High-pressure experiments, phase transitions

D. Rubie, Characterising the sample environment in multianvil high-pressure experiments. Phase Transitions 68, pp. 431—451 (1999). 

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]. 

As shown in Fig. 12.11 (p. 126). upon heating at ambient pressure, /3-quartz will experience phase transitions to /3-tridymite and then to /3-cristobalite at 870 °C and 1470 °C. respectively. Is it feasible to achieve a direct interconversion /3-quartz /3-cristobalite by temperatui e variation at high pressure ... 

How water is transported into the mantle and the depths to which it can be carried have been investigated in high-pressure experiments. In brief, these show that whilst hydrous phases in basalt and subducted sediment can transport water down to the top of the transition zone at about 410 km, hydrous phases in mantle peridotite are capable of transporting water even deeper, certainly into the lower mantle and maybe into the core. 

Suppose, on the other hand, that we have followed some unimolecular reaction up to its high pressure limit, and we then continue to increase the pressure still further what might we expect to happen Most very high pressure experiments in chemical kinetics have been conducted in the liquid phase [41.G 70.K 81.1] and the results appear to be interpretable quite satisfactorily in terms of transition state theory we write that... 

The topic of polyamorphism, i.e. phase transitions between amorphous phases of different structure, has been briefly outlined above (Sect. 9.1.1) but is worth mentioning here because practically all proven cases of it involve high pressure experiments. While it has been accepted for a long time that amorphous solids can resemble the structure of different crystalline polymorphs, until recently it was usually taken for granted that the liquid state is unique for a given substance. High-pressure studies... 

In all the above mentioned experiments, hydrostatic (i.e. isotropic) pressure was applied to materials. In high-pressure experiments, this is usually assumed to be the case, but it is true only when the stress environment is purely hydrostatic. In other cases the stress state of the sample should be described by stress tensors, which are very difficult to determine. The mean pressure could be some average of the normal stress components over the sample, but one cannot neglect the effects of shear stress, differential stress, and stress inhomogeneity on many physical properties [191]. Influence of different conditions of compression on has been studied in detail on the bcc to rhombohedral phase transition in vanadium. Under a non-hydrostatic compression the phase transition occurred at 30 GPa at ambient temperature and at 37 GPa at 425 K. Under quasi-hydrostatic compression in the Ar pressure medium, Ptr increased to 53 GPa. When Ne was used as the medium, Ptr increased to 61.5 GPa, still short of the ideal value of 65 GPa [192]. 

Fig. 4. Calculated phase diagram of silicon including the diamond, / -tin, and the simple hexagonal (sh) structure, which was discovered recently (In high pressure experiments, Refs. 142 and 143). The c/a ratios of both metallic phases are calculated at each point shown by zeroing the shear stress. The / -tin to sh transition pressure is 143 Kbar, in good agreement with experiment, (from Ref. 32)...

It should be mentioned that for 80CB the high-pressure smectic A and the reentrant nematic are only observed as supercooled, metastable phases that are not always found in a high-pressure experiment. Metastability has often been noticed in the study of liquid crystals. The retarded onset of a phase transition is frequently accompanied by a so-called exothermic anomaly in DTA experiments, where the undercooled phase transforms on reheating. In some cases monotropic meso-phases have been found, which are only observed on cooling, for example, smectic phases for 7PCH and 5CCH (cf. Table II). ... 

Jones, O.E. and Graham, R.A., Shear Strength Effects on Phase Transition Pressures Determined from Shock Compression Experiments, in Accurate Characterization of the High Pressure Environment (edited by Lloyd, E.C., National Bureau of Standards Special Publication 326, US Government Printing Office, Washington, DC, 1971, pp. 229-242. 

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