Site occupation and electric field gradient in acentric neptunite-measurements and evaluations concerning powder- and single crystal - Mössbauer spectroscopy and x-ray diffractometry

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