Thermal response of structure and hydroxyl ion of phengite-2M(1): an in situ neutron diffraction and FTIR study

The thermal dependence of the structure of natural phengite-2M(1) with chem ical formula (K0.95Na0.05)(Al0.76Fe0.14Mg0.10)(2)(Si3.25Al0.75)O-10(OH1.96F 0.04) has been studied by in situ neutron diffraction. The short-range corr elated behaviour of the hydroxyl group was probed by FTIR spectroscopy whil e the long-range correlated hydroxyl structure was studied by neutron diffr action. Changes in long-range ordering of Si and Al on the tetrahedral site s were not observed from neutron diffraction. Structure refinement of the n eutron diffraction data by the Rietveld method suggested that the apparent average hydroxyl bond length decreases on heating. The infrared data show a decrease in the stretching frequency of hydroxyl group, however. Possible explanations for these results are explored. It seems most likely that the apparent shortening of the hydroxyl bond length may be an artefact due to a n increase in its vibrational amplitude. The anisotropic vibration of the h ydroxyl bond as revealed by the anisotropic displacement parameters of H, i ncreases so much that the average length (shown by the neutron refinement) appears to decrease at high temperatures while the local length of the bond , as indicated by FTIR results, increases.