Crystal structure transitions, with high pressures

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

As is known in the case of binary systems, when their composition is close to equiatomic, d-ina-via metals form, as a rule, stable phases with Bl-type structures. But, as was observed by Vereshchagin and Kabalkina (1979) using a high-pressure treatment, it is possible to get B1 B2 (NaCl-CsCl)-type structural transitions with some oxides. The question of whether this could happen with d-met carbides was discussed by Ivanovsky et al (1988). These authors carried out LMTO band structure calculations for hypothetical TiC, VC and CrC compounds with a B2 structure. The lattice parameters were determined from the condition that the unit cell volumes of the CsCl- and NaCl-type phases were equal. In order to consider the influence of uniform isotropic compression the B2 VC calculations were carried out for crystal lattice volumes of 5 and 10% less than the equilibrium one. 

In many cases the crystal structure of a rare-earth compound studied under high pressure is a priori known. In such studies the quality of the theoretical link between structure and spectra can be tested. However, a different possibility would be to use the experimentally determined spectral variations in connection with a theoretical approach to derive information about the local structure of the rare-earth ions. Such an attempt has been made in sect. 4.4.2, where the local distortions have been derived either directly from the spectra or by applying the superposition model. Similarly, high pressure studies have been used to get information about the structure in more complicated cases of multiple sites or glasses. In addition, the spectra of rare-earth ions have been used to detect phase transitions that often occur under pressure. Results of such studies will be discussed in the next two sections. 

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