Inorganic Crystals Oxides

KNbOi (LB Number 1A-2). This crystal is ferroelectric below about 418 °C. Further phase transitions take place at about 225 °C and about — 10°C, retaining ferroelectric activity. The crystal has large electromechanical coupling constants and is useful in lead-free piezoelectric elements and SAW (surface acoustic wave) filters in communications technology (Fig. 4.5-13,4.5-14). 

KTaO (LB Number 1A-5). KTa03 is cubic at aU temperatures, and its dielectric constant becomes very large at low temperatures without a phase transition (Fig. 4.5-14). It is generally believed that this behavior is related to the zero-point lattice vibrations. Replacement of Nb by Ta generally lowers drastically the ferroelectric Curie temperature, as seen by comparing Fig. 4.5-14 with Fig. 4.5-12. (This effect is well demonstrated later in Figs. 4.5-39 and 4.5-40). 

SrTiOj (LB Number 1A-8). This crystal is cubic at room temperature and slightly tetragonal below 105 K. The phase transition at 105 K is caused by softening of the lattice vibration mode at the (1/2, 1/2, 1/2) Bril- 

BaTIOj (LB Number IA-IO). BaTiOs is the most extensively studied ferroelectric crystal. It is ferroelectric below about 123 C, where the crystal symmetry changes from cubic to tetragonal. Further phase transitions take place from tetragonal to orthorhombic at about 5 and to rhombohedral at about —90 °C (Figs. 4.5-18 to 4.5-20). It is believed that the ferroelectric transition 

When the temperature is raised above 1460 °C, cubic BaTi03 performs another phase transition to a hexagonal structure. This hexagonal phase can be quenched 

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Family Nr. Inorganic Crystals Oxides [5.1,2] Name Family Nr. Inoi anic Crystals other than Oxides [5.3] Name Orgi Family Nr. uiic Crystals, Liquid Crystals, and Polymers [5.4] Name... 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

Inorganic non-oxide materials, such as III-V and II-VI group semiconductors, carbides, nitrides, borides, phosphides and silicides, are traditionally prepared by solid state reactions or gas-phase reaction at high temperatures. Some non-oxides have been prepared via liquid-phase precipitation or pyrolysis of organometallic precursors. However, amorphous phases are sometimes formed by these methods. Post-treatment at a high temperature is needed for crystallization. The products obtained by these processes are commonly beyond the manometer scale. Exploration of low temperature technique for preparing non-oxide nanomaterials with controlled shapes and sizes is very important in materials science. 

The immense number of chemical compounds formed by the halogens provides chemists with an extraordinary database from which numerous chemical and physical phenomena can be correlated with respect to various periodic trends. From databases like Inorganic Crystal Structure Data (ICSD, http //www.fiz-karlsruhe.de ) and International Centre for Diffraction Data (ICDD, http //www.icdd.com) with 67 000 and 25 000 entries, respectively, one can easily make out that halides are one of the dominant classes of compounds besides oxides. Even within the subset of inorganic solids, there is tremendous diversity of composition, stracture, and properties and to summarize this would create its own encyclopedia. Therefore, the discussion in this article is limited primarily to binary halides, their structures, and some of their properties, except halides of elements which are nonmetals. Binary actinide hahdes are discnssed elsewhere see Actinides Inorganic Coordination Chemistry). Complex hahdes (sohd phases containing two or more kinds of metal ions), ... 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

Two points should be emphasized. First, according to classical structure theory, all the equivalent positions of a given set should be occupied and moreover they should all be occupied by atoms of the same kind. In later chapters we shall note examples of crystals in which one or both of these criteria are not satisfied an obvious case is a solid solution in which atoms of different elements occupy at random one or more sets of equivalent positions. (The occupation of different sets of equivalent positions by atoms of the same kind occurs frequently and may lead to quite different environments of chemically similar atoms. Examples include the numerous crystals in which there is both tetrahedral and octahedral coordination of atoms of the same element—in the same oxidation state—as noted in Chapter 5, and crystals in which there is both coplanar and tetrahedral coordination of Cu(ii), p. 890, or Ni(ii), p. 965.) The second point for emphasis is if a molecule (or complex ion) is situated at one of the special positions it should possess the point symmetry of that position. A molecule lying on a plane of symmetry must itself possess a plane of symmetry, and one having its centre at the intersection of two planes of symmetry must itself possess two perpendicular planes of symmetry. If, therefore, it can be demonstrated that a molecule lies at such a position as, for example, would be the case if the unit cell of Fig. 2.13 contained only one molecule, (a fact deducible from the density of the crystal) this would constitute a proof of the symmetry of the molecule. Such a conclusion is not, of course, valid if there is any question of random orientation or free rotation of the molecules. Moreover, there is another reason for caution in applying this type of argument to inorganic crystals. 

Finally, the selective growth of inorganic crystals was demonstrated on a SAM-templated silver surface. Hsu and coworkers [58] preferentially grew zinc oxide (ZnO) from an aqueous solution on silver surfaces. The silver surface was patterned by 4CP with a carboxyl acid (-COOH)-terminated SAM to inhibit ZnO growth. Figure 5.5.9(b) and (c) contain micrographs that demonstrate ZnO growth only... 

The Inorganic Crystal Structure Database (ICSD) [7] contains information on all compounds containing at least one nonmetallic element but no C-C or C-H bonds (because these are covered by the CSD). Each reported crystal structure has a separate entry. Information provided in the database includes the chemical name, phase designation, unit cell dimensions, density, space group, and the oxidation state of the elements. Atomic information includes three-dimensional coordinates. Also listed are the R value, temperature, pressure, method of measurement and the full journal reference. 

This book serves as an introduction to advanced inorganic fibers and aims to support fundamental research, assist applied scientists and designers in industry, and facilitate materials science instruction in universities and colleges. Its three main sections deal with fibers which are derived from the vapor phase such as single crystal silicon whiskers or carbon nanotubes, from the liquid phase such as advanced glass and single crystal oxide fibers, and from solid precursor fibers such as carbon and ceramic fibers. 

Brown ID, Altermatt D (1985) Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database. Acta Cryst B 41 244-247 Brown ID, Shaimon RD (1973) Empirical bond-strength bond-length curves for oxides. Acta Cryst A29 266-282... 

It can be concluded that in the majority of crystalline halides and oxides, a degree of ionicity is between 0.5 and 1.0. The effective sizes (radii) of atoms change with ionization in a non-linear manner (see Sect. 1.5), this translates into < 10 % deviation from the perfectly ionic radii, which explains the efficiency of the ionic radii in inorganic crystal chemistry. 

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