Lanthanides under pressure

Fig. 5. Critical radius ratios r, = for phase transitions in the regular lanthanides under pressure...

The Mott-like transition, a central concept for the description of the actinide metal series, causes the sudden increase of the atomic volumes, encountered when between Pu and Am (Fig. 3). All other properties indicate the onset of a 5f localized behaviour at Am (see Part V) the 5 f pressure, which had contained to smaller values the equilibrium interactinide distance, suddenly gives in, with the withdrawal of the 5f s within the atomic core. The occurrence of such a transition within a series characterized by an unsaturated shell, is a unique phenomenon of the actinide series. In lanthanides, it does not occur except perhaps under pressure in cerium metal the approaching of cerium atoms induces suddenly the itineracy of 4f orbitals and a sudden volume collapse - see Chap. C. Neither it occurs in d-transition metal series, where the atomic volumes have an almost parabolic behaviour when plotted vs. Z (see Fig. 3 and Chap. C). The current... 

The next two elements, berkelium and cahfornium, were recently found to have identical structural sequences under pressure (Fig. 2 b, c). The first high pressure transition for both Bk and Cf is dhcp ccp as in the lanthanides. Thus the lanthanide character of heavy actinides again seems confirmed. But a second transition to the low symmetry a-uranium type structure follows in both metals. This transition reflects the start of 5 f participation in bonding. The transition pressures increase monotonically on going from Am to Bk and Cf 5, 7 and 17 GPa for the dhcp ccp transition, 10, 25, 30 GPa for the ccp An III (low symmetry phase) transition. The second transition in Cm occurs at 18 GPa this transition pressure fits well into the sequence of delocalization pressures. But the dhcp hep transition in Cm occurs at 12 GPa and thus does not fit into the increasing Z sequence with respect to both structure type formed and transition pressure. ... 

Transitions from a localized to an itinerant state of an unfilled shell are not a special property of actinides they can, for instance, be induced by pressure as they rue in Ce and in other lanthanides or heavy actinides under pressure (see Chap. C). The uniqueness for the actinide metals series lies in the fact that the transition occurs naturally almost as a pure consequence of the increase of the magnetic moment due to unpaired spins, which is maximum at the half-filled shell. The concept has resulted in re-writing the Periodic Chart in such a way as to make the onset of an atomic magnetic moment the ordering rule (see Fig. 1 of Chap. E). Whether the spin-polarisation model is the only way to explain the transition remains an open question. In a very recent article by Harrison an Ander-... 

R. Ferreira Paradoxial Violations of Koop-man s Theorem with Special Reference to the 3 d Transition Elements and the Lanthanides. Cfionne/fe.-Band and Localized States in Metallic Thorium, Uranium, and Plutonium, and in some Compounds, Studied by X-Ray Spectroscopy V. Gutmann, H Mayer The Application of the Functional Approach to Bond Variations under 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. 

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. 

Comparing only the two lanthanides, a very similar behavior under pressure could be established. In the case of U3+ at least the qualitative behavior, except for Bp, also matched that of the lanthanides. Quantitatively, the absolute shifts for the actinide ion were larger, a result that could be expected from the much more expanded wavefunctions of the 5f shell. It should be noted, however, that the relative changes were also very similar compared to the lanthanides. 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

Simple silica matrices To further illustrate the limit of this chapter, we briefly discuss one type of materials that will not be systematically reviewed layered lanthanide silicates obtained by hydrothermal synthesis. An alkaline solution of sodium silicate and lanthanide chlorides was stirred to produce a gel which was subsequently put into an autoclave under pressure at 230 °C during seven days, a procedure during which lamellar photoluminescent silicates... 

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. 

In a previous study Kotsuki s group reported reactions of / -ketoesters with enones or acrylates under 800 MPa (Scheme 10.12) [41]. Table 10.11 compares the yields obtained under biactivation conditions (pressure -h lanthanide catalysis) on one side and in the presence of a catalytic system consisting of Yb(OTf)3 -I- silicagel at ambient pressure on the other. Both methods reveal similar efficiency, although operation under pressure reduces reaction time. However, association of pressure and lanthanide catalysis proves its usefulness when acrylates are involved as Michael acceptors. In this case, no reaction occurs with the ambient pressure method even after prolonged reaction times. 

It catalyses the aminolysis of epoxides in an extraordinarily efficient manner in aprotic solvents (e.g. toluene, CH2CI2) with complete trans stereoselectivity and high regioselectivity [Chini et al. Tetrahedron Lett 35 433 1994], It also catalyses the trans addition of indole (at position 3) to epoxides (e.g. to phenoxymetltyloxirane) in >50% yields at 60° (42 hours) under pressure (10 Kbar) and was successfully applied for an enantioselective synthesis of (+)-diolmycin A2 [Kotsuki Tetrahedron Lett 37 3727 799(5]. Of the ten lanthanide triflates, Yb(OTf)3 gave the highest yields (> 90%, see above) of condensation products by catalytically activating formaldehyde, and a variety of aldehydes, in hydroformylations and aldol reactions, respectively, with trimethylsilyl enol-ethers in THF at room temperature. All the lanthanide triflates can be recovered from these reactions for re-use. [Kobayashi Hachiya J Org Chem 59 3590 1994.]... 

The nature of the 4f electrons in lanthanides and their compounds may be broadly characterized as being either localized or itinerant and is held responsible for a wide range of physical and chemical properties of both the elements and compounds. The localized states are marked by tightly bound shells or narrow bands of highly correlated electrons near the Fermi level and are observed at ambient conditions for all of the lanthanide elements. The pressure variable has a dramatic effect on the electronic structure of lanthanides, which in turn, drives a sequence of structural phase transitions under pressure. A very important manifestation of this change is the pressure-induced s to d electron transfer that is known to give rise to the... 

Superconductivity is no stranger to the rare earths, in particular La and several of its compounds. Under pressure, Ce also becomes a superconductor as do Y, Lu (see Probst and Wittig 1978) and certain alloys however, for those lanthanides in which well-defined local 4f moments are present, superconductivity is inhibited by the pair breaking associated with the spins. And yes,- rare earths play a role, albeit not central, in what has been described as the condensed matter event of the 1980s -the remarkable discovery of superconductivity above 90 K in YBaCuO (Wu et al. 1987, Cava et al. 1987). In this case the rare earth is electronically isolated from the superconducting electrons, and as a result the substitution of heavy lanthanides with large 4f spins for the yttrium (Fisk et al. 1987) has virtually no dfect on the superconducting properties. 

L absorption in the divalent lanthanides under high pressure 523... 

Work on correlating the crystal structure sequence in the pure metals and intra rare earth alloys started in about the mid-1960s, shortly after the discovery by Spedding et al. (1962) of the formation of the Sm-type structure in alloys between a light lanthanide metal and a heavy lanthanide (or yttrium) metal. The next major impetus was provided by the high pressure work of Bell Telephone Laboratory s research group (Jayaraman and Sherwood, 1964a, b McWhan and Bond, 1964) who found that under pressure hep Gd transformed to the Sm-type structure Sm (rhombohedral, Sm-type structure) transformed to the dhep structure and dhep La transformed to the fee structure. Thus it became evident that the crystal structure sequence in the lanthanide series at 1 atm pressure varied from fee dhep Sm-type(or 8)- hep, while pressure caused the reverse structure sequence. 

Both above-mentioned particularities of actinides with itinerant electrons, high density and low-symmetry structures, can be induced in actinides and lanthanides with localized f electrons by the action of pressure. Compression leads to hybridization and overlapping of f electron orbitals and, thus, generates also in these regular metals states which are similar to those found in U, Np, and Pu at ambient pressure. Phase transitions under pressure to low-symmetry structures have up to now been reported in Am, Cm, Bk, and Cffor the actinides, and inC Pr, Nd, and Sm for the lanthanides. 

Fig. 4. General diagram of the phase relations of the lanthanide metals under pressure. experimentally observed phase transitions (upstroke transition pressures) or orthorhombic mcl 1,2,3 three different...

X-ray diffraction studies on Y under pressures up to 50 GPa at room temperature (Vohra et al. 1982, Grosshans 1987) gave crucial experimental evidence for the hypothesis that the structural sequence of the regular lanthanides can be attributed purely to the progression of the s to d transfer under pressure, without any (significant) contributions from f electrons. In fact, the typical structural sequence of the regular... 

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