Phase transitions under pressure

The dimer structure in Sm2 78C70 also causes a unique structural phase transition under pressure at 1.5 GPa [89]. The volume compressibility of the... 

F. Zipoli, M. Bernasconi, and R. Martohak (2004) Constant pressure reactive molecular dynamics simulations of phase transitions under pressure The graphite to diamond conversion revisited. Eur. Phys. J. B 39, p. 41... 

Xiao P, Cheng J-G, Zhou J-S, Goodenough JB, Henkelman G (2013) Mechanism of the Calr03 post-perovskite phase transition under pressure. Phys Rev B 88 144102... 

The nature of the 4f electrons in lanthanides and their compounds may be broadly characterized as being either localized or itinerant and is held responsible for a wide range of physical and chemical properties of both the elements and compounds. The localized states are marked by tightly bound shells or narrow bands of highly correlated electrons near the Fermi level and are observed at ambient conditions for all of the lanthanide elements. The pressure variable has a dramatic effect on the electronic structure of lanthanides, which in turn, drives a sequence of structural phase transitions under pressure. A very important manifestation of this change is the pressure-induced s to d electron transfer that is known to give rise to the... 

Phase transitions under pressure are best researched for Cs. Here the following succession of transformations has been established (the pressures of transitions in GPa are shown above the arrows, the volume changes in per cent under the arrows and Ac in parentheses) ... 

Both above-mentioned particularities of actinides with itinerant electrons, high density and low-symmetry structures, can be induced in actinides and lanthanides with localized f electrons by the action of pressure. Compression leads to hybridization and overlapping of f electron orbitals and, thus, generates also in these regular metals states which are similar to those found in U, Np, and Pu at ambient pressure. Phase transitions under pressure to low-symmetry structures have up to now been reported in Am, Cm, Bk, and Cffor the actinides, and inC Pr, Nd, and Sm for the lanthanides. 

Phase transitions under pressure were observed in UP2, UAs2, and UAsS. These... 

Besides total energy consumption for mechanochemical reactions, the level of local parameters of temperature and pressure arising in the mill is also important. It is very difficult to measure these parameters directly during activation. Therefore, an indirect method based on P and T determination of phase transitions under mechanical activation of the... 

Polymorphic transitions are known to be induced by an external field such as tensile stress, hydrostatic pressure or shearing stress, high electric field and others. Crystalline phase transitions under tensile stress are discussed in Chapter 6 in relation with the observed changes in microhardness. 

Theoretical predictions can be made of structural phase transitions under very high pressure (compressions down to 30% of the nbient volume), and bcc tungsten was demonstrated to be the only stable modification over the whole pressure range [1.28]. Recently, electronic structure calculations were also applied to assess the linear coefficient of thermal expansion [1.29]. 

A phase diagram is a graphical representation of phase transitions under different pressures and temperatures. By analyzing the phase diagram of a... 

Once a specific event in the load-displacement curve has been associated with a particular phase transition, the pressure at which it occurs can be estimated by considering the elastoplastic behavior of the material under the indenter. For the point-force contact, the Sneddon s solution [39] to the problem of the penetration of an axisymmetric punch into an elastic half space predicts the following relation between the applied load P and the indenter displacement A ... 

LnHs (Ln = Sm, Gd, Ho, Er, Lu) and YH3 undergo hep-fee phase transitions under high pressures from 2 up to 12 GPa Ptr increases with the decrease of the cation radius [120]. In search of the cause of the observed hep-fee transition, XRD study of hydrogen separations in the structure before and after transition was performed. The H-H distance is considered to be important since it affects repulsive interactions between hydrogen species. As is known, H-H hydrogen repulsion influences the increase of the ideal c/a ratio from 1.63 to approximately 1.80 of hep structure that... 

The studies of the shock transitions by the X-ray pulsed analysis have shown [211-214] that at low pressure the compression of a lattice is mono-axial, at medium pressure the compression evolves toward isotropic, and at high pressure the compression is strictly isotropic, being of a purely hydrostatic character. It results in similar values of the pressure for phase transitions under static and dynamic regimes, which in fact is the case. Generally speaking, the dynamic values usually must exceed the static characteristics because the sample under shock loading is heated, whereas... 

Two underlying mechanisms of color change in a piezochromic material have been advanced (/). In a polymorphic crystalline solid diat has one phase at ambient pressure and another at hi pressme, the corresponding colors or optical properties of the two phases can be observed by pressme-induced phase transition. Alternatively, pressure induced changes in molecular geometry and intermolecular interactions of the molecular solid can cilitate die continuous variation of optical properties with pressure. The latter mechanism appears to... 

If the substance in the state of interest is a hquid or gas, or a crystal of a different form than the perfectly-ordered crystal present at zero kelvins, the heating process will include one or more equilibrium phase transitions under conditions where two phases are in equilibrium at the same temperature and pressure (Sec. 2.2.2). For example, a reversible heating process at a pressure above the triple point that transforms the crystal at 0 K to a gas may involve transitions from one crystal form to another, and also melting and vaporization transitions. 

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