Diffraction study of actinides under pressure

Abstract

Uranium and thorium have sufficiently low radioactive dose rates to allow their study at synchrotrons and neutron facilities. Correspondingly, numerous compounds of these two actinides have been studied under pressure by synchrotron x-ray diffraction. The maximum pressures reached were on the order of 60-80 GPa, and 300 GPa in one case.

The situation is much more difficult for all other actinides. Their high level of radioactivity has up to now prevented their study at synchrotrons, except in a few special cases. In contrast, all actinide metals available in sufficient quantities, and a large number of compounds of highly radioactive actinides, have been studied in highpressure laboratory facilities.

Recent examples of in situ high pressure x-ray diffraction work will be described.     

Electron-energy-loss spectroscopy and X-ray absorption spectroscopy as complementary probes for complex f-electron metals: cerium and plutonium

In this paper, we demonstrate the power of electron-energy-loss spectroscopy (EELS) in a transmission electron microscope by investigating the electron structure of two f-electron metals: Ce and Pu. It is shown that EELS in a transmission electron microscope may be used to circumvent the difficulty of producing single-phase or single-crystal samples owing to its high spatial resolution, and that diffraction patterns and images can be acquired, providing unambiguous phase determination when acquiring spectra. EELS results are supported by synchrotron-radiation-based X-ray absorption, multielectron atomic spectral simulations, and local density approximation calculations based on density-functional theory with the generalized gradient approximation. For Ce, it is shown that changes in {111} stacking sequences can drive substantial modifications in the electronic structure of close-packed phases of Ce that have similar atomic volumes, contrary to previous assumptions in literature. For Pu, it is shown that Russell–Saunders (L–S) coupling fails for the 5f states and that either a j–j or an intermediate scheme must be used for the actinides because of the considerable spin–orbit interaction in the 5f states. We present a model showing how the 5f states behave along the light actinide series.     

Structural and electronic relationships between the lanthanide and actinide elements

The similarity and difference between the solid state properties of the 4f and 5f transition metals are pointed out. The heavier 5f elements show properties which have direct correspondence to the early 4f transition metals, suggesting a localized behaviour of the 5f electrons for those metals. On the other hand, the fact that Pu metal has a 30% lower volume than its neighbour heavier element, Am, suggests a tremendous difference in the properties of the 5f electrons for this element relative to the heavier actinides. This change in behaviour between Pu and Am can be viewed as a Mott transition within the 5f shell as a function of the atomic number Z. On the metallic 5f side of the Mott transition (i.e., early actinides), the elements show most unusual crystal structures, the common feature being their low symmetry. An analogous behaviour for the lanthanides is found in cerium metal under compression, where structures typical for the light actinides have been observed experimentally. A generalized phase diagram for the actinides is shown to contain features comparable to the individual phase diagram of Ce metal. The crystal structure behaviour of the lanthanides and heavier actinides is determined by the number of 5d (or 6d) electrons in the metallic state, since for these elements the f electrons are localized and nonbonding. For the earlier actinide metals electronic structure calculations - where the 5f orbitals are treated as part of the valence bands - account very well for the observed ground state crystal structures. The distorted structures can be understood as Peierls distortions away from the symmetric bcc structure and originate from strongly bonding 5f electrons occupying relatively narrow 5f states. High pressure is an extremely useful experimental tool to demonstrate the interrelationship between the lanthanides and the actinides. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pressure induced phase transition behaviour in f-electron based dialuminides

The rare-earth and actinide based compounds are endowed with several exotic physical and chemical properties due to the presence of f-electrons. These properties exhibit interesting changes under the action of various thermodynamic fields and hence continues to be a subject of extensive research. For instance, under pressure, the nature of f-electrons can be changed from localized to itinerant, leading to a variety of changes in their structural, physical and chemical properties. The present review on the high pressure phase transition behaviour of dialuminides of rare earths and actinides is an outcome of research in our laboratory during the last five years using a unique combination of a Guinier diffractometer and a diamond anvil cell built in-house. To bring out the correlations between the compressibility and structural behaviour with the electronic structure, we have also carried out electronic structure calculation. Further, the usefulness of Villars’ three parameter structure maps in predicting pressure induced structural transitions has been explored and this has been illustrated with the available phase transition data.     

Ce-La-Th合金高压相变的第一性原理计算

采用第一性原理计算对Ce_(0.8)La_(0.1)Th_(0.1)在高压下fcc-bct的结构相变、弹性性质及热力学性质进行了研究讨论.通过对计算结果的分析,发现了合金在压力下的相变规律,压强升高到31.6 GPa附近时fcc相开始向bct相转变,到34.9 GPa时bct相趋于稳定.对弹性模量的计算结果从另一角度反映了结构相变的信息.最后,利用准谐德拜模型对两种结构的高温高压热力学性质进行了理论预测.     

High pressure physics research at BARC

Abstract

High pressure physics research at BARC spans a period of more than two decades and covers the area of both static and dynamic pressures on the one hand and both theoretical and experimental aspects on the other. The experimental facilities available include Diamond Anvil Cells for X-ray diffraction and Raman spectroscopy and a light gas gun. Phase transformations investigated in metallic systems includethose in Cd-Hg, transition metals, rare earths and actinides and these investigations have been supported by detailed band structure calculations and studies of the mechanismof phase transformations. Extensive equation of state studies over a wide range of pressures include a new model for the intermediate pressure range between 0.5 and 10 TPa and interpretation of experimental shock Hugoniots.     

High Pressure Theoretical Studies of Actinide Dioxides

Actinide dioxides (ThO 2 , UO 2 , Pu 2 etc.) compounds have the CaF 2 -type structure at ambient pressure and temperature. Under high pressure, they exist in the PbCl 2 -type structure, belonging to space group Pnma [1]. We have studied crystal structures under high pressure in actinide dioxides by means of first-principles self-consistent total-energy calculations with the non-local Perdew, Burke and Ernzerhof (PBE) exchange correlation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. The atomic equilibrium volume, bulk modulus and transition pressure for actinide dioxides were calculated, covering the full pressure range for which the mentioned experiments have been done [2].     

High pressure phase diagrams of binary lanthanide alloys between La,Ce and Pr

Abstract

In comparison with the “regular” binary alloy phase diagrams between “regular” trivalent lanthanide metals, binary intralanthanide alloys of Ce exhibit many irregularities typical for Ce under pressure due to its f electron delocalization. For comparison with the La-Ce and Ce-Pr high pressure phase diagrams also the more regular La-Pr data are presented for the pressure range up to 40GPa.     

A high pressure distorted α-uranium (Pnma) structure in plutonium

Under pressure many rare earths and actinide metals transform to α-U type structure or its lower symmetry distorted forms. We have reinterpreted the diffraction data of Dabos et al. for Pu [S. Dabos et al. J. Alloys Compd. 190 (1993) 237] and find that an Am IV type distorted α-U structure in Pnma space group can explain its high pressure phase. The structures of both the high pressure Am IV type phase and α-Pu, the 0.1 MPa phase, are shown to have a distorted hcp topology. The upturn in the atomic volume of Pu at 0.1 MPa can also be rationalized on the basis of this proposal.     

Data-base for structual phase transitions at high pressures

Abstract

The data on 700 structual phase transitions at high pressure are collected. The 420 fully described phase transitions are reviewed in the form of statistical table showing the number of given change of phase symmetry at the transition. Some other statistical data are presented.     

High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.     

5f-Electron structure in light actinides from Mössbauer studies

The 5f-electron shell in light actinides exhibits properties often quite different from those of their 4f-counterparts in the lanthanides. The main reason is the wider radial extent of the 5f-electrons, which can give rise to a reasonable overlap with other orbitals. The consequence is the formation of 5f-bands and the possibility of significant hybridization. Also, spin-orbit coupling and crystalline field splitting may well be of the same magnitude and relativistic effects play oa more dominant role. The specific electronic structure properties of the elemental metals, of intermetallics and of non-conducting compounds of the light actinides have been investigated thoroughly by M?ssbauer spectroscopy, mainly on²³⁸U and²³⁷Np. Chemical compounds can be formed with widely different (formal) charge states of the actinide ion. They often have a strong covalent bonding character. Features of such bonds have been studied by systematic investigations of the hyperfine coupling parameters and pertinent results are discussed. The possibility of forming 5f-bands of different width and hybridization in metals and intermetallics leads to a broad spectrum of magnetic properties. In particular, we find both localized and itinerant electron magnets. The system of cubic Laves phases NpX₂ renders itself best for a detailed study of magnetic behavior. M?ssbauer measurements under applied high pressure demonstrate that the actinide-actinide separation is one crucial parameter controlling the amount of 5f-delocalization. In addition, it is shown that spin and charge dynamical processes play an important role. These findings are further augumented by new compressibility data on NpAl₂ and NpOs₂.     

Ionization potentials of the lanthanides and actinides – towards atomic spectroscopy of super-heavy elements

A study on the systematic of the atomic ionization potentials for both, the lanthanide and actinide elements have been performed. The existing experimental basis, predominantly relying on results from resonance ionization spectroscopy, has been extended by novel laser spectroscopic investigations on the elements Au, Dy, Pr and Pa. Conclusive results of suitable precision for the ionization potentials could be obtained except for Pa, due to the complexity of its atomic spectrum. Nevertheless, a consistent interpretation of the observed trends for the ionization potentials of lanthanides and actinides was attempted. The series of lanthanides depicts the two well-known, completely smooth, linear trends above and below half-shell closure, from which an expectation value for the missing ionization potential of the all radioactive element promethium of IP _Pm= 44985(140) cm ^?1 was derived. In contrast, the lighter members of the actinide series below the half-filled shell exhibit a significant deviation from predictions, which are ascribed dominantly to relativistic influences affecting the energetic position of the multitude of low-lying configurations. With the assumption of removal of a 6d electron during the ionization process agreement between theory and experiment and a smooth, even though not linear behavior, is obtained also in this region of the Periodic Table. This new interpretation could help to better predict similar trends and systematics for elements heavier than the actinides. Particularly relevant in this respect are the super-heavy elements, which are produced only in minuscule atom numbers and thus were not accessible for any atomic physics study yet.     

High pressure Raman spectra of Gd,Lu and Y

Abstract

The pressure dependence of Raman phonons was studied for Gd, Lu and Y. The TO phonon mode of the hcp phase goes down in frequency for all three metals on approaching the hcp → Sm-type phase transition. Phonon softening also occurs in the high pressure Sm-type phase.     

Sorption of actinides on magnetite and goethite under reducing conditions

The present study investigated the sorption of ²³⁷Np, ²³⁹Pu and ²⁴¹Am onto magnetite and goethite as a typical iron corrosion product under reducing conditions as a function of solution pH, both in the presence and absence of CO₂ at 25°C. The sorption experiments were carried out for each actinide independently and for the simultaneous presence of the three actinides. Alpha-spectrometry of the liquid phase was performed to determine the equilibrium concentrations of the actinides after the sorption period. The sorption was strongly pH dependent. The sorption of ²³⁷Np onto iron minerals was enhanced by the presence of CO₂ to some extent. In other cases, the presence of CO₂ did not affect or prevented the actinides from sorption. There seems to be no simple tendency in the observed data under the conditions examined, probably due to the effects of aqueous and surface complexation reactions with carbonate ions.     

斜锆石（ZrO2）高温高压相变的Raman光谱研究

 以Ar作压力介质，在0~23 GPa压力范围内，利用金刚石压腔装置（DAC）和激光加温技术，采用显微拉曼光谱进行原位测试，对处于准静水压力条件下的斜锆石开展高温高压相变研究。研究结果表明：室温下斜锆石ZrO₂于3.4 GPa时开始发生相变，到10.4 GPa时其明显转变成一个空间群为Pbca的斜方相。此新相随着压力升高，直到15.3 GPa，仍稳定存在。通过研究，首次获得了Pbca相的拉曼谱图。随后在15.3 GPa压力下进行了激光加温后淬火，结果发现，加热前的Pbca相又转变成了空间群为Pnam的PbCl₂结构类型的高压相，该相直到实验最高压力23 GPa仍稳定存在。     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

Discussion of the generalized phase diagram for ‘regular’ lanthanides under pressure (Abstract)

Abstract

X-ray diffraction studies on the kinetics and hysteresis of the structural phase transitions in lanthanides under pressures up to 58 GPa and temperatures between 200 and 520 K are presented. Estimates of the 0 K equilibrium transition pressures are derived from the pressure and temperature dependence of the activation free energies Δ_aG. A comparison of critical radius ratios, R_ws/R_5p, for all the regular lanthanides at the various phase transitions shows simple systematics in the high pressure behaviour of the lanthanides. The “volume collapse” transitions in lanthanides are compared with the behaviour of the actinides and discussed with respect to f-electron delocalization.     

On localization of f-electrons in high temperature phases of early actinides

Abstract

It has been suggested by Vohra and Holzapfel that the f-electrons in early actinide elements become localized at high temperatures close to melting. Their conclusion is based on the unusually large thermal expansions of Np and Pu and their observation of the high temperature β-Np structure under pressure in trivalent 3d Sc, which does not have any f-electrons. We show that these have alternative explanations. Further, the atomic volumes of the high temperature bcc phases of U, Np and Pu are considerably lower than the ones calculated from band theory assuming localized f-electrons. We conclude that the f-electrons continue to be delocalized at high temperatures in U, Np and Pu except Pa.     

Electronic structure and physical properties of bcc selenium under high pressure

Abstract

Here we report a theoretical calculation of the band structure and superconductivity of Se in the bcc phase. The energy band structure and the effect of pressure on the band structure is obtained by means of the Linear Muffin-Tin Orbital method within the atomic sphere approximation. The superconducting transition temperature (T_c) is calculated using McMillan's formula and we predict the value of T_c at 115.3 Gpa as 2.3 K. Further increase in presssure decreases the T_c values. The normal state electrical resistivity at 115.3 Gpa is 1.43 fl cm, with further increase in pressure the resistivity decreases, which is a typical behaviour of number of elemental metals under pressure.