Impurity stabilized

The primary phases all contain impurities. In fact these impurities stabilize the stmctures formed at high temperatures so that decomposition or transformations do not occur during cooling, as occurs with the pure compounds. For example, pure C S exists in at least six polymorphic forms each having a sharply defined temperature range of stability, whereas alite exists in three stabilized forms at room temperature depending on the impurities. Some properties of the more common phases in Portland clinkers are given in Table 2. 

A useful source of details such as likely impurities, stability and tests for homogeneity of amino acids is Specifications and Criteria for Biochemical Compounds, 3rd edn. National Academy of Sciences, USA, 1972. 

Despite the occurrence of binary AIB2 borides (see also Fig. 2), no ternary representatives are known (Mn, Mo)B2 found from isothermal sections is a stabilized high-T phase by conversion to lower T by a statistical ( ) metal-metal substitution. Both MnB2 and M0B2 are high-T compounds stable above 1075°C and 1517°C respectively WB2 is claimed but is either metastable or impurity stabilized. Similar examples are observed with (W, Pd>2B5 (M02B5 type) as well as (Mo, Rh),, (B3 and (W, Ni), B3 (Mo,., 83 type). The phase relations in the B-rich section of the Mo(W)-B binaries, however, are not known precisely. 

These non-existent allotropes, which are impurity-stabilized phases, are fee Sc, fee Y-Ce, the bcc Ho, Er, Tm and Lu and fee phases of Nd, Sm, Gd and Dy, some of which have been described as formed at room temperature during mechanical milling. A number of fee high-pressure polymorphs, for instance, are actually compounds, with a structure related to the NaCl-type, formed by reaction with O, N and/or H during mechanical milling (see also Alonso et al. 1992). 

In essence the adstoms simulate the missing half of the substrate. These impurity-stabilized unreconstructed surfaces [e.g., Pt(lOO) and Au(lOO)] have the structure known for the other stable clean unreconstructed metal surfaces. 

Foaming. The emulsion flow regime is particularly sensitive to foaming (17,85). The presence of foaming impurities stabilizes the emulsion and therefore brings about a premature capacity limitation. 

The existence of three crystalline and one amorphous modification of boron has been established (Table 2). Reports on further modifications are uiu eliable and refer to uncharacterized and/or impurity stabilized material, a-tetr. boron, also denoted tetr. I boron (a = 8.75 A, c = 5.06 A),... 

Insofar as small crystals of nonreducible oxides dispersed on the internal interfaces of the basic structural units (platelets) will stabilize the active catalyst surface Fe(lll), the paracrystallinity hypothesis will probably hold true. But the assumption that this will happen on a molecular level on each basic structural unit is not true. The unique texture and anisotropy of the ammonia catalyst is a thermodynamically metastable state. Impurity stabilization (structural promotion) kinetically prevents the transformation of platelet iron into isotropic crystals by Ostwald ripening [154]. Thus the primary function of alumina is to prevent sintering by acting as a spacer, and in part it may also contribute to stabilizing the Fe(lll) faces [155], [156], [298],... 

Nonhydrocarbon and Oxygenated Solvents. Most industrial solvents that are not hydrocarbons are pure chemical compounds. As such, they have sharp boiling points and weU-defined properties. Specifications for these solvents focus mostiy on impurities such as water and other contaminants. This also means that a solvent from one manufacturer should perform the same as the same solvent from another manufacturer any differences are probably the result of impurities, stabilizer content, etc, rather than the properties of the overall solvent. 

Specification of material proof of structure, purity, proportions of impurities, identity of major impurities, stability of raw material and formulation... 

The specification for a new drug substance should include a list of impurities. Stability studies, chemical development studies, and routine batch analyses can be used to predict those impurities that are likely to occur in the commercial product. The selection of impurities in a new drug substance specification should be based on the impurities fotmd in batches manufactured by the proposed commercial process. Those individual impurities with specific acceptance criteria included in the specification for a new drug substance are referred to as specified impurities in this guidance. Specified impurities can be identified or unidentified. 

Details of the degradation products, impurities, stabilizers, etc. for the various evaluated chemical warfare agents discussed in this chapter are given in Tables A1-A6. 

The cleavage plane of silicon is 111 and a cleaved surface formed under UHV conditions is reconstructed in a 2 x 1 surface lattice. Above 750 K, this converts irreversibly to a 7 x 7 structure, which is the same as that produced if a clean surface is generated by ion bombardment and annealing or by simple heat cleaning. Much of the earlier work on clean 111 silicon surfaces has been reviewed by Monch [133], but it is worth noting here that of the many reported structures for this surface, these two are now firmly believed to be intrinsic, and not impurity stabilized. 

The sesquioxides R2O3 crystallize in three forms, A-type(hexagonal), B-type(monoclinic) and C-type(cubic) structures, according to the ionic radius of the rare earth ion. Lighter rare earth ions, from La to Nd give A-form. These ions have happened to be seen to form the C-type stmcture, but this observation seems to be due to impurity stabilization or a metastable phase. An example of the B-type oxide is given by Sm203. Other rare earth sesquioxides yield the C-type oxides [3-6]. 

Fig. 105. Plausible arrangement of the Ga-induced Ge(l 11)2x2 honeycomb structure consisting of 2x2 domains of Ge adatoms (shown by open circles) separated by c(4x2) domain boundaries also built of Ge adatoms (shown by filled circles). AU adatoms occupy T) sites. Small open circles show Ge atoms of the top layer of the unreconstructed Ge(lll) surface [94M1]. The surface is plausibly similar to the high-temperature 2x2 phase of the clean Ge(lll) surface. The presence of Ga (or Al, Au, In) impurities stabilizes it at room temperature.

Impurity atoms sometimes can stabilize a phase, which otherwise might not form. This has been established for R3AI phases, where a small amount of carbon will stabilize the AuCuj type phase, but neither nitrogen nor oxygen would. Buschow and van Vucht (1967) found that only CejAl and Pt3Al will exist without the presence of carbon but the R3AI phases of R = Nd, Sm, Gd, Tb, Dy, Ho and Er will only form if some carbon is present. In the impurity-stabilized AuCu3 structure the carbon atoms occupy the open body-centered site, which is normally vacant, while the Au atoms occupy the comers and the Cu atoms the face-centered positions. 

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