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 absor ption near edge structure (XANES) in hexagonal and cubic boron nitride crystals were carried out. A central role is revealed of: (i) the ato mic B and N ls(-1)2p resonances in the formation of the main resonance features, (2) the splitting of these atomic excitations by an anisotr opic surroundings potential into single pi and double degenerated sigm a components in hexagonal relative to cubic BN crystal in which they r emain triple degenerated, and (3) the localization region size determi ning 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 simulati on of the core excitations in the crystals is discussed. It is shown t hat the B K-excitations are essentially localized within the nearest n eighbors whereas the N K ones are more delocalized. The experimental m easurements with high energy resolution evidence that the full width a t 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 appe ars. Its origin is rationalized taking into account the local atomic r earrangement allowing the out-of-plane displacements of excited-core B atoms from their regular positions (polaron mechanism).