Pressure phase transitions with

The sample eapsule is plaeed in a tight-fitting 4340 steel fixture that serves to support the eopper eapsule. Pressure from the detonation of the explosive is transmitted to the eopper eapsule through a mild steel driver plate. This plate is also lapped optically flat on both surfaces. The mild steel acts to shape the pressure pulse due to the 13 GPa structural phase transition. With proper choice of the diameter of the driver plate and beveled interior opening of the steel fixture, shock deformation of the driver plate acts to seal the capsule within the fixture. 

For solid nitrogen five modifications are known that differ in the packing of the N2 molecules. Two of them are stable at normal pressure (transition temperature 35.6 K) the others exists only under high pressure. At pressures around 100 GPa a phase transition with a marked hysteresis takes place, resulting in a non-molecular modification. It presumably corresponds to the a-arsenic type. Electrical conductivity sets in at 140 GPa. 

Bulk moduli and pressure derivatives. Results for the bulk modulus and its pressure derivative for all three HMX polymorphs obtained from fitting simulation-predicted isotherms to the equations of state discussed above are summarized in Table 7. For all data sets, we include fits to the Us-Up form (Eq. 18) and both weighting schemes for the third-order Birch-Mumaghan equation of state (Eqs. 20 and 21). In the case of the experimental data for /THMX, values for the moduli based on Eqs. 18 and 20 were taken from the re-analysis of Menikoff and Sewell. Two sets of results are included in the case of Yoo and Cynn, since they reported on the basis of shifts in the Raman spectra a phase transition with zero volume change at 12 GPa. Simulation data of the /T HMX isotherm due to Sorescu et al. were extracted by hand from Fig. 3b of their work. 

AF Compounds. The stmctures of the dkali fluorides are all of the NaCl type however, the sensitivity to moisture for the K, Rb, and Cs compounds and their high-pressure phase transitions to the CsCl type show an increasing tendency to adopt the higher coordination number 8 with increasing ionic radius. In ternary compounds, it is mostly easier to adopt a more suitable coordination sphere of fluoride ligands Li+ may have tetrahedral coordination such as in scheelites LiMp4 and a few other compounds (see Section 3.2.13) but has normally CN 6. Na+ shows CN 6-8 with a certain favoring of CN 7. K+ clearly prefers CN 8, and Rb+ and Cs+ CN 10 to 12, in some cases up to 18. 

Zirconia also undergoes high pressure phase transitions from the monoclinic to two orthorhombic structures. A thermochemical study of these phases, combined with phase equilibrium observations (Ohtaka et al. 1991) suggests that surface energy changes the apparent position of phase boundaries, as well as enhances the kinetics of transformation. However Ohtaka et al. (1991) were unable to quantify these effects. 

Temperature-induced phase transitions are typically associated with small volume changes so that the change of the internal energy is dominated by the entropy term TAS. For pressure-induced phase transitions with volume changes of the order of 10%, the PAF term dominates and the entropy term can be neglected at room temperature. The corresponding work AW is defined as ... 

Although the transition pressures and the starting phases were reported differently, it is plausible that there is a (second order) phase transition with an onset pressure of about 18 GPa and completion at about 25-26 GPa. This... 

Phase transitions with temperature and pressure have mostly been studied with simple (binary) compounds. 

Such behavior of Ptr from the structural point of view depends on the balance between two effects of opposite sense. A reduction of the particle size increases the surface energy which acts as an additional (quasi) pressure and thus reduces the external pressure required for the phase transition. On the other hand, surface atoms have lower coordination number than those in the interior, hence the mean decreases with the particle size, and a phase transition with an increase of Ac requires more compression, i.e. higher pressure. In agreement with this model, if a material is stable in a wide range of pressures (e.g. MgO, e-Fe or Ni), its compressibility is rather insensitive to the particle size. 

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