M1-site occupancy in 3T and 2M1 phengites by low temperature neutron powder diffraction: reality or artefact?

Abstract

Two low temperature (15 K) neutron powder diffraction data sets from 2 M 1 and 3 T phengite samples were collected on the D2B beamline at ILL (Grenoble, France), to investigate the cation ordering and the occurrence of fractional occupants at the M 1-site. Low temperature has been used in order to reduce the thermal motion. The complexity of this study poses problems to property quantify our results, but on the basis of the issues from several refinement schemes we obtained indications in favour of occupancy factors other than zero for the formally empty in dioctahedral micas M 1-sites (roughly ≈ 0.12 and ≈ 0.18 atom per M 1-site, in case of 2M 1 and 3 T polytype, respectively). The atomic displacement parameters, and the large uncertainties upon the M 1-O bond lengths, suggest the large M 1 site to be positionally disordered in both polytypes. These results are to be taken up with due care as stacking faults resulting in staggering of the TOT layers might mimic a fictitious M 1-O site occupancy.