Substoichiometric compositions

The chemistry of iron(IV) in solid-state materials and minerals is restricted to that of oxides, since other systems such as iron(IV)-halides are not stable [186]. Iron(lV) oxides are easy to handle because they are usually stable in air, but they often have a substoichiometric composition, with oxygen vacancies contributing to varying degrees. Moreover, the samples may contain different amounts of iron(lll) in addition to the intended iron(IV) oxide, a complication which may obscure the Mossbauer data [185]. Even iron(V) was found in iron(IV) oxides due to temperature-dependent charge disproportionation [188, 189]. 

Fig. 33 Illustration of the two extremes, homogeneous and segregated, of the distribution of hydrogen-bonded side chains along the backbone in the case of a substoichiometric composition. System PAA(LC) investigated consists of poly(acrylic acid) and side chains containing cyanobiphenyl mesogens connected with an alkyl chain to imidazole-based hydrogen-bond acceptors (Scheme 10c). Reprinted with permission from [242]. 2006 American Chemical Society...

Oxygen is so tightly bound, especially in the substoichiometric compositions, that its analysis can be difficult. Activation analysis (y, n) or the dissolution of the carbide in C-saturated Pt at 2200°C, thereby releasing the oxygen as CO, are used. Dissolved oxygen... 

The crystal structures of the phases in these systems are characterized by destabilization of the octahedral structure elements and the appearance of trigonal prismatic structural elements. F.c.c. monocarbides MeCj v in the Mo-C and W-C system exist only at high temperatures and with substoichiometric compositions. The stoichiometric monocarbides WC and MoC are hexagonal. 

The microhardness of the rvB group carbonitrides (Fig. 23d, e) increases with increasing [C]/([C] + [N]) ratio. For Zr(C, N) and Hf(C, N) the increase is steep on the nitrogen-rich side and flattens on the carbon-rich side, so that a significant positive deviation from linearity can be observed. Contrary, for Ti(C, N) this deviation is only very small for 50 at-% nonmetal but significant for a substoichiometric composition of 45 at-% nonmetal. 

Titanium mononitride S-TiN c (0.42 < x < 1.0) has wide solubility from 10.93 to 22.63% of interstitial nitrogen [ 11. The TiN is stable golden compound of face-centered cubic (Bl) structure. Strictly speaking, stoichiometric composition TiNi.o cannot exist under atmospheric pressure, but substoichiometric composition TiNo.97 is stable. For this reason, the atomic vacancies [2] in sublattices of Ti and N reduce height of Fermi level based on valence electron concentration (VEC) of transition metal compounds [3], resulting in stabilization of band structure. It was experimentally reported that the VEC value of the transition metal compounds is stable at 8.8 under atmospheric pressure [4]. In case of titanium nitride, TiNo.ge is the most stable composition. 

The corrosion resistance of TiN can be presented by anodic polarization curves. The polarization result of TiNo.gr sintered body determined at room temperature is shown in Figure 11.3.1 [11]. Since its electrode potential directly after immersion in dilute sulfuric acid is positive (4-0.016 V), it is not soluble for the dilute sulfuric acid. The current density increases rapidly with increasing the voltage, and then decreases reversibly from 0.1 to 0.5 V, showing passivation of TiN. This behavior resembles to anodic polarization curve of titanium. This would be due to substoichiometric composition TiN with excess amount of titanium. Dissolution of TiN is possible in hot fluoric nitric acid solution alone. 

Unlike the interstitial monocarbides, MC,, where C is never >1, the interstitial mononitrides, MN,, can have a composition where x >1. In substoichiometric compositions (x < 1), the sublattice of nitrogen is predominantly deficient while at hyperstoichiometric compositions (x > 1), the metal lattice is predominantly deficient. The lattice parameter is at a maximum at stoichiometry. Even at stoichiometry, a substantial fiaction of both nitrogen and metal sites are usually vacant. 

Group 4 is characterized by a single cubic compound having a very wide range of composition which extends almost to MC, o- In each system, the phase melts congruently at a substoichiometric composition and at a temperature which is much higher than the melting point of the... 

Within Group 5, the phase relationship becomes more complex. Between the metal and the MC compound, a M2C phase forms with a narrow composition range at room temperature. This decomposes peritectically into a liquid and the MC phase with a composition near MCo.g. The cubic phase extends only to MCo.gs in the V C system, but NbC and TaC approach MCj 0 as the temperature is lowered. Melting of NbC and TaC is congruent at a substoichiometric composition. In fact, TaCo.88 is the highest melting substance known. 

---