Melts oxide, standard states

However oxide activities are by definition ratios of the fugacities of 02 in the melt to those in the pvire liquid oxide standard state ... 

Simplifying this infinite set of equilibria, Masson obtained an equation relating the molar fraction of silica in an Si02-MO melt (with MO basic oxide completely dissociated at standard state to give M + + 0 ) to the activity of dissolved oxide 2 yjQ. 

Although in crystalline phases determination of the enthalpy of formation from the constituent elements (or from constituent oxides) may be carried out directly through calorimetric measurements, this is not possible for molten components. If we adopt as standard state the condition of pure component at T = 298.15 K and P = bar , it is obvious that this condition is purely hypothetical and not directly measurable. If we adopt the standard state of pure component at P and T of interest , the measurement is equally difficult, because of the high melting temperature of silicates. 

Einally, it should be noted that phase changes can be accommodated in the Ellingham diagram. When the temperature moves above the melting point of the metal or metal oxide, their corresponding standard states must change. For example, above 1100°C, copper metal is no longer solid, and the oxidation reaction of interest is ... 

The origin of this dilemma appears to lie in the standard states assumed by both the Toop - Samis and Masson models for the "basic" oxide component. These mixing models assume 100 dissociation of the basic oxide constituent of the binary. Thus in the pure liquid end-member oxide melt the ion fraction of free oxide Xq2- is assiuned to be 1.0. In oxide melts containing strongly polarizing cations this is unlikely to be correct since the dissociation reactions e.g. 

In attempting to use the polymer or quasi-chemical models to calculate the mixing properties of ternary and more complex melts, the differences in the standard state values of X 2- assumed for the end-member oxide components may be very important. In the absence of any additional cross-interactions the polymerization constants in a multicomponent system containing a mols of AO, b mols BO etc. will be given by... 

Thus, we find that - G /n is proportional to E". E" is an intrinsic quantity that permits the direct comparison of compounds of different formulae. Figure 5.12 is a plot of G vs. T for simple binary oxides. Here, G is taken as a positive value so that more stable oxides are represented by upper curves. The knee in the curves for MgO and CaO occurs at the metal boiling point, reflecting a change in standard state of the metal. The end of the curves signifies the melting point of the compound. 

Hard blue-white metal body-centered cubic crystal density 7.19 g/cm melts at 1,875°C vaporizes at 2,199°C electrical resistivity at 20°C, 12.9 microhm-cm magnetic susceptibility at 20°C, 3.6x10 emu standard electrode potential 0.71 V (oxidation state 0 to -i-3). 

Iodine — Iodine, L, is a halogen which occurs naturally mainly as iodide, I- [i]. Iodine (Greek ioeides for colored violet ) is a black solid with a melting point of 113.6 °C which is readily undergoing sublimation to form a violet gas. Iodine occurs in the oxidation states -1,0, +1, +3, +5, +7 and it possesses a rich redox chemistry [ii]. In aqueous solution the formation of I2 from I- occurs with a standard potential of 0.621V vs. SHE and this oxidation process is preceded by the formation of I3 with a standard potential of 0.536 V vs. SHE. For the reaction I2(cryst) + 2e - 21 E = 0.535 V. The I—/I3 redox couple is employed, for example, in solar cells [iii] and in long-lived lithium-iodine battery systems. The oxidation of I2 in organic solvents results formally in I+ intermediates which is a powerful oxidant and useful, for example, in electro-synthetic chemical processes [ii]. 

Because of the importance of microstructure on dielectric and ferroelectric properties, the transformation pathway associated with conversion of the amorphous film into the crystalline state has been studied extensively. The basic mechanism involved is one of nucleation and growth, although the formation of intermediate phases that can impact the thermodynamic driving forces associated with the transformation frequently occurs. " Another key aspect of CSD films is that crystallization occurs well below the melting point of the materials. Therefore, compared to standard mixed-oxide processing of bulk materials, the thermodynamic driving forces associated with the transformation are much greater and the kinetics of mass transport are much less. 

Figure 25 shows the general process steps for the melting route. All components of the alloys must he in metallic form, either as elements, or as master alloys which may he available more economically. Examples of the latter are RE-TM eutectic alloys (except with Sm) prepared by electrowinning, and Fe-Zr or Fe-Ti which are standard products for the steel industry. The RE metals used are made either metallothermically by reducing RE-oxides with calcium (and in the case of Sm with La or mischmetal) as the reductant, or by molten-chloride electrolysis. Electrolytic methods do not work with samarium because of its stable divalent state. Samarium is usually further refined by vacuum distillation, which is easy because of the low boiling point. 

B low standard electrode potentials C variable oxidation states D high melting and boiling points... 

In the assessment of the refining performance of uranium, systematic data has been reported for the chemical properties of uranium complex in various alkali chlorides such as LiCl-RbCl and LiCl-CsCl mixtures [3-5], Information on the coordination circumstance of solute ions is also important since it should be correlated with stability. The polarizing power of electrolyte cations controls the local structure around neodymium trivalent Nd " " as an example of f-elements and the degree of its distortion from octahedral symmetry is correlated with thermodynamic properties of NdClg " complex in molten alkali chlorides [6]. On the other hand, when F coexists with Cr in melts, it is well-known that the coordination circumstances of solute ions are drastically changed because of the formation of fluoro-complexes [7-9]. A small amount of F stabilizes the higher oxidation states of titanium and induces a negative shift in the standard potentials of the Ti(IV)ITi(ni) and Ti(III)ITi(II) couples [7, 8], The shift in redox potentials sometimes causes specific electrochemical behavior, for example, the addition of F to the LiCl-KCl eutectic leads to the disproportionation of americium Am into Am " and Am metal [9],... 

The standard red-ox potential of tellurium in alkali chloride melts is more positive than that of palladium. Addition of elemental tellurium to NaCl-CsCl-U02Cl2 melt at 550 °C resulted in a quite slow uranium reduction reaction. After 180 min the oxidation state of uranium decreased to 5.92 and the concentration of uranium in the melt from 0.85 to 0.79 wt%. The absorption spectra contained two well pronounced bands corresponding to the uranyl(V) complex, U02Cl4 . 

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