KAMB-B 65-0860 Structure of Ice VI Barclay Kamb Repriated frent Science. Oct.her 8, IH.'). Vel. 150, Xe. 3693, ...." .i-", powder data, to search the capillaries for crystals suitable for single-crystal study, and to orient and collect com­ plete diffraction data from such crys­ tals. Ice VI, prepared and examined in this way, gives a powder pattern (Ta­ ble 2) in general agreement with data obtained at high pressure and also with data from other experiments on quenched samples (6). Single crystals are easily obtained and are found to be tetragonal with unit cell dimensions a = 6.27 ± 0.01 A, c = 5.79 ± 0.01 Structure of Ice VI A. The diffraction patterns show extinc­ tions for reflections of type hkO with Abstract. Ice VI, a high-pressure form of density 1.31 g cm-J , has a tetrag­ h + k odd and for hhl with I odd. The onal cell of dimensions a = 6.27 A, c = 5.79 A, space group P4jnmc, each space group is thus uniquely determined cell colltaining ten water lIlolecules. The structure' is built up of hydrogen-bonded as P4~ / nmc (7) . The foregoing cell c//{lins of water molecules that are analogs of the tectosilicate chains out of which accounts well for the observed powder the fibrous zeolites are constructed. The chains in ice VI are linked laterally' to spacings (Table 2). one another to form an 0 en; zeolite-like framework. The cavities in this r ne­ Recently single crystal data for ice war ' are filled with a second fralllewor I enflca Wit 1 t 7e rst. The two frame­ VI, obtained at high pressure by means l . pene ra e £It 0 not interconnect and the complete structure call thus of a pressure cell mounted on a pre­ be con.fldered a "sell-clathrate." 1 h~ctural feature is a natural way to achieve cession camera, have been published high density in tetrahedrally linked framework structures. by Block, Weir, and Piermarini (8). These authors report an orthorhombic The stability conditions of the quenched by immersing the cylinder in cell with dimensions a' = 8.38 A, b' known forms of ice (Fig. 1) are a re­ liquid nitrogen, the pressure is released, = 6.17 A, and c' = 8.90 A, and for flection of the thermodynamic relations and the sample is forced out of the this cell they report diffraction extinc­ among the various phases, and these cylinder and into a bath . of liquid tions for reflections of type h' k'O with relations are in turn a macroscopic re­ nitrogen. At the temperature of liquid h' odd (diffraction aspect p •• a). This flection of the molecular architecture nitrogen (-196°C), high-pressure ice identification of the unit .cell is errone­ of the individual forms. Conversely, samples are metastable and can be OLIS. The cell chosen by Block el al. the structural features of the indi­ kept indefinitely at atmospheric pres­ (8) is related to the true tetragonal vidual forms in relation to their stabil­ sure. When the capillaries are removed cell as follows. The b' axis is one of ity fields and to the thermodynamic into the open aii', they explode, because the tetragonal a-axes, and the a' axis variahles provide some insight into the of the volume increase upon inversion is the tetragonal cell diagonal [10 I], forces of interaction between water of the high-pressure ice to ordinary that is, a' = a + c. The c' axis chosen molecules, and in particular into the ice J. by Block et al. (8) should evidently disruption of these forces that is caused Individual capillaries were mounted correspond to the other cell diagonal by packing the molecules more closely for x-ray study in a precession camera, [lOT]. The angle between the a' and c' together, as is achieved under high where they were maintained at a tem­ axes is 94.5°, according to the dimen­ pressure. Some structural information perature of -175°C by a stream of sions of the tetragonal cell, rather than for most of the forms of ice is now cooled nitrogen gas. The precession 90° as apparently assumed by Block availahle (Table 1), and the detailed camera can he used to obtain x-ray et al. (8). structures of ice T. Ie, II, I If, and VTJ are known (1-5). I here describe the structure . of ice VI, determined by x­ Table I. X-ray structural data for ice polymorphs. ray diffraction, and give preliminary x-ray .structural information for ice V Density" Crystal Space Cell dimen- Ice Zt Reference (Table 1). (g cm"" ) syste{J1 group sions (A)" The samples were prepared as fol­ lows. Short glass capillaries, filled with I 0.92 Hexagonal P6,, / nZlllc a 4.48, c 7.31 4 (I) l. Ie 0.92 Cubic Fd3m a 6.35 8 (2) water, were placed in a hollow steel II 1.17 Rhombo- R3 a 7.78, Ot Il3.to 12 (3) cylinder fitted with pistons at both hedral ends. The chamber thus formed was III 1.14 Tetragonal P4,212 a 6.79, C 6.79 12 (4) filled with gasoline, to transmit pres­ IV 1.28 (14) sure from the pistons to the water sam­ V 1.23 Monoclinic A2/ a a 9.22, b 7.54 28 ~ C 1035, {J 109.2· ples. By loadirig the pistons and control­ VI 1.31 Tetragonal P4"/ llmc a 6.27. e 5.79 10 :j: ling the cylinder temperature appropri­ VII 1.50 Cubic . PI/3m a 3.41 2 (5) ately (Fig. 1), the desired i~e phase • At atmospheric pressure and -175°C. i Number of molecules in the unit cell. :j: Data given can be formed. The sample is then in accompany ing text. Fig. I. Phase diagram for water. The solid phases (ice polymorphs) are identi­ fied by the roman numeral designations assigned by Bridgman (14, 9, 15), upon 20 whose data the diagram is based. The field of ice IV in relation to ice VI and liquid is plotted by dotted lines within the ice V field, and is plotted by analogy to 0,0 ice, for which the field was ac­ tually measured (14); ice IV is everywhere unstable relative to ice V. Ice Ic ("cubic ; ice") is shown sc hematically below the tem­ If) 15f- 0 L perature of about _105 C, below which 0 -<> it forms by vapor condensat.ion (16) and 0 above which it inverts to ice I (17) : it is Y: not known to have afield of actual sta­ <ll bility. L ::J If) If) <ll L 1-0 0.. Proof of the correct unit cell is es­ sential to determination of the struc­ ture; hence in Fig. 2 there is shown a precession photograph of the hkO plane of ice VI. The photograph was made i without use of a layer-line screen and 5:- hence shows also the symmetry of the II Li quid upper layers parallel to hkO. It is thus analogous to a Laue photograph in ex­ hibiting the diffra~tion symmetry of Ie I the crystal ' about its c-axis. The 4 -fold oL------~OL-O-----------5~O----------~O-----------5~O----------~100 axis and the mirror planes are clearly visible. There are no 4-fold axes paral­ Temperature (OC) lel to the plane of the photograph . Their absence cannot be judged from the photograph itself, but is determined by manipulating the original crystals. It is unlikely that the crystals studied by Block el al. (8) at high pressure were essentially different from the quenched samplcs of ice VI described here. The unit cell chosen by these au­ thors is compatible with essentially the same powder pattern as is given by the quenched samples (Table :2), and the dimensions of the "orthorhombic" and tetragonal cells are compatible, taking into accou nt the differing cell orienta­ tions and the different experimental conditions and possible sources of ex­ perimental error. The lI'k'O photo­ graph published by Block el at (Fig. 1 of Ref. 8) appears identical, to the ex­ tent that the diffraction pattern can be seen in the photograph, with precession photographs of the hkh plane of the tetragonal crystals. The two crystals (icc vr under pressure, and quenched) can therefore differ at most by a slight distortion. It is unlikely that a low- Fig. 2. Precession photograph about the c-axis of ice VI, made with unfiltered molybuen'um radiation and without a 0 layer-line screen, /L = 15 • The II kO plane shows as single spots in the central por­ tion of the photograph. ilk I and higher layers as double spots and hyperbolic streaks in the oute r portion. Note 4-folu symmetry about the c-axis. 2 temperature (quenched) form would tail. The parameter values x and z Table 2. X-ray powder data for ice VI. be more symmetrical (tetragonal) than given must be accurate (probably to hkl the high-temperature form (supposedly ± 0.002), since the "fractional dis­ orthorhombic) from which it inverted crepancy" ~ I ~F ! /~IF()I is only 0.07, 4.44 4.47 110 by a displacive transfor~ation upon which is about as low as is commonly 81z 4.3 4.27 4.26 101 81z 3.6 cooling. achieved for refined structures based 51t 3.4 The density of ice YI at 6175 at­ on photographically measured x-ray 13 3.12 3.14 3.14 200 m~ospheres and 0 2°e (the triple point intensities. 8 2.91 2.92 2.90 002 3 90 2.75 2.76 2.76 201 ice V-ice VI-Ii ui cm- Positions of the hydrogen atoms 50 2.63 2.64 2.63 102 .