the Advanced Photon Source (APS) for the second experiment. The spectrometer collected photons with ∼410 eV energy loss (nitrogen edge) with respect to incident beam energy near 10 keV. The sample commenced in the WZ phase and upon pressurization above ∼15 GPa, the sample converted into the high pressure RS form as evidenced by visual darkening of the sample and a marked change in the XRS pattern suggesting fundamental changes in intramolecular bonding. Upon pressure release, the sample remained in the high pressure RS phase and partially reverted to the low pressure WZ phase [confirmed with X-ray diffraction (XRD)]. Theoretical calculations qualitatively agree with experimental observations.