SITE OCCUPATION AND ELECTRIC-FIELD GRADIENT IN ACENTRIC NEPTUNITE-MEASUREMENTS AND EVALUATIONS CONCERNING POWDER-CRYSTAL AND SINGLE-CRYSTAL- MOSSBAUER-SPECTROSCOPY AND X-RAY-DIFFRACTOMETRY

Mossbauer measurements on neptunite (KNa2Li(Fe,Mn,Mg)(2)Ti2Si8O24) at 400 K reveal the distribution of Fe-ions on the crystallographic sites in agreement with neutron diffraction results published elsewhere. Ev en the previously postulated small amount of Fe-ions on the Ti(2) site has been detected, combined with a charge transfer which is in agreem ent with optical absorption investigations by other authors. A qualita tive site occupation model is able to explain the different features o f our observations. Single crystal Mossbauer measurements with the k-v ector of incident gamma-rays parallel to the crystallographic b-axis ( space group Cc) of neptunite at different temperatures yield the angle beta between the main component of the electric field gradient (EFG), V-zz and b. This angle is in close accordance with a calculated value of beta for the Fe(1) position from electron density maps. The latter also reveal an absolute value of V-zz which is in satisfactory agreem ent with V-zz derived from Mossbauer spectroscopy.