Phase transition reconstructive

Reconstructive phase transitions Chemical bonds are broken and rejoined the reconstruction involves considerable atomic movements. Such conversions are always first-order transitions. 

The coordination numbers of the atoms increase from 6 to 8 in the NaCl —> CsCl type conversion therefore, it is a reconstructive phase transition which can only be a first-order transition. 

Changing the bond graph—reconstructive phase transitions... 

Reconstructive phase transitions occur when major changes are made in the topology, i.e. when the bond graph is reorganized. The transitions usually observed in structures with lattice-induced strain are displacive and often second order (no latent heat). Reconstructive transitions arise when two quite different structures with the same composition have similar free energies. Unlike the displacive transitions they involve the dissolution of one structure and the recrystallization of a quite different structure. These phase transitions possess a latent heat and often display hysteresis. 

Different experimental methods have been used in order to fully characterize pure compounds and binary systems, such as DTA, X-ray, electrochemical techniques, etc., the latter making it possible to identify the nature of phase transitions that take place in the solid state. They have enabled distinguishing between the formation of compounds in the solid state (reconstructive phase transition) and their structural transition (non-reconstructive phase transition). 

In the electrical conductivity versus temperature plot of the first group of compounds, a significant (two orders of magnitude) conductivity jump is observed at the same temperature as determined for the reconstructive phase transition. A second, but smaller jump happens approximately at an identical temperature for all compounds (835-845 K). This effect is not observed in the DSC thermograms. 

No doubt that the progress in both sensitivity and methodology will increase the range of applicability of time-resolved in situ NMR studies, which are important for the comprehension of first-order and reconstructive phase transitions, but also in solid-state reaction kinetics. 

Clean, Low-Indexed Surfaces. The structures of clean surfaces are of fundamental interest because they are the basis for more complicated systems offering practical applications. On their own, clean surfaces have importance as quasi-2D systems, which can show special effects like relaxations, reconstructions, phase transitions, surface-specific defects, local mass fluctuations, and roughening transitions. In the following we concentrate on face-centered cubic (fee) metals. The geometry of the three low-indexed fee surfaces is shown in Figure 44. 

For even larger pressures, the atomic coordination numbers begin to increase. This corresponds to formation of new chemical bonds, which make the atoms to be closer in space. In order to make the new bonds, the atoms, when squeezed, open their closed core electronic shells (e.g., under sufficiently high pressures, even the noble gas atoms open their electronic shells cf. Chapter 14). Some new pha.se transitions are accompanying these changes, known as reconstructive phase transitions. 

Reconstructive phase transitions occur between markedly different crystal structures. The two phases have no symmetry relationships. Bonds between the atoms are broken and remade in the transition. Large latent heats obtain if the coordination number changes during constructive transformations. Transition rates are low. 

P. Tolddano, V.P. Dmitriev, Reconstructive Phase Transitions (World Scientific, Singapore, 1996)... 

CsSnIj is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature... 

Katzke H, Bismayer U, Toledano P (2006) Reconstructive phase transitions between carbon polymorphs limit states emd periodic order-parameters. J Phys Condens Matter 18 5129-5134... 

Fig. 9.1 An example of reconstructive phase transition transformation of hexagontil boron nitride (h-BN) into cubic boron nitride (c-BN) is impossible without complete atomization of the parent solid (here achieved by detonating a mixture of BN and an explosive). The coordination number changes from 3 to 4...

For a reconstructive phase transition Wits — where A f l corresponds to... 

Toledano P, Dmitriev V (1996) Reconstructive phase transitions. World Scientific, Singapore... 

Mason WP (ed) (1964) Physical acoustics. Academic, New York, NY Nelson DF (1979) Electric, optic and acoustic interactions in dielectrics. Wiley, New York, NY Thomson W (Lord Kelvin) (1878) On the piezoelectric property of quartz. Phil Mag 5 4 Tichy J, Gautschi G (1980) Piezoelektrische Messtechnik. Springer, Heidelberg Toledano P, Dmitriev V (1996) Reconstructive phase transitions. World Scientific, Singapore Valasek J (1920) Piezoelectric and allied phenomena in Rochelle salt. Phys Rev 15 537-538 Valasek J (1921) Piezoelectricity and allied phenomena in Rochelle salt. Phys Rev 17 475-481 Valasek J (1922) Piezo-electric activity of Rochelle salt under various conditions. Phys Rev 19 478 Valasek J (1924) Dielectric anomalies in Rochelle salt crystals. Phys Rev 24 560 Voigt W (1890) General theory of the piezo- and pyroelectric properties of crystals. Abh Gott 36 1-99... 

Correlation between reconstructive phase transitions and transport properties from SrCo02.s brownmillerite a neutron diffraction study. Solid State Sci 10,... 

The computational studies on surface chemistry of Co catalysts have offered significant supports to the investigation of FTS on Co catalysts. However, the work is far from decent. As the experimental studies indicated, surface reconstruction and phase transition were certain to take place under practical FTS conditions. The theoretical studies about surface reconstruction and phase transition of Co catalysts, however, are fairly rare. In addition, cluster models were less studied in the previous theoretical work compared to slab models. However, practical catalytic reactions do not always happen as proposed in ideal plane models, nor the active sites distribute homogeneously on the surface. The investigation on cluster models is acting a crucial role in the study of heterogeneous catalysis. Accordingly, more considerations on surface reconstruction, phase transition, and cluster models should be taken into account in future work. 

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