Fig. 1. a) A section of single ditrigonal ring formed by the TO4 polyhedra perpendicular to the z* direction (at 100°C). The interlayer potassium/sodium (not shown for clarity) sits at similar x, y position coordinate as that of the hydroxyl group (O(3)-H(1)) in the figure. The octahedral cations are omitted for clarity. Apical oxygen atoms, labelled O(1) and O(2) are part of T(1) and T(2) respectively and those labelled O(4), O(5) and O(6) are basal oxygen atoms. O(3) is the part of the hydroxyl group and is roughly at the center of the ring and at the same z coordinate as that of the O(1) and O(2) atoms. b) Two ditrigonal rings viewed perpendicular to (a). The major axis of the hydrogen thermal ellipsoid is at high angles to the hydroxyl bond. Structures are from our refinements of neutron data collected at 100°C.
Fig. 2. Comparison of measured (crosses) and calculated (solid line) diffraction patterns (from Rietveld refinements of the crystal structure) of phengite (at 100°C). The lower curve shows the differences, and the tick marks show the peak positions.
Fig. 4. Variation of a, b, c, volume and ß with temperature. The two data points for each of the lattice parameters, at 300, 400 and 500°C corresponds to the heating and cooling respectively. Note: the error bars corresponding to a, b, c, and volume are smaller than the size of data points.
Fig. 5. a.Variation of ditrigonal distortion, with temperature. With no distortion, O(4)-O(5)-O(6) = 120°C and hence is zero. The open circle represents the data collected while heating whereas the open square represents the data collected while cooling. b. Variation of , the angle that O(3)-H vector makes with the (001) plane, with temperature. The open circle represents the data collected while heating whereas the open squre represents the data collected while cooling. c. Variation of the average O(3)-H bond length with temperature as obtained from the neutron diffraction data. d. Variation of the H...O(2) (possible) hydrogen bond length with temperature.
Fig. 6. Variation of the hydroxyl stretching frequency with temperature. The open circle represents the data collected while heating whereas the open square represents the data collected while cooling.
Fig. 7. Variation of the equivalent Uiso of the hydrogen atom with temperature, from the refined anisotropic temperature factor. The open circle represents the data collected while heating whereas the open square represents the data collected while cooling.
Fig. 8. ORTEP generated figures for the thermal ellipsoid of OH group at 20°C and 650°C. The ellipsoid shows the 50 % probability distribution of the scattering density, given by the anisotropic displacement parameters, and demonstrates the increase in librational amplitude of the hydroxyl group with the temperature.
is zero. The open circle represents the data collected while heating whereas the open square represents the data collected while cooling. b. Variation of 
, the angle that O(3)-H vector makes with the (001) plane, with temperature. The open circle represents the data collected while heating whereas the open squre represents the data collected while cooling. c. Variation of the average O(3)-H bond length with temperature as obtained from the neutron diffraction data. d. Variation of the H...O(2) (possible) hydrogen bond length with temperature.
