The electric field gradient in natural iron-doped chrysoberyl Al 2 BeO 4 and sinhalite MgAlBO 4 single crystals

This work reports on the evaluation of the electric field gradient (EFG) in natural chrysoberyl Al 2 BeO 4 and sinhalite MgAlBO 4 using two different procedures: (1) experimental, with single crystal Mössbauer spectroscopy (SCMBS) on the three principal sections of each sample and (2) a “fully quantitative” method with cluster molecular orbital calculations based on the density functional theory. Whereas the experimental and theoretical results for the EFG tensor are in quantitative agreement, the calculated isomer shifts and optical d–d-transitions exhibit systematic deviations from the measured values. These deviations indicate that the substitution of Al and Mg with iron should be accompanied by considerable local expansion of the coordination octahedra.