High-Pressure Electronic Structures of Boron Nitride Using Synchrotron Inelastic X-Ray Scattering

Yue Meng¹, Ho-kwang Mao², Chichang Kao³, Michael Hu¹, Russell J. Hemley², Peter Eng⁴, Tom Trainor⁴, and Matt Newville⁴

¹HPCAT & Carnegie Institution of Washington, APS Bldg. 434E, Argonne National Laboratory, 9700 S. Cass Av., Argonne, IL 60439
²Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW Washington DC 20015
³NSLS, Brookhaven National Laboratory, Upton, NY 11973
⁴GSECARS, Argonne National Laboratory, Argonne, IL 60439

X-ray Raman spectroscopy (XRS) is the result of core-level electron transition to unoccupied valence states induced by hard x-ray photon. Hence XRS yields detailed information about electronic structure and bonding in a material, comparable to those provided by core-level excitation using soft X-ray absorption (XAS) and electron energy-loss spectroscopy (EELS) techniques. However, unlike XAS and EELS, XRS is free of problems related to surface contamination and does not require vacuum condition; therefore, it is a unique and powerful probe in studying materials under high-pressure conditions. Such a technique was not possible for research under very high pressures until recently as a result of enhanced brightness of synchrotron sources, new development of micro-focusing optics and improved diamond anvil cell methods.
Here we present the first measurements of synchrotron x-ray Raman spectra of both B and N 1s core electrons of the phases of boron nitride (BN-graphite and BN-wurtzite) under high pressures up to 14 GPa. At high pressure, g-BN crystals develop strong preferred orientation with c-axis aligned parallel to the DAC compression direction. The preferred orientation actually allow us to distinguish the transitions from 1s core to s* state and to p* state of the g-BN phase by measurements with DAC axis parallel and perpendicular to the direction of the momentum transfer q. Pressure-induced structural transformation from the g-BN phase to the w-BN phase is detected at 14 GPa from the appearance of new s* states starting at 198 eV in the B(1s) spectra and at 410 eV in the N(1s) spectra. The instability of the g-BN phase at high pressure is manifested by the reduced intensities of both s* and p* states. The measurements on the w-BN phase suggest that the bonding in the w-BN structure is, at least, nearly isotropic and is similar to that of the c-BN structure. Our study illustrates the capability of this new experimental technique in the study of high-pressure phase transformation and related electronic properties.