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.
HNOa.H20, orthorhombic, P2xen (nonstandard setting of Pna2x, No. 33), Z= 4, a = 5.4647 (1), b=8.6439 (1), c=6.2308 (1) A, V=294.32 A a, Dx= 1-829 g cm -a at 85 K and a = 5.4759 (2), b = 8.7242 (2), c=6-3275 (2) A, V=302.28 A 3, Dx= 1.779 g cm -3 at 225K, Dm=l'816 g cm -a at 78K [Biltz, Hfilsmann, & Eickholz. Nachr. Ges. Wiss. Gb'ttingen (1935). No.3, 1, 95-102]. The crystal structure at both 85 and 225K consists of H30 + and NO~" ions each of which has pseudo-threefold symmetry. The oxonium ion is hydrogen bonded to three different nitrate ions to form infinite layers.
The crystal structure of H2804. H20 has been determined from three-dimensional single-crystal X-ray data obtained at -135°C. The crystals are monoclinic, space group P21/e, with four formula units in a cell with the dimensions: a=7.062, b=6"948, c--8.139/~,, and p= 106.22 °. The structure can be considered as consisting of H30 + and HSO4 ions. The HSO~-ions are linked by hydrogen bonds (2.657/~) to form infinite chains. These chains are coupled together by hydrogen bonds from the H30 + ions (2-538, 2.566, and 2.649/~) in such a way that infinite 'double-layers' are formed. The S-O distances within the HSO~" ion are 1.434, 1.449, 1.462, and 1.560/~.
The crystal structure of H2SO4·2H2O has been determined from three-dimensional single-crystal x-ray data recorded at −190°C. The crystals are monoclinic, space group C2 / c, with 12 formula units in a cell with the dimensions: a = 13.008, b = 7.979, c = 14.881, and β = 101.60°. The structure consists of H3O+ and SO42− ions. Each of the three independent H3O+ ions is hydrogen bonded to three different SO42− ions (0···0 distances, 2.520–2.590 Å) is such a way that a three-dimensional hydrogen-bond pattern is formed. The average S–O distance in the sulfate ion is 1.474 Å.
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