Crystallization under hydrostatic

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

The equation of state of a cubic crystal under hydrostatic pressure is given by... 

Again, detailed structural studies have been carried out on polymerized single crystals. Moret et al. [82] polymerized crystals under hydrostatic conditions... 

Garcia -Revilla S, Rodriguez F, Valiente R, Polnau M (2002) Optical spectroscopy of AI2O3 Ti single crystal under hydrostatic pressure. The influence on the Jahn-Teller coupling. J Phys Cond Matter 14 447... 

Draustic changes in reaction kinetics au e observed to occur for (a) polymerization of TS crystals under hydrostatic pressure (37,38),... 

FIGURE 70 Thermal expansion coefficient of CeNi single crystal under hydrostatic pressure. The maximum in is shown by an arrow (Uwatoko et al., 1993). 

The absorption spectrum of an SmSe film on KCl substrate between 0.6 and 2.2 (xm (-2 to 0.5 eV) is studied up to 7 kbar by Vedam, Kirk [15], for a figure, see the paper. Both the peaks, Ei at 1.06 eV and E2 at 1.88 eV, shift with pressure towards lower photon energies with rates of -12 meV/kbar and -8.6 meV/kbar, respectively [15]. The reflectance of SmSe single crystals under hydrostatic pressure up to 70 kbar is shown in Fig. 68 from Welber, Jayaraman [5]. 

Fig. 68. Reflectance of SmSe single crystals under hydrostatic pressure.

In spite of the presence of ECC, the sample exhibiting a domain structure remains unoriented on the macroscopic level. Figure 3 c shows a great difference in the structures obtained, if molecular orientation exists and if hydrostatic compression is applied. Although the method of hydrostatic compression of the melt is of paramount importance from the scientific view point just for samples crystallized under pressure it was possible to prove unequivocally the existence of ECC), it does not allow a direct preparation of oriented samples of high strength (they are brittle and readily crumble to powder under minimum strain). However, the material obtained in this way can probably serve as a semi-finished product for further technological treatment that would improve its mechanical properties. 

Fig. 16.4 (a) The crystal ceiis of anatase Ti02. (b) View of the octahedra-packing of anatase Ti02 the thick red lines indicate the axis in representative octahedra which, under stress, results in a change of band gap. (c) Band gap variation in anatase Ti02 under hydrostatic, epitaxial, and uniaxial stress. Adapted from Yin etal. [72] with kind permission from American Institute of Physics (2010). 

Incoherent Clusters. As described in Section B.l, for incoherent interfaces all of the lattice registry characteristic of the reference structure (usually taken as the crystal structure of the matrix in the case of phase transformations) is absent and the interface s core structure consists of all bad material. It is generally assumed that any shear stresses applied across such an interface can then be quickly relaxed by interface sliding (see Section 16.2) and that such an interface can therefore sustain only normal stresses. Material inside an enclosed, truly incoherent inclusion therefore behaves like a fluid under hydrostatic pressure. Nabarro used isotropic elasticity to find the elastic strain energy of an incoherent inclusion as a function of its shape [8]. The transformation strain was taken to be purely, dilational, the particle was assumed incompressible, and the shape was generalized to that of an... 

Because structural phase transitions are often ferroelastic or coelastic in character it is essential to have a well-defined stress applied to the crystal at high pressures. In effect, this means that a hydrostatic pressure medium must be used to enclose the crystal. A 4 1 mixture by volume of methanol ethanol remains hydrostatic to just over 10 GPa (Eggert et al. 1992) and is convenient and suitable for many studies. If the sample dissolves in alcohols, then a mixture of pentane and iso-pentane which remains hydrostatic to 6 GPa (Nomura et al. 1982), or a solidified gas such as N2, He, or Ar can be employed. Water appears to remain hydrostatic to about 2.5 GPa at room temperature, just above the phase transition from ice-VI to ice-VII (Angel, unpublished data). The solid pressure media such as NaCl or KCl favoured by spectroscopists are very non-hydrostatic even at pressures below 1 GPa and have been shown to displace phase transitions by at least several kbar (e g. Sowerby and Ross 1996). Similarly, the fluorinert material used in many neutron diffraction experiments because of its low neutron scattering power becomes significantly non-hydrostatic at -1.3 GPa. Decker et al. (1979) showed that the ferroelastic phase transition that occurs at 1.8 GPa in lead phosphate under hydrostatic conditions is not observed up to 3.6 GPa when fluorinert was used as the pressure medium. At pressures in excess of the hydrostatic limit of the solidified gas and fluid... 

The following sections develop three subjects the classical approximations for the strain/stress in isotropic polycrystals, isotropic polycrystals under hydrostatic pressure and the spherical harmonic analysis to determine the average strain/stress tensors and the intergranular strain/stress in textured samples of any crystal and sample symmetry. Most of the expressions that are obtained for the peak shift have the potential to be implemented in the Rietveld routine, but only a few have been implemented already. 

Table 3.5. CB parameters of three cubic crystals with camel s back structure near the CB minimum Xi at the X point of the BZ. The values for GaAs apply under hydrostatic pressure above 4 GPa. The effective masses are in me units...

The relevant numerical values are given in Table 3.5 and the value for to// deduced from these values is 3.06m.e. The direct-gap crystal GaAs has been added to this table as it is possible to convert it under hydrostatic pressure into an indirect-gap structure with a minimum at the Xi point [44]. 

Hall effect measurements are reported for three single crystals of the charge transfer salt HMTSF-TCNQ in the temperature range 1.4-200 K at ambient pressure and under hydrostatic pressures of approximately 6 Kbars. There is evidence that the high conductivity of this material at low temperatures arises from a small number of electrons with a high mobility and a low degeneracy temperature as suggested by other experiments and a recent band-structure calculation. 

Extended chain crystals (ECC) (formed when polymer crystallizes under high hydrostatic pressure, or a crystalline polymer is annealed under pressure) (31)... 

J., Hu, J., Mao, Ho-Kwang. Single crystal X-ray diffraction in monohydrate L-asparagine under hydrostatic pressure. Science and Technology of High Pressure. Proceedings of AIRAPT-17, Ed. M.H. Manghnani, W.J. Nellis, and M.F. Nicol, University Press, Hyderabad, India (2000) 502-505. 

In summary, we calculate that the low clinoenstatite is not stable under hydrostatic conditions and enstatite has a comparatively small stability field. The energy differences are so small that they are within the reliability of the simulations and thus the precise positions of the phase boundaries are not well located. The primary reason for this problem is the reliability of the potential models. Hence, calculating phase relationships represents the most difficult challenge for free energy minimization techniques. However, the simulations do provide valuable insights into the mechanisms of phase transitions and the effect of pressure and/or temperature on the crystal structures and the relative phase stabilities. 

The electrical resistivities of the dense Kondo systems CeNiln, CePdln, and CePtln have been measured under hydrostatic pressures up to 19 kbar (Kurisu et al., 1990). The Kondo temperature of CeNiln and CePtln shifts linearly with pressure to higher temperatures at rates of 2.3 and 1.5 K/kbar, respectively. For CePdIn, the pressures were not high enough to reach the CePtln or CeNiln state. Measurements of the elastic properties of CePdln reveal that all elastic constants exhibit softening at low temperatures due to the crystal electric field effect and the antiferromagnetic ordering (Suzuki et al., 1990). 

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