Structural phase transformations

In this subsection, we describe several examples of applications of the total energy pseudopotential method to structural phase transformations induced either by pressure or temperature. 

From the defintion of P, we see that the diamond-3-Sn transition occurs at the volume where the first common tangent (Gibbs line) can be drawn between the diamond curve and the 3-Sn curve. In Fig. 5, the Gibbs line is given by the dashed line. Thus, the points 2 and 3 label the transition volumes between the structures and the slope of the Gibbs line provides the transition pressure. 

A calculation of the energetics of Ge in various crystal structures showed that Si and Ge behave very similarly under pressure. Both materials transform to a metallic 3-Sn phase at a pressure around 100 kbars. The theoretical results l (Table XI) are- in excellent agreement with experiment particularly for the transition volumes. These results are remarkable considering that the only input to 

Several calculations along the same line have also been performed 

Another important class of studies is to investigate temperature-induced structural transitions. This has been performed for the metal Be. At ambient pressure. Be transforms from a low temperature hexagonal close-packed (hep) structure to a high temperature body-centered cubic (bcc) structure at approximately 1530 K before it melts at about 1560 This transition is interesting in several 

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The shock-compression induced structural phase transformation in iron from the low pressure bcc phase to the high pressure hep phase is one of the most visible problems studied in shock-compression science, and its discovery was responsible for widespread recognition of the capabilities of the high pressure shock-compression experiment. The properties of many shock-induced phase transitions are summarized in Duvall and Graham [77D01]. 

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

E. Yu. Batyan, S. V. Matveichuk, and G. A. Branitskii, Structural phase transformations in silver-ceramic systems and their relation to catalytic properties in the process of methanol partial oxidation, Kinet. Catal. 136, 816-820 (1995). 

M. Khayyat, G. Banini, D. Hasko, and M. Chaudhri, Raman microscopy investigations of structural phase transformations in crystalline and amorphous silicon due to indentation with a Vickers diamond at room temperature and at 77 K, J. Phys. D—Appl. Phys. 36, 1300-1307 (2003). 

From the experimental results concerning internal structures, phase transformation, and fracture toughness of the sinter, the following information can be summarized. 

A. G. Khatchaturyan, Theory of Structural Phase Transformations in Solids, John Wiley Sons, New York, 1983). 

Keywords interstitial solid solutions, crystal structure, phase transformation, order-disorder, isotopic effect, antiphase domains, neutron diffraction, TiN026Hoi5, TiN026Doi5, TiN0.MH0.075D0.075 ... 

The differences in the structural-phase transformations during sintering under identical conditions of the UDD powder in the initial state and UDD powder after hydrogen treatment consist in the quantitative fractions of the formed graphite, dense aggregates, and clusters. As a result of the increase in the compressibility of the powder after the treatment, the fraction of dense aggregates and clusters in sintered specimens increases. 

The presence of several structural phase transformations in glassy Ge02 suggests the respective structural changes in the melt, which is already partially confirmed by both experimental study [117] and computer simulation study [118]. 

Vlasova, M., G. Dominguez-Patino, N. Kakazey, M. Dominguez-Patino, D. luarez-Romero, and Y. Enriquez Mendez. 2003. Structural-phase transformations in bentonite after acid treatment. Sci. Sintering 35 155-166. 

Recent efforts involving these systems include (i) high-pressure studies and structural phase transformation stabilization of mixed valence main group metal halides (iii) synthesis of low oxidation state halides and (iv) characterization of mixed metal halides. 

The first two peaks at 110 and 280 °C describe a structural phase transformation and the melting of the material. The main amount of hydrogen (80 wt.%) is liberated from the molten LiBH4 in a temperature range between 320 and 380 °C. Under these conditions, one hydrogen atom remains under in the structure of LiH. 

The decomposition of LiAlH4 involves structural phase transformations and several decomposition steps and has been intensively investigated [69-73]. First, the endothermic melting of LiAlH4 between 165 and 175 °C is observed, followed by the exothermic decomposition (AH= —lOkJ mofi H2) and recrystallization of Li3AlHfi between 175 and 220 °C (Eq. (5.18)). During this first step the hexahydride structure. 

Kikuchi, Structural Phase Transformations in C70, J. Chem. Soc., Chem. Commun. 1676-1677 (1992). 

Statistical Mechanics of the Harmonic Oscillator. As has already been argued in this chapter, the harmonic oscillator often serves as the basis for the construction of various phenomena in materials. For example, it will serve as the basis for our analysis of vibrations in solids, and, in turn, of our analysis of the vibrational entropy which will be seen to dictate the onset of certain structural phase transformations in solids. We will also see that the harmonic oscillator provides the foundation for consideration of the jumps between adjacent sites that are the microscopic basis of the process of diffusion. 

In sections 5.22 and 5.24, we have made schematic evaluations of the nature of the vibrational spectrum (see fig. 5.5). At this point, it is convenient to construct approximate model representations of the vibrational spectrum with the aim of gleaning some insight into how the vibrational free energy affects material properties such as the specific heat, the thermal expansion coefficient and processes such as structural phase transformations. One useful tool for characterizing a distribution such as the vibrational density of states is through its moments, defined by... 

This chapter has shown how the zero-temperature analyses presented earlier in the book may be extended to incorporate finite-temperature effects. By advancing the harmonic approximation we have been able to construct classical and quantum mechanical models of thermal vibrations that are tractable. These models have been used in the present chapter to examine simple models of both the specific heat and thermal expansion. In later chapters we will see how these same concepts emerge in the setting of diffusion in solids and the description of the vibrational entropies that lead to an important class of structural phase transformations. 

Uglov, V. V., Anishchik, V. M., Astashynski, V. V. et al. 2004. Structure-phase transformation of high speed steel by various high intensity ion-plasma treatments. Surface and Coatings Technology 180-181 108-112. 

A structural phase transformation causing a change from semiconducting behavior to metallic conduction, which causes a large decrease in p over a small temperature range (NTC). 

Crisan O, Crisan AD, Skorvanek I, Kovac J. Magnetism and structural phase transformation in Fe/Fe oxide nanopowders. J Phys Conf Ser 2009 144 012027-012031. 

Shape memory is the ability of a material to remember its original shape, either after mechanical deformation, which is a one-way effect, or by cooling and heating, which is a two-way effect. This phenomenon is based on a structural phase transformation. 

An increasing deformation of Lai j.Rj Ga03 perovskite structure wlwn La is substituted by other rare earth cations leads to a shift of the Pbnm-R3c phase transformation from 420 K in lanthanum gallate towards higher temperatures. Transition temperatures of the respective transformation in Lai RxGa03 increase linearly with the concentration of R-cations, for example, 1 at.% substitution of La by Ce, Pr, Nd, Sm, or Gd increases the temperature of the Pbnm-R3c transformation by 8,14, 21, 27, and 38 K, respectively (Vasylechko et al., 2001a Vasylechko, 2005). The relationship between the temperature of structural phase transformations and the deformation of the perovskite structure will be discussed in Section 3.4. 

Luo H, Greene RG, Ghandehari K et al (1994) Structural phase transformations and the equations of state of calcium chalcogenides at high pressure. Phys Rev 850 16232-16237 Zimmer H, Winzen H, Syassen K (1985) High-pressure phase transitions in CaTe and SrTe. Phys Rev 832 4066-4070... 

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