A quasi-atomic treatment of chemical and structural effects on K-shell excitations in hexagonal and cubic BN crystals

Experimental and theoretical investigations of the K-shell X-ray absorption near edge structure (XANES) in hexagonal and cubic boron nitride crystals were carried out. A central role is revealed of: (i) the atomic B and N ls(-1)2p resonances in the formation of the main resonance features, (2) the splitting of these atomic excitations by an anisotropic surroundings potential into single pi and double degenerated sigma components in hexagonal relative to cubic BN crystal in which they remain triple degenerated, and (3) the localization region size determining the essential difference between the B and N K-shell excitations, The quasi-atomic approach to inner-shell photoprocesses is applied to compute the XANES. The applicability of molecular models for simulation of the core excitations in the crystals is discussed. It is shown that the B K-excitations are essentially localized within the nearest neighbors whereas the N K ones are more delocalized. The experimental measurements with high energy resolution evidence that the full width at half maximum for the low-lying excitation in h-BN is approximate to 0.37 eV and in its vicinity a near-pi-resonance structure containing a shoulder at 192.3 eV and a well-resolved narrow peak at 193.1 eV appears. Its origin is rationalized taking into account the local atomic rearrangement allowing the out-of-plane displacements of excited-core B atoms from their regular positions (polaron mechanism).