Iron pressure/temperature transition

In fact, iron exhibits all three common metallic crystal structures bcc, fee, and hep within its pressure-temperature phase diagram, as is shown by the inset of Fig. 8.14. The transition from the bcc a phase to the hep e phase... 

That iron is the dominant element in the core has been well established. Recent experimental and theoretical work has substantially expanded and refined our knowledge on the physics and chemistry of iron under the pressure-temperature regime that is relevant to the core. New data on high-pressure, high-temperature phase transitions and thermal EOS of iron promise to provide a better estimate on the density deficit in the core. 

A rule to bear in mind is that, at the same pressure, temperature and particle number, the entropy of a body will be greater, the heavier the atoms and the weaker the bonding forces. Diamond, which consists of atoms that are rather light and very firmly linked in four directions, has an unusually low entropy per mole. Lead, on the other hand with its heavy, loosely bound atoms, is rather rich in entropy. The characteristics of iron lie somewhere in between it has a medium value of molar entropy. Using the example of water, the table shows how entropy increases by transition from a solid to a liquid state and even more by transition from a liquid to a gaseous state. 

The examples of phase transitions mentioned above occur at nonzero temperature. At these so-called thermal or classical transitions, the ordered phase (the ice crystal or the ferromagnetic state of iron) is destroyed by thermal fluctuations. In the last two decades or so, considerable attention has focused on a very different class of phase transitions. These new transitions occur at zero temperature when a nonthermal parameter such as pressure, chemical composition, or magnetic field is changed. The fluctuations that destroy the ordered phase in these transitions cannot be of a thermal nature. Instead, they are quantum fluctuations that are a consequence of Heisenberg s uncertainty principle. For this reason, these zero-temperature transitions are called quantum phase transitions. 

The discussion above has been directed principally to thermally induced spin transitions, but other physical perturbations can either initiate or modify a spin transition. The effect of a change in the external pressure has been widely studied and is treated in detail in Chap. 22. The normal effect of an increase in pressure is to stabilise the low spin state, i.e. to increase the transition temperature. This can be understood in terms of the volume reduction which accompanies the high spin—dow spin change, arising primarily from the shorter metal-donor atom distances in the low spin form. An increase in pressure effectively increases the separation between the zero point energies of the low spin and high spin states by the work term PAV. The application of pressure can in fact induce a transition in a HS system for which a thermal transition does not occur. This applies in complex systems, e.g. in [Fe (phen)2Cl2] [158] and also in the simple binary compounds iron(II) oxide [159] and iron(II) sulfide [160]. Transitions such as those in these simple binary systems can be expected in minerals of iron and other first transition series metals in the deep mantle and core of the earth. 

Somewhat unusual pressure dependence of the nature of the spin transition curve has been found for chain-like SCO systems containing substituted bridging triazole ligands [163, 164]. Although the transition is displaced to higher temperatures with increase in pressure, the shape of the transition curve, unusually, is effectively constant, i.e. there is no significant change in the hysteresis width and the transition remains virtually complete. This has been taken to indicate that the cooperativity associated with the transitions in these and related systems is confined within the iron(II) triazole chains. 

Dual nickel, 9 820—821 Dual-pressure processes, in nitric acid production, 17 175, 177, 179 Dual-solvent fractional extraction, 10 760 Dual Ziegler catalysts, for LLDPE production, 20 191 Dubinin-Radushkevich adsorption isotherm, 1 626, 627 Dubnium (Db), l 492t Ductile (nodular) iron, 14 522 Ductile brittle transition temperature (DBTT), 13 487 Ductile cast iron, 22 518—519 Ductile fracture, as failure mechanism, 26 983 Ductile iron... 

If we look back to the experimental studies on natural expandable minerals at high pressures, it can be recalled that the production of a chlorite-phase occurred when interlayering in the natural dioctahedral mineral had reached about 30% interlayering. It is possible that below this transition only expandable phases are present for most magnesium-iron compositions one is dioctahedral, the other would be trioctahedral. Thus, at temperatures below the transition to an ordered allevardite-type phase, dioctahedral mixed layered minerals will coexist with expandable chlorites or vermiculites as well as kaolinite. The distinction between these two phases is very difficult because both respond in about the same manner when glycollated. There can also be interlayering in both di- and... 

The phase transformations in the catalyst play an important role in determining the activity, attrition resistance, and deactivation of this catalyst. Activation of this precipitated catalyst transforms single crystals of hematite to smaller crystallites of carbide. While the transformation from hematite to magnetite is extremely rapid, the magnetite to carbide transition is much slower under the conditions of temperature and pressure employed in this study. As carbon deposits on the carbide particles, it serves to further prise the carbide particles apart. In a commercial slurry phase reactor the carbide particles break away leading to catalyst attrition. The implication of this work for the attrition resistance of iron FT catalysts is explored in detail elsewhere.18... 

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