Crystal-structure-existence diagrams

Sn2Sl2, was said (385) to exist in an a and a )8 form. The crystal structures of both forms have been elucidated (119, 239, 240, 388). Apart from questions that still remain concerning the true relationship of a-and )8-Sn2Sl2, a publication by Fenner (121) on the synthesis and structure of a new ternary phase, Sn4SIe, contradicts both of the phase diagrams already mentioned. 

When two metals A and B are melted together and the liquid mixture is then slowly cooled, different equilibrium phases appear as a function of composition and temperature. These equilibrium phases are summarized in a condensed phase diagram. The solid region of a binary phase diagram usually contains one or more intermediate phases, in addition to terminal solid solutions. In solid solutions, the solute atoms may occupy random substitution positions in the host lattice, preserving the crystal structure of the host. Interstitial soHd solutions also exist wherein the significantly smaller atoms occupy interstitial sites... 

Bivariant behavior, for which both temperature and pressure are allowed to vary independently, fixes two degrees of freedom and restricts the system to a single phase. This is represented on the phase diagram as an open area. Since there is only one component, there may only be single areas for the liquid and vapor phases, but several areas corresponding to solids of different crystal structure may exist. 

Crystalline cellulose is believed to exist in at least four crystalline variations. Cellulose I occurs in the majority of plants. Its crystal structure conforms with the diagram in Fig. 75. 

The X-ray crystal structure of the hexachloride salt of the protonated macrocycle shows that it adopts a cleft-like conformation as shown in the diagram, and modelling studies support the existence of this conformation in solution. The electrostatic nature of the anion binding by 4.12-nH+ means that it binds particularly strongly to multiply changed anions, and ATP, with its 4- charge, is bound in water at pH 4, with logXn = 11 for the hexaprotonated host. 

Although phase diagrams show the existence of ABr,, AB.,, and ABn compounds, congruent-melting type compounds belonging to these stoichiometries are relatively few in number and their crystal structures are poorly characterized. Thus, no correlation can be made between them. 

As early as 1943, Sommer (101) reported the existence of a stoichiometric compound CsAu, exhibiting nonmetallic properties. Later reports (53, 102, 103,123) confirmed its existence and described the crystal structure, as well as the electrical and optical properties of this compound. The lattice constant of its CsCl-type structure is reported (103) to be 4.263 0.001 A. Band structure calculations are consistent with observed experimental results that the material is a semiconductor with a band gap of 2.6 eV (102). The phase diagram of the Cs-Au system shows the existence of a discrete CsAu phase (32) of melting point 590°C and a very narrow range of homogeneity (42). 

Three possibilities exist when a salt with a polyphosphate x-ray pattern crystallizes from a melt containing an excess of phosphorus pentoxide. 1. The phosphorus pentoxide is incorporated into the polyphosphate chains converting the chains to crystalline ultraphosphates. 2. The excess phosphorus pentoxide does not enter the polyphosphate crystal structure, but forms an amorphous phase between the crystals of polyphosphate. The amorphous phase is not detected by x-ray. 3. The excess phosphorus pentoxide does not enter the crystal structure of the polyphosphate, but forms as an ultraphosphate between the crystalline polyphosphate crystals as a eutectic phase. (This latter case is precisely what happens in the calcium sodium ultraphosphate system from which calcium phosphate fibers are grown (21) and the phase diagram of Hill et. al. is obeyed as it should be.)... 

Another example of the use of polarized radiation in imaging studies is the analysis of poly(vinylidene fluoride)(PVDF) films, which have been uniaxially elongated at different temperatures. Depending on the thermal, mechanical and electrical pretreatment, PVDF can exist in different modifications [59]. The crystal structure of the cmmpled 11(a) modification can be converted into the aU-tra s 1(P) form by tensile stress below 140°C (see Figure 9.27a). Figure 9.27b shows the stress-strain diagrams of PVDF films in the 11(a) form which have been elongated to 400 % strain at 100 and 150°C. The observed decrease in stress upon elevation of the... 

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