Rare-earth compounds studied under pressure

Non-metallic rare-earth compounds studied under high pressure. In almost all cases the energy level shifts as a function of pressure have been determined. The second column gives details concerning the measurements and evaluations made. In particular the following abbreviations are used L Luminescence-, A Absorption-, E Excitation-, S Site-selective spectroscopy, O Other methods, EPC Electron-Phonon Coupling, Int Intensities, LT Lifetime, CFP Crystal-Field Parameters, FIP Free-Ion Parameters, IP Intrinsic Parameters, ET Energy Transfer... 

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. 

In the next section the rare-earth compounds that have been studied by optical means under pressure so far will be reviewed. Then, after a brief introduction of the most commonly used high pressure device, the diamond anvil cell, sect. 4 presents a discussion of the pressure-induced changes of the crystal-field levels and their interpretation. In sects. 5 and 6 some aspects of the dynamical effects under pressure are discussed. These include lifetime and intensity measurements, the influence due to excited configurations and charge transfer bands, and the electron-phonon coupling. 

To provide an overview of the rare-earth compounds which have been studied under pressure so far, table 1 lists the compounds, with respect to the doped ion and with the respective references. Obviously, Eu has been studied under pressure in much more host matrices than any of the other elements. This situation is similar to the observations made by Gorller-Walrand and Binnemans (1996), who reviewed the experimental data on spectroscopic properties of trivalent lanthanide ions doped into crystalline host matrices at ambient pressure. They found that Nd and Eu alone built up around 50% of all studies. 

While ambient pressure studies must rely on discrete changes of crucial parameters, the high pressure method is capable of generating continuous changes of interatomic distances or relative energies of different electronic states. Moreover, at the same time the chemical composition of the rare-earth compound is conserved under pressure, while ambient pressure studies usually have to consider different compounds. In this sense, the application of high pressure can solve physical problems which can not be accessed by any other method. 

Optical Studies on Non-Metallic Rare Earth Compounds under Pressure by Thomas Troster, University of Paderborn, Germany... 

Pressure studies have been able to unravel a lot of the physics of the rare earths. Not only have pressure experiments seen changes of valence from divalent to trivalent, but also changes in the structural properties. In the case of Ce and Ce compounds, the valence changes under pressure from trivalent to tetravalent or from one localized f-state to a delocalized state have been observed. This will be discussed in greater detail in Section 4 of this chapter. 

The anhydrous alkali double carbonates of the rare earths have been synthesized from mixtures of M2CO3 (M = Li, Na, K) and rare earth oxalate hydrate under carbon dioxide pressure of 200-300 MPa and at temperatures of 350-500°C (fig. 26). The sodium and potassium compounds can also be synthesized by dehydration of MR(C03)2 H20 under the same experimental conditions. At lower pressures (20 MPa) litliium forms an oxycarbonate, LiROC03 (Kalz and Seidel, 1980). The compounds have been characterized from powder samples by IR and X-ray investigations and by thermal decomposition studies. 

Structural Studies of Rare Earth Compounds using Diffraction Techniques Under High Pressure 4... 

STRUCTURAL STUDIES OF RARE EARTH COMPOUNDS USING DIFFRACTION TECHNIQUES UNDER HIGH PRESSURE... 

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. 

Similar to the rare-earth trichlorides, also different ternary MYX compounds have been studied thoroughly under high pressure. The results for the pressure-induced changes of the Slater parameter F2 and the spin-orbit coupling parameter of these and other compounds are presented in table 5. Due to the difficulties with the DS model, the evaluation of the parameter shifts has been performed only in terms of the two covalency models. Assuming small changes for the free-ion parameters, the relative changes were approximated by ... 

Troster, T., 2003. Optical studies of non-metallic compounds under pressure. In Gschneidner Jr., K.A., Biinzli, J.-C.G., Pecharsky, V.K. (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 33. Elsevier, Amsterdam, pp. 515-589 (chapter 217). Tsukube, H., Shinoda, S., 2002. Chem. Rev. 102, 2389. Tsukube, H., Shinoda, S., 2006. Near infrared emissive lanthanide complexes for anion sensing. ICFE 6 Conference, Wroclaw, September 4-9, 2006, paper AI-5. 

The best catalyst for the synthesis of methanol from CO + H2 mixtures is copper/zinc oxide/alumina. Intermetallic compounds of rare earth and copper can be used as precursors for low-temperature methanol synthesis as first reported by Wallace et al. (1982) for RCu2 compounds (R = La, Ce, Pr, Ho and Th). The catalytic reaction was performed under 50 bar of CO + H2 at 300°C, and XRD analyses revealed the decomposition of the intermetallic into lanthanide oxide, 20-30 nm copper particles and copper oxide. Owen et al. (1987) compared the catalytic activity of RCux compounds, where R stands mainly for cerium in various amounts, but La, Pr, Nd, Gd, Dy and even Ti and Zr were also studied (table 4). The intermetallic compounds were inactive and activation involved oxidation of the alloys using the synthesis gas itself. It started at low pressures (a few bars) and low temperatures (from 353 K upwards). Methane was first produced, then methanol was formed and it is believed that the activation on, for example, CeCu2, involved the following reaction, as already proposed for ThCu2 (Baglin et al. 1981) ... 

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