Unit cell rock salt

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

FIGURE 5.39 rhe arrangement of ions in the rock-salt structure, (a) The unit cell, showing the (tacking of the individual ions, and (b) a representation of the same structure by dots that identify the centers of the ions. 

Calculate the number of cations, anions, and formula units per unit cell in each of the following solids (a) the rock-salt unit cell shown in Fig. 5.39 (h) the fluorite (CaF2) unit cell shown here, (c) What are the coordination numbers of the ions in fluorite ... 

The sodium chloride structure, AX systems. Cubic Fm3m (Space Group 225) The sodium chloride or rock salt, NaCl, structure has a simple face-centered cubic unit cell (Figure 8) with alternating cations-anions along the three cubic axes. 

The description of this structure is more complicated than that of Ba2YCu3Ox. There are six layers in the unit cell of this structural type and they can be viewed in two quite different ways. In the first interpretation, we divide the six layers into two blocks of three layers each, the first being (AX)0(BX2)C(AX)0 and the second (AX)C(BX2)0 (AX)C. These layers and these sequences are typical of perovskite and, therefore, in this description the structure is considered to be made of two perovskite blocks related to one another by a shift of origin of t = (l/2)(a + b). We may also regard the structure, however, as containing alternate blocks of perovskite (layers (BX2)0 c) and rock salt (layers (AX)co(AX)oc). As before, the unit cell is made of two... 

Figure 2 Tetragonal and orthorhombic unit cells of La2 3tMxCu04 (e.g., M = Sr or Na) and I CuOj, respectively. Metal atoms are shaded and Cu-O bonds are shown. The structure may be viewed as an intergrowth of perovskite and rock salt blocks.

The simple cubic structme, sometimes called the rock salt structure because it is the structme of rock salt (NaCl), is not a close-packed structure (see Figure 1.20). In fact, it contains about 48% void space and as a result, it is not a very dense structure. The large space in the center of the SC structme is called an interstitial site, which is a vacant position between atoms that can be occupied by a small impurity atom or alloying element. In this case, the interstitial site is surrounded by eight atoms. All eight atoms in SC me equivalent and me located at the intersection of eight adjacent unit cells, so that there me 8 x (1/8) = 1 total atoms in the SC unit cell. Notice that... 

There are four formula units of FeO in a perfect unit cell with the rock salt structure. The mass of these four units can be calculated from their relative atomic masses Fe, 55.85... 

Not all ionic compounds crystallize in the same way as rock salt. Figure 10 shows the lattice of CsGl in which each positive ion is not surrounded by six, but by eight Cl- ions. The smallest unit, or unit cell of the crystal, is a cube in which the Cs+ ions occupy the corners,... 

Figure 9-22. The crystal structure of the rock salt after Shubnikov and Koptsik [32], (a) A unit cell (b) Projection of the structure along the edges of the unit cell onto a horizontal plane (c) Projection of some symmetry elements of the Fm3m space group onto the same plane. The vertical screw axes 2 and 42 are marked by their respective symbols. Used with permission.

Figure 3.10. The cubic rock-salt, or sodium chloride, unit cell consists of two interlocking FCC sublattices, one of sodium cations and one of chloride anions, displaced by (1 /2 a) along (1 00).

Figure 3 Unit cell of the rock salt structure adopted by MO for the group 2 elements (M Be), depicting edge-sharing of octahedra...

Figure 3 Unit cells for (a) Rock salt, (b) Wurtzite, (c) Fluorite (d) Rutile...

Figure 21 Two complete hysteresis cycles for 4.5 mn CdSe NQDs presented as unit cell voliunes for the wurtzite sixfold-coordinated phase (triangles) and the rock-salt fourfold-coordinated phase (squares) versus pressure. Sohd arrows indicate the direction of pressure change, and dotted boxes indicate the mixed-phase regions. Unlike hulk phase transitions, the wurtzite to rock-salt transformation in nanocrystals is reversible and occurs without the formation of new high-energy defects, as indicated by overlapping hysteresis loops. The shape change that a sliding-plane transformation mechanism (see text) would induce is shown schematically on the right. (Reprinted figure with permission from J.N. Wickham, A.B. Herhold, and A.P. Alivisatos, Phys. Rev. Lett., 2000, 84, 923. 2000 by the American Physical Society)...

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