Stoichiometric materials

Va.na.dium (II) Oxide. Vanadium(II) oxide is a non stoichiometric material with a gray-black color, metallic luster, and metallic-type electrical conductivity. Metal—metal bonding increases as the oxygen content decreases, until an essentially metal phase containing dissolved oxygen is obtained (14). 

Ordinarily, the energy gap (Eg) between the filled and empty bands is appreciably greater than kT. The concentration of conduction electrons n in the pure stoichiometric material is equal to the concentration of holes p and is given by... 

Point defects are only notionally zero dimensional. It is apparent that the atoms around a point defect must relax (move) in response to the defect, and as such the defect occupies a volume of crystal. Atomistic simulations have shown that such volumes of disturbed matrix can be considerable. Moreover, these calculations show that the clustering of point defects is of equal importance. These defect clusters can be small, amounting to a few defects only, or extended over many atoms in non-stoichiometric materials (Section 4.4). 

NMR time scale (ca. 10 s), the lithium spins see an average manganese oxidation state of 3.5 (i.e., Mn 5+ ions), and thus there is only one magnetically inequivalent lithium site (the 8a site). The NMR spectra are clearly sensitive to this hopping mechanism, and NMR spectroscopy may be used to follow the partial charge-ordering process that occurs just below room temperature in the stoichiometric material. 

The spectra of the doped materials (Cr, Ni, Zn +, Li+, Co +, AP+) are similar to those seen for the nominally stoichiometric materials, and sets of resonances between 500 and 700 ppm are seen on cation doping in addition to that of the normal spinel environment (at ca. 500 ppm). Again, these resonances are assigned to lithium ions near manganese-(IV) cations. The lower intensity of the additional resonances seen on Cr + substitution, in comparison to Zn + or Ni + substitution, is consistent with the oxidation of fewer manganese ions near the depart ions. For the Li- and Zn-doped spinels, resonances at ca. 2300 ppm were also observed, which are assigned to lithium ions in the octahedral sites of the spinel structure. In the case of Zn doping, it is clear that the preference of Zn + for the tetrahedral site of the spinel structure forces the lithium onto the octahedral site. 

Although this compound has the nominal formula FeO, stoichiometric material cannot exist as a stable phase at low pressures or at pressures in excess of 10 MPa. The non-stoichiometry is accommodated by oxidation of a proportion of the metal ions and the creation of cation vacancies (Lindsley, 1976). 

As expected for a material with cubic sites, the RT spectrum of stoichiometric wiis-tite consists of a single line. The spectrum of the non-stoichiometric material shows an asymmetrically broadened doublet with contributions from Fe " resonance and from two quadrupole split Fe" doublets. The component peaks which contribute to this spectrum have not been fully resolved owing to the range of Fe" environments in the structure (i. e. the variation in Fe content and vacancy level). At 77 K the Mossbauer spectrum consists of a broad doublet with contributions from Fe" and Fe ". Fe(OH)2 shows a sextet corresponding to a Bhf of 16.6 T at 20 K (Miyamoto, 1976 Genin et al. 1986) and to -20 Tat 4K (Refait et al. 1999). The Tn is at 34 K (Miyamoto et al., 1967). The spectrum of high pressure FeOOH at room temperature consists of a sextet (Peret et al. 1973). 

Figure 17.1 shows the molar free energy, F, as a function of temperature for a pure, or stoichiometric, material at fixed volume. The material has a first-order phase transformation at the temperature where the molar free energies cross. The equilibrium free energy is a function of temperature only. The corresponding order parameter, , which is also a function of T as illustrated in Fig. 17.1b, is a subsidiary parameter introduced by the series expansion... 

Table 14.12 Stoichiometric material balance for trioleine methanolysis.

Schottky and Frenkel defects do not alter the stoichiometry of the material as they are intrinsic. In non-stoichiometric materials, both types of point defect occur, but ... 

Such behaviour is distinct from that of stoichiometric materials with fixed ratios such as Fe304, and FCjOj, which have different structures. 

Uranium(IV) oxide, UOj, crystallizes with the fluorite structure as shown in Figure 6.5. On heating in oxygen, additional oxygen can be taken up by the lattice to form the non-stoichiometric material 002+. There is a gradual increase in the lattice parameters as additional oxygen is added to the structure. 

Because of the absence of 5qq quenching by simple ion pair interactions, high Eu concentrations and stoichiometric materials should be usable for laser action. 

Ho3+ is the second most extensively exploited lanthanide laser ion in terms of different transition lased, it is the most exploited. Stimulated emission is observed for 12 transitions with wavelengths ranging from 0.55 to 3.91 ym and in hosts including crystals, three stoichiometric materials (HoF3-LiHoF4, H03AI5O-12) (19, 2lj, thin films (52), and silicate glass (75). 

There are also many oxides that are non-stoichiometric. These commonly consist of arrays of close-packed oxide ions with some of the interstices filled by metal ions. However, if there is variability in the oxidation state of the metal, non-stoichiometric materials result. Thus ferrous oxide generally, has a composition in the range FeO0.90-FeO0.95, depending on the manner of preparation. There is an extensive chemistry of mixed metal oxides (see also page 54). 

Nickel oxide is a non-stoichiometric material with cationic vacancies. The concentration of its vacancies is naturally associated with the oxygen pressure of its... 

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