Nonstoichiometric composition

It must be noted that a significant improvement of the chemical durability of lithium disilicate glass-ceramics was achieved later in the development of glass-ceramics with nonstoichiometric compositions. 

The neutral silver is formed by this reaction. During the subsequent heat treatment of the glass at approximately 600 C, colloids of metallic silver are formed (Beall 1992). This colloidal silver forms heterogeneous nuclei of approximately 80 A for the subsequent crystallization of a lithium metasilicate primary crystal phase, Li2Si03. Lithium metasilicate possesses a chain silicate structure. The crystallization of this compound proceeds dendritically. The dentritic growth of crystals in glass-ceramics is discussed in Section 3.2.4. 

Lithium disilicate glass-ceramics demonstrate a relatively high linear coefficient of thermal expansion of approximately 105 x 10 K h This property is favorable for the fabrication of special composite materials, e.g., for sealing to metal substrates in the electrical industry (Beall 1993). 

Beall (1993) and Echeverrfa (1992) achieved notable results in the development of a new lithium disilicate glass-ceramic. The new material is distinguished by the following three characteristics  

Composition of Lithium Disiiicate Glass-Ceramics with Excellent Chemical Durability (wt%) 

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X 264 pm = 792 pm) and implies instead the nonstoichiometric composition Hg2.s2(AsF6) or more generally Hg3 3(Asp6) since the composition apparently varies with temperature. Partially filled conduction bands formed by overlap of Hg orbitals produce a conductivity in the a-b plane which approaches that of liquid mercury and the material becomes superconducting at 4 K. 

The accurate composition of the gas hydrates for a long time remained a controversial subject, since direct analysis leads to ambiguous results owing to decomposition of the hydrate and/or inclusion of mother liquor in the crystals. Thus it was firmly believed that the nonstoichiometric compositions of gas hydrates found experimentally were all due to errors in the analysis. But more recent determinations of the composition by the indirect... 

Improvement of Mm(Ni - Co - A1 - Mn)x type alloys has been achieved in various ways. It was reported that alloys with a nonstoichiometric composition [Mm(Ni - Co - Mn - Al), 4.5 < jc < 4.8] had a larger discharge capacity than those with stoichiometric alloys [26-27J. Using X-ray diffraction analysis, it was found that the larger capacity is dependent on an increase in the unit cell volume of alloys with x=4.5-4.8. It was also reported that annealing treatment improved the durability of this type of alloy. 

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Little information is available on homogeneity ranges and defect structures in the dodecaborides. The only variation from stoichiometry in these borides is for YB,2i the limiting phase determined by density measurements is Yq92B,2. This result can be attributed to the size of Y which is the maximum for metals that form the dodecaborides. Attempts to prepare DyB,2 with a nonstoichiometric composition are conclusive. ... 

There are many ways in which these square antiprism and cuboctahedral defect clusters can be arranged. A nonstoichiometric composition can be achieved by a random distribution of varying numbers of clusters throughout the crystal matrix. This appears to occur in Ca0.94Y0.06F2.06> which contains statistically distributed cuboctahedral clusters. 

The likelihood of finding that a nonstoichiometric composition range is due to the presence of interpolated atoms in a crystal will depend upon the openness of the... 

On the substrate side, the same process occurs for the holes, but on a different energy level. The holes are injected by a high work function metal or semiconductor like the transparent ITO, which consists of a nonstoichiometric composite... 

As a reference state for the surface composition, the electrode surface of stoichiometric composition of MX (cx, = Cm, ) is chosen at this surface composition the dissolution rate is dmioted by and the potential of the compact layer is denoted by Mr- Thus, we obtain the dissolution rate v x. of the interface of nonstoichiometric composition 6is a function of or — Mh s shown... 

Iron(II) sulfide exhibits nonstoichiometric composition. It may be characterized by x-ray. Iron may be analyzed by various instrumental techniques. (See Iron). 

However, its reactions with heavy metals at elevated temperatures form hard refractory interstitial nitrides of nonstoichiometric compositions. 

Tungsten reacts with oxygen at high temperatures. The finely-divided powder is pyrophoric. But the bulk metal begins to oxidize at about 400°C. The metal oxidizes rapidly when heated in air or oxygen at red heat. Two simple oxides are known, a blue monoclinic dioxide, WO2, and a lemon yellow trioxide, WO3. The trioxide, WO3, is the most stable oxide and the ultimate product of heating the metal in oxygen. Many other oxides also are known, but they are of nonstoichiometric compositions and are unstable. The metal also is oxidized by water vapor at red heat. 

Heating the metal with ammonia at elevated temperatures (at about 700°C) yields nitrides of nonstoichiometric compositions. With nitric oxide, uranium is oxidized at about 400°C, forming triuranium octaoxide, UsOs-... 

Nonstoichiometric composition producing impurity levels can arise in two ways, either (1) excess atoms in interstitial positions or (2) holes in the lattice. Both methods [(1) and (2)] are theoretically possible in n- and p-type semiconductors. 

Homogeneous nonideal, e.g. open networks, obtained from the same chemistry as the previous ones. These networks contain dangling chains as a result of incomplete cure, nonstoichiometric composition, or presence of monofunctional monomers. 

An isolated CS plane is referred to as a Wadsley defect and a random array of CS planes is considered to constitute planar (extended) defects which are entirely different from point defects. It is obvious that when CS planes occur at regular intervals, the composition of the crystal is stoichiometric, whereas a random array of CS planes results in nonstoichiometric compositions. While we have invoked anion vacancies which are later annihilated in our description of CS plane formation, we must point out that vacancies are not essential precursors for the formation of CS planes. Accommodating anion-deficient nonstoichiometry through CS mechanism is a special feature restricted to d° metal oxides such as W03, Nb205 and TiOz which exhibit soft phonon modes. Soft phonon modes in metal oxides arise from soft metal-orxygen potentials which permit large cation relaxation. The latter... 

It had the nonstoichiometric composition YbH2i55 and a fluorite-type structure, and, therefore, appears to be analogous to the first class of rare earth hydrides. 

Cdi) and Cd vacancies (VC(l), and an excess of one over the other gives Cd-rich or Te-rich CdTe. In CdS it was presumed that Cd vacancies and sulfur vacancies (Vs) are the predominant defects and here again nonstoichiometric compositions arise from an excess of one type of defect over the other. Other combinations of interstitials and vacancies are possible and in Bi2Te3 (15) indications are that stoichiometric deviations arise from the presence of one type or atom upon the sublattice of the other (BiTe and TeBi ). 

This section covers some other heterometallic rare earth oxides, including Al, Ti, Zr, Sn, Mo, W, Mn, Fe, Co, Ni, and Cu complex oxides, while certain well-known oxysalts, Y-Ba-Cu-O, for example, will not be specifically discussed. For these heterometallic compounds, due to their relatively complex compositions, it is usually difficult to obtain phase-pure products, especially when some dopant ions are added. At elevated temperatures, some of these oxides undergo phase transitions, which may significantly change their physical and chemical properties such as thermal expansion coefficient and ionic conductivity. And for fhose oxides with variable metal valencies, different nonstoichiometric compositions may also result in distinct functionalities in magnetism and catalysis. 

The thermal conductivity of is much higher than that of the fee transition metal carbides. The lower thermal conductivity of fee carbides has been attributed to their generally nonstoichiometric composition and hence the higher concentration of phonon scattering defects in the lattices, but can probably be explained by the stronger covalent bonding in WC and hence the higher efficiency of the heat transport by phonons. 

Other variables refined in the LS procedure are the scale factor K (Section 3.3.5) always, atomic site occupancies in the cases of disorder and/or nonstoichiometric composition and the absolute structure parameter for noncentrosymmetric structures. If the sample contains atoms with sufficiently strong anomalous scattering, the latter parameter indicates whether the assumed absolute structure of the crystal is right or wrong. 

Although hydrogen can react with transition metals to form compounds such as UH3 and FeH6, most of the interstitial hydrides have variable compositions (often called nonstoichiometric compositions) with formulas such as LaH2.76 and VHo.56- The compositions of the nonstoichiometric hydrides vary with the length of exposure of the metal to hydrogen gas. 

W. Nachbar and G. B. Cline, Jr., The Effects of Nonstoichiometric Composition and Particle Size on the Burning Rates of Composite Solid Propellants, Tech. Note 3-64-62-1, Contract No. AF 49 638)-4I2, Lockheed Missiles and Space Company, Palo Alto (1962) presented at Fifth AGARD Combustion and Propulsion Colloquium, Braunschweig (1962). 

NaCl-type phase with nonstoichiometric composition and y-MojN-type phase, are formed. The WC-type region and the other region can be separated simply by r lr, eg., in the region of larger than 0.53, only the WC-type phase is formed, in which there exists not only simple nitrides and carbides, such as WN, MoN, OsC and RuC, but also solid-solution compounds, such as MoN-NbN, MoN-TiN, TiN-CoN and TiN-NiN systems. It is of interest to note that Tio 7C00 3N and Tip yNif, 3N have the WC-structure while the end members, TiN, CoN and NiN do not take the WC structure. 

Since the reaction rate at potentials above Vp is limited by the dissolution of oxide and the dissolution rate depends on the nature of the oxides (see Chapter 4), the change of current with potential indicates that oxide composition/structure varies with the formation. The nonstoichiometric composition of an anodically formed oxide film can be expressed as SiO with a higher value of n close to the oxide/electrolyte interface and a lower value of n close to the Si/oxide interface.A thicker oxide film has a bulk composition closer to the stoichiometric SiOa. The dissolution rate of anodic oxide depends on the composition of the electrolyte as shown in Fig. 5.46. " (also Fig. 5.7 ). It depends little on the type of material and doping levels. ... 

BaFBr is an important storage phosphor material which forms solid solutions and gives rise to nonstoichiometric compositions such as (BaF)i iBro.g. The principal F MAS NMR resonance in BaFBr occurs at 150.9 ppm but the non-stoichiometric sample shows an additional line at 145.3 ppm with an intensity of 9%. The similarity of this intensity to the 10% excess fluorine in the sample led to the assignment of this resonance to fluorine on the antisites (the bromine sites) (Schweizer etal. 1998). 

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