High pressure studies compounds

Sugiura, H. and Yamadaya, T., High Pressure Studies on the Perovskite-Type Compound BaBiOs. Physica 139 140B 349 (1986). 

The aim of this chapter is to present a review of the high pressure optical studies on rare-earth ions in non-metallic compounds. Other methods, as for example neutron scattering, magnetic resonance techniques or MoBbauer spectroscopy will not be considered here, unless they provide additional valuable information to the optical studies. It will be demonstrated that the problem of host lattice structural dependence of 4f/v states can be effectively tackled by high pressure techniques and hopefully the interest for further, more refined high pressure studies of this problem can be stimulated. 

Before continuing, some words must be said with regard to the terms rare earths and f elements used in this chapter. The term rare earths includes the elements Sc, Y and the lanthanides La through Lu. However, this chapter solely deals with divalent or trivalent rare-earth ions which are optically active, i.e., possess a partially filled f-shell. Thus, although the term rare earths is used in this chapter, it should be kept in mind that the elements Sc, Y, La, and Lu are excluded. In some exceptional cases the more general term f elements will be used, as for example when high pressure studies on actinide ions with a partially filled 5f shell are discussed. There are only few studies on 5f elements in non-metallic compounds under pressure, however, it seems interesting to compare the results found for these ions with those for the 4f-elements. 

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. 

The high pressure studies presented so far only considered compounds with a single, well-defined site for the rare-earth ion. However, in contrast to this case, many materials exhibit either a discrete variety of different sites or even a continuous distribution of sites, a situation typically encountered in glasses. 

The knowledge of fundamental relations between the properties of rare earths and the electronic and physical structure of the host compound is essential for the development of improved or new materials. However, up to now only a rather small amount of high-pressure studies compared to ambient pressure studies has been performed. It is hoped that this chapter has demonstrated the ability of high-pressure physics in exploring fundamental relationships especially for rare-earth ions and will stimulate further experimental and theoretical work in this area. 

A. Jayaraman, High pressure studies metals, alloys and compounds 707... 

J.D. Thompson and J.M. Lawrence, High pressure studies - Physical properties of anomalous Ce, Yb and U compounds 383... 

Figure 5. A plot of ATC (= Tc+ - Tc" ) vs m2(Tc+) with data obtained from high-pressure studies on pure UPt3 (specific heat and neutron diffraction, open symbols) and data for a series of U(Pt,Pd)3 compounds with Pd concentrations up to 0.5 at% (closed symbols) figure from ref. [20].

Furthermore, azo dienophiles have been employed in diene-transmissive hetero Diels-Alder reactions of cross-conjugated trienes which allow the straightforward construction of polycyclic compounds [297]. Theoretical interest has been directed to the hetero Diels-Alder reaction of diethyl azo di-carboxylate with 1,3-cyclohexadiene whose concerted course was demonstrated by means of a high pressure study [298]. 

HIGH-PRESSURE STUDIES OF NITROGEN-CONTAINING COMPOUNDS... 

We are grateful to our colleagues, Z. Liu, M. Somayazulu, J. Hu, Q. Guo, J. Shu, O. Tschauner, A. F. Goncharov, V.V. Struzhkin, P. Dera, C. Prewitt, J. Lin and O. Degtyareva for experimental assistance and helpful discussions. We also thank K. Christe and W. Wilson for samples and helpful discussions. Our high pressure studies of nitrogen compounds have been supported by LLNL (subcontract to Harvard), AFSOR, DARPA, NSF and DOE. 

This chapter introduces the physical chemistry of materials under high pressures. Space limitations permit only a broad-bmsh introductory survey. High-pressure studies range from designing equipment to generate, to coniine and to measure high pressures to spectroscopic studies from 10 Hz to beyond 10 Hz at temperatures from below 1 K to 10 K and beyond for all sorts of elements, compounds, solutions and mixtures. To say that these are extreme ranges of conditions is an understatement. 

HIGH PRESSURE STUDIES METALS, ALLOYS AND COMPOUNDS... 

High pressure studies on magnetic transitions in rare earth metals, intra-rare earth alloys and compounds (the monochalcogenides, ferrites, spinels (RFe204) and garnets (R3Fe50i2 where R is a rare earth element)) have been numerous and all the work prior 1969 has been reviewed by Bloch and Pavlovic (1969). For the discussion on pressure studies of magnetic transition in this chapter, much material has been drawn from the above review. 

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