Types unit cell

Type Unit cell Density Melting point, °C T °C... 

Structural type Unit cell dimensions (rounded values) for the reported prototype (pm) Percentage of atoms in the centre of a polyhedron with CN ... 

Figure 7.4b. cI2-W structure type. Unit cell and characteristic projections. Open circles represent atoms on the reference plane, dashed circles atoms on the other parallel planes. 

FIGURE 10.6 The A-type unit cell of the perovskite structure for compounds ABOs, such as CaTiOs. 

Draw a packing diagram (Chapter 1, Section 1.4.5) of the perovskite A-type cell (CaTiOs/ABOs), determine the number of ABO3 formula units, and describe the coordination geometry around each type of atom. Repeat this procedure for the perovskite B-type unit cell. 

Type Unit cell composition Void volume (ml/ml) Pore diameter (A) Si/AI ratio... 

Fig. 35. Hexagonal and trigonal systems. (See also Figs. 24, 27 and 29.) a. Hexagonal- type unit cell. 6. Apatite, 3Ca3(P04)I.CaFB. Class 6/rm c. riydrocinchonine sulphate hydrate, (C19H940N9)9.H2S04. llHaO. Class 6m. d. Rhombohedral-type uriit cell, e. A habit of calcite, CaC()s. Class 3m. /. KBr03. Class 3m.

Perovskite crystallizes in the cubic space group — Pm3m. The Ti4+ ions are located at the corners of the unit cell, a Ca2+ ion at the body center, and O2- ions at the mid-points of the edges this so-called A-type cell is shown in Fig. 10.4.1(a). When the origin of the cubic unit cell is taken at the Ca2+ ion, the Ti4+ ion occupies the body center and the O2- ions are located at the face centers this B-type unit cell is shown in Fig. 10.4.1(b). 

The first-discovered oxide superconductor is La2Cu04 and it has the K2NiF4 structure type, as shown in Fig. 10.4.3(a). This structure can be derived from one B-type unit cell of the perovskite stmcture in combination with two A-type unit cells, each with one layer removed. In this manner, a tetragonal unit cell with c 3a is generated. The resulting structure belongs to space group — I4/mmm, in which the Cu atom has octahedral coordination and the La atom has nine-coordination. The CuC>6 octahedra share vertices and form infinite layers. 

D-3942 Determination of the Unit Cell Dimension of a Faujasite-type Unit cell dimension of synthetic faujasite type zeolite by x-ray diffraction... 

Type Unit-cell compound (typical, fully hydrated) Pore structure Typical Si/Al ratio Void volume/ cm cm Kinetic diameter/ nm... 

There are two main types of ionic oxides which are empirically formulated MOg. Where the metal ion is large (Th, 0.95 A Ce +, 0.92 A U, 0.89 A) the crystals are built up of fluorite-type unit cells with 8 4 co-ordination. But where the metal ion is smaller (Sn, 0.71 A Ti, 0.68 A) the structure is based on the rutile lattice with 6 3 co-ordination. Other examples of this structure are VOg, RuOg, PbOg and TeOg. The rutile lattice is slightly deformed in MoOg and WOg. 

In many systems, 3D Me-S bulk alloy formation is connected with considerable changes of the lattice type, unit cell, and/or interatomic distances producing significant internal mechanical stress. This is illustrated, for example, for the electrochemical formation of the p phase of the Li-Al alloy in the system A1 (polycrystalline)/molten Li, IC, cr at AEfs 260 mV [3.345]. As a result, acoustic emission is generated during the 3D Me-S bulk alloy formation as illustrated in Fig. 3.62 [3.345]. 

Fig. 7 The K2Nip4-type unit cell of La2 "Ni 04 (left) and the superstructure, induced by cation ordering on the octahedral sites, of solids La2 "Lii/2 Mi/2 04 (right hatched polyhedra predominantly, occupied by M " = Ni, Co cations) - adopted from [18]...

Zeolite Type Unit Cell Composition Si/AI Molar Ratio Unit Cell Volume (nm ) Void Volume " (cm /cm ) Pore Openings (A) Supercage Diameter (A) Kinetic Diameter (nm) tc ( C)... 

Table 1 Hydrate structure types, unit cell contents, and cage types...

Structure type Unit cell content Space group Cage Symmetry of cages... 

Fig. 2 The replacement of a square planar center (open circle) hy a tetrahedral (solid circle) one the open square denotes an octahedral center. (Left top) Flat network in the Hofmann-type. (Right top) Twisted network in the Hofmann-Td-type. (Left middle) A rectangular box cavity in the Hofmann-type. (Right middle) A twisted biprismatic cavity in the Hofmann-Td-type. (Bottom) A couple of Hofmann-type unit cells (left) makes a unit cell (right) of the Hofmann-Td-type.

The internal stracture of the unit cell also plays a role in the definition of the lattice type. Unit cells containing a... 

Unit cell type Unit cell dimensions (nm) ... 

Here the narrow prescription of Chapter 1 is widened to deal with more chemically complex phases, in which the materials may contain mixtures of A, B and X ions as well as chemical defects. In these cases, using an ionic model, it is only necessary that the nominal charges balance to obtain a viable perovskite composition. In many instances these ions are distributed at random over the available sites, but for some simple ratios they can order to form phases with double or triple perovskite-type unit cells. The distribution and valence of these ordered or partly ordered cations and anions are often not totally apparent from difEraction studies, and they are often clarified by use of the bond valence sums derived from experimentally determined bond distances. Information on the bond valence method is given in Appendix A for readers unfamiliar with it Point defects also become significant in these materials. The standard Kroger- fink notation, used for labelling these defects, is outlined in Appendix B. 

NdFeSi was claimed to be single-phase with an orthorhombic TiNiSi-type of structure. The lattice parameters (a = 11.18, b = 6.89 and c = 5.32), however, do not correspond to a TiNiSi-type unit cell. The existence of a compound NdFeSi was confirmed by Bodak et al. (1970) but at variance with Mayer and Felner (1973) a PbFCl-type of structure [P4/nmm, a = 4.057(3), c = 6.893(5)] was obtained from arc-melted alloys heat treated at 800 ° C for 3 months (low-temperature phase ). Due to the high temperature of preparation and homogenization the TiNiSi-type phase as reported by Mayer and Felner (1973) is likely to represent a high-temperature modification. 

Fluorite-type unit cell (Reproduced from Ref. 64.)... 

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