Bismuth phase diagrams

The potassium/caesium phase diagram is an example of a system involving the formation of mixed crystals with a temperature minimum (Fig. 4.4). The right and left halves of the diagram are of the same type as the diagram for antimony/bismuth. The minimum corresponds to a special point for which the compositions of the solid and the liquid are the same. Other systems can have the special point at a temperature maximum. 

Cadmium (m.p. 321° C) and bismvuh (m.p. 271° C) do not form solid solutions nor compounds with one another. Their eutectic point lies at 6 weight percent bismuth and C. Sketch their phase diagram, and label each region to show what phases are present. 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

The maximum in catalytic activity observed for the multiphase region of the phase diagram necessarily arises from interactions between the separate phases. The bismuth rich and cerium rich solid solutions can readily form coherent interfaces at the phase boundaries due to the structural similarities between the two phases which can permit epitaxial nucleation and growth. A good lattice match exists between the [010] faces of the compounds, this match is displayed in Figure 6. We have also shown that regions of an [010] face of a Ce doped bismuth molybdate crystal resembles cerium molybdate compos tionally. This means that the interface between the two compounds need not have sharp composition gradients. It is structurally possible for the Bi-rich phase to possess a metal stiochiometry at the surface that matches that of the Ce-rich phase. 

Figure V-13 The phase diagram bismuth-selenium. After [94CHI/SHE]. Reprinted from [94CH1/SHE] with permission from the authors and Kluger/Plenum Publishers.

Fig. 2. Phase diagrams of bismuth. The sample containers were not wrapped with lead foils, (a) Upstroke. Three sample setups were used, b) Downstroke. Three sample setups were used however, a run marked 0 was the same sample setup as that in (a), (c) Up-stroke corrected by the volumetric method described in the text. Only a few representative spots were marked on the diagram.

The phase diagrams of bismuth corrected by the volumetric method are shown in Figs. 2c and 3c. The corrections were applied to the up-stroke phase diagrams. As seen in the figures the volumetric corrections do not produce a satisfactory result. Moreover, it is difficult to analyze the physical and mathematical relations for the calibration values obtained by the volumetric method to find the true pressures on the sample and/or the pressure losses previously defined. The case treated is confined to that of a sample under pressure in a cylinder. These difficulties cause the author to recommend that for calibration, the up-stroke diagrams should be used. 

Because bismuth has several well-characterized solid, crystalline phases, it is used to calibrate instruments employed in high-pressure studies. The following phase diagram for bismuth shows the liquid phase and five different solid phases stable... 

Often the catalyst is bonded or compounded with silica, and the presence or absence of silica appears to have little effect on the reactions. Likewise, results are reported both with and without phosphorus which do not differ greatly. Studies of the effect of the Bi/Mo ratio have been made in efforts to find the reasons for the markedly different catalytic behavior of bismuth molybdate, so-called, relative to bismuth and molybdenum oxides. The phase diagram for this system was published by Belyaev and Smolyaninov 144), and was confirmed and refined by... 

Bismuth is used to calibrate instruments errqiloyed in high-pressure studies because it has several weU-charactaized crystalline phases. Its phase diagram (right) shows the liquid phase and five solid phases that are stable above 1 katm (1000 atm) and up to 300°C. 

Fig. 124. Main features of the phase diagrams of bismuth with rare earths and actinides.

As with azeotropes, eutectics maybe ternary, quaternary, and so on, but their phase diagrams get very complex very quickly. A few important eutectics have an impact on ordinary life. Ordinary solder is a eutectic of tin and lead (63% and 37%, respectively) that melts at 183 C, whereas the melting points of tin and lead are 232 C and 327 C. Wood s metal is an alloy of bismuth, lead, tin, and cadmium (50 25 12.5 12.5) that melts at 70 C (lower than the boiling point of water ) that can be used in overhead fire sprinkler systems. NaCl and H2O make a eutectic that melts at — 21 C, which should be of some interest to communities that use salt on icy roads in the winter. (The composition of this eutectic is about 23 weight percent NaCl.) An unusual eutectic exists for cesium and potassium. In a 77 23 ratio, this eutectic melts at —48 C This eutectic would be a liquid metal at most terrestrial temperatures (and be very reactive toward water). 

Bismuth (Bi) is not very common in the Earth s crust ( 9 ppb) and consequently, tends to be expensive ( 39 US /100 g for pure element). The Bi-Li phase diagram shows that LiBi and Li3Bi alloys can be formed [119], so that the maximum capacity is 385 mAh/g, roughly equivalent to that of graphite. However, the volumetric capacity of Bi is 3768 mAh/cm, about 4.5 times that of graphite. 

The stability and solubility constants derived at 25 C for zero ionic strength have been used to create a predominance speciation diagram for bismuth(III). The diagram is illustrated in Figure 15.8. It shows that there is a predominance region for each of the monomeric species as well as for the hexameric species Bi5(OH)i2 -The solid phase considered was bismite, Bi203(s), and the minimum solubility calculated for bismuth(III) is just greater than 10 mol kg . ... 

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