Phase width

Further effects seen with the substitution of Cl for T in the [(Zr6Z)Ii2] type are a reduction of inter-cluster I I repulsions which allows for a reduction in Zr-I inter-cluster bond lengths. The [110] section of this rhombohedral structure is shown in Fig. 5.3 and illustrates that the clusters can be described as a cubic-close-packed array, with the 3 axes running vertically through Z. Phase widths found are 0[Pg.63]

Compounds which have the NiAs structure often exhibit a certain phase width in that metal atom positions can be vacant. The composition then is M X. The vacancies can have a random or an ordered distribution. In the latter case we have to deal with superstructures of the NiAs type they are known, for example, among iron sulfides such as Fe9S10 and Fe10Sn. If metal atoms are removed from every other layer, we have a continuous series from Mj 0X with the NiAs structure down to M0 5X (= MX2) with the Cdl2 structure phases of this kind are known for Co Te (CoTe NiAs type CoTe2 Cdl2 type). 

Shortly after, we recognized that ScCu4Ga2 (Im3) [70] might also be tuned to a QC, but the correct stoichiometry and reaction conditions were not achieved in our limited experiments. Recently, Honma and Ishimasa [71] have reported that i-QC phase forms almost exclusively from a rapidly quenched ScisCu48Ga34 composition, emphasizing a very narrow phase width and its thermodynamic metastability at room temperature. However, the failure turned us to other Ga intermetallics, which led to the pseudogap tuning concepts that follow. 

Earlier experience has shown that stoichiometric reactions of ABa(V, A (A electropositive metal, B and C electronegative main-group or late transition metals) with e/a of about 1.7-2.3 are a good rule-of-thumb way to generate Tsai-type AC/QCs. This time, suitable reactions of CaAuxGa6-x provided both 1/1 and 2/1 ACs [87]. In addition, investigations of phase widths for both ACs also led to the discovery of the first Al-free 1/0 AC, CaAu3Ga [88]. 

Figure 17 shows a portion of the composition-phase distribution obtained from Ca-Au-Ga samples that were either slowly cooled from melts or annealed at 500° C for 3 weeks and quenched. Both ACs have considerable phase widths in Au/Ga proportions relative to previous studies, as reflected in the variations of lattice parameters [14.6732(7)-14.790( 1) A for the 1/1 AC and 23.8829(9)-23.9816(9) A for the 2/1... 

A different phenomenon is the appearance of the so-called blue phases in case of chiral LC s, first observed at cholesteric LC s. Sometimes, one or more blue phases occur close below the clearing point with a very small phase width and bright blue or green colours. Figure 15 shows some typical representatives containing one or more X=Y groups in... 

Complexation can have different effects. In some cases, the phase widths were decreased upon complexation while stabilizing effects were observed in complexed... 

The liquid crystalline properties of 43a-d were interesting as all derivatives showed very stable mesophases with phase widths of 57-109 K. It is remarkable that Schiff bases 43c,d are stable at temperatures above 300 °C. It also strikes that, in contrast to conventional calamitic liquid crystals, the Schiff bases 43c,d possess significantly higher phase transition temperatures compared to azo linked 43a,b. Additionally, the trans compounds tend to possess higher clearing temperatures due to the elongated shape of the molecules. 

Akopova reported liquid crystalline phthalocyanines 116ac (Scheme 63) with eight peripheral crown ethers devoid of alkyl chains [132]. Nonetheless, discotic nematic phases could be observed as derived from miscibility experiments with the discotic nematogen hexa(cyclohexanebenzoyloxy)triphenylene and optical textures. In the case of metal-free 116a, a phase width of 43 K was found. Complexation with Zn(II) (116b) decreased the phase range to 35 K and with... 

The technique obviously allows one to measure the stoichiometry range and also the free formation enthalpy. The actual nonstoichiometry within the phase-width can be directly calculated from the defect model. As discussed in Part I, for a simple ionic disorder the nonstoichiometry 8 is a sinh-function in the difference of the chemical component potential to the value of the stoichiometric point (A = 0) (see Part I,2 Section IV). Owing to... 

Figure 2. The thermodynamic situation upon solidification of a multemary system. The vertical lines designate principal reaction pathways, the dashed tangent lines illustrate the compositional changes arising from an equilibrium solidification at the respective pathways (interrupt lines on the vertical arrows). The narrow areas of existence designate stable phases with a finite phase width, the area designated metastable indicates the existence of a single phase solid which is unstable at ambient conditions.

At the elevated temperatures the line width for CeCd 6 and CeCd 4i5 indicates the phase widths approximately. Because microphases of different stability may be produced in CeCd 4 5, there is a variability to both phase widths. Elliott and Lemons (6, 7) give more complete composition ranges. In particular Table I (6) may be used to calculate phase limits for CeCd 6 in the presence of different CeCd 4 5 structures. 

Figure 5. Internal redox and acid-base chemistry in a solid MO]+<5 within the phase width at a given temperature.

Based upon the experimental data it was deduced that the chemical bonding in Zintl phases should be a mixture of covalent, ionic and metallic contributions ". As metallike systems the B32-type compounds Aj Bj possess a distinct phase width in the range of approximately 0.45 < x 0.55 . Furthermore one finds a metallic conductivity in these systems and as in metals the electrical resistivity increases with temperature . ... 

In Sect. B, structural properties such as phase widths and bonding distances are considered. The bond distances in B32 type Zintl phases have been discussed extensively in the literature because it is expected from the arrangement of the two joined diamond sublattices that both kinds of atoms should have the same size ... 

For the most part the relation between the neat and middle phase line widths noted at 100°C. holds at all temperatures. The neat phase widths remain larger than the middle phase widths in all the systems listed in... 

Table IV except in the dimethyldodecylamine oxide (DC12AO) system. The line widths of this system are shown in Figure 14. Around 60°C. the middle phase width becomes greater than the neat phase width. The abrupt changes in the two widths occur at the mesomorphic-isotropic tran-...

Summarizing, CoOi fN f oxynitride formation appears to be a kinetically controlled phenomenon [371], in accordance with the reported experimental observation of its thermal sensitivity [366]. The phases decompose into rock-salt CoO, metallic Co, and molecular nitrogen at higher temperatures. The broad phase width is also in accordance with theoretical calculations. Further entropy effects will additionally favor, but only very weakly, the formation of ternary phases due to substitutional entropy (O/N). In addition, there is no indication of anionic ordering in the calculations, and a random distribution of N and O is clearly preferred. 

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