HIGH-TEMPERATURE INTERGROWTH STRUCTURES IN Fe2Oa-TiO2viewed along [012] r. It is seen that it comprises two interpenetrating sets of (111) strings of edge-linked octahedral face-shared dimers, the directions of both strings being different from that in the low-temperature (121)r precursor phase, Fig. 14(a). In transforming_ to the high-temperature phases, the integrity of the [111 ]r strings is lost, but the l l0i], strings of corner-linked face-shared octahedral pairs persist.A high-resolution electron-imaging and diffraction study of the transformation mechanism has been carried out and will be reported elsewhere (Bursill & Grey, 1977). This study shows (121)~ and (i2i) r CS structures intersecting to form (101)~ interfaces containing the [10i]r strings common to both high-and low-temperature structures. These [10i]~ structural features (imaged edge-on in Figs. 3 and 4) provide the key to the structural relations and the reaction mechanism. It is interesting to note that the hightemperature structures may be derived mathematically as follows:
I (020)r= (121)r + (i2i).II (Okl)r=P(020) r + q(011) r.The element (011)r represents the same anti-phase component required for swinging in the lowtemperature structures.
The crystal structure of trifluoromethanesulphonic acid monohydrate has been determined from threedimensional single-crystal X-ray diffraction data recorded on a Stoe-Philips two-circle Weissenberg diffractometer at 298 and 83°K. The crystals are monoclinic, space group P21/c, with four formula units in a unit cell of dimensions a=5.9634 (3), b=9.975 (3), c=9.708 (1).~, fl=98.661 (7) ° at 298°K and a=5"8416 (3), b=9.855 (3), c=9.548 (1)A and fl=99.711 (7) ° at 83°K. The structure comprises oxonium ions and trifluoromethanesulphonate ions hydrogen-bonded together to form layers. The oxonium ion is hydrogen-bonded to three sulphonate groups in a pyramidal bonding arrangement. The O-H...O distances obtained from a full-matrix least-squares refinement are, at 83 °K, 2.546 (3), 2-588 (3) and 2.672 (3) /~. The structure at room temperature is essentially the same as that found at 83 °K, with only those minor changes expected to accompany an increase in temperature.
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