Richard G. Haire

Photoluminescence and Raman studies of Sm3+ and Nd3+ ions in zirconia matrices: example of energy transfer and host-guest interactions.

Photoluminescence and Raman studies on Sm(3+)- and Nd(3+)-doped zirconia are reported. The Raman studies indicate that the monoclinic (m) phase dominates up to a 10 at.% lanthanide level, while stabilization of the cubic phase is attained at approximately 20 and approximately 25 at.% of Sm(3+) and Nd(3+), respectively. Both systems are strongly luminescent under photo-excitation. The emission spectrum at 77 K of the ZrO(2):Sm(3+) system consists of a broad band at 505 nm, that corresponds to the zirconia matrix. At room temperature the band maximum blue-shifts to 490 nm. Sharper bands corresponding to f-f transitions within the Sm(3+)ion are also exhibited in the longer wavelength region of the spectrum. Exclusive excitation of the zirconia matrix provides sensitized emission from the acceptor Sm(3+) ion. The excitation profile is dominated by a broad band at 325 nm when monitored either at the zirconia or at one of the Sm(3+) emissions. A spectral overlap between the 6H(5/2)-->(4)G(7/2) absorption of the Sm(3+) ion with the zirconia emission leads to an efficient energy transfer process in the systems. Multiple facets of the spectral behavior of the Sm(3+) or Nd(3+) in the zirconia matrices, as well as the effects of compositions on the emission and Raman properties of the materials, and the role of defect centers in photoluminescence and the energy transfer processes are discussed.

Transformation of monoclinic californium bromide to orthorhombic CfBr3 by the application of pressure

A new synthetic route to the orthorhombic form of CfBr/sub 3/ is reported. A few micrograms of monoclinic CfBr/sub 3/ is loaded and studied in a triangular-shaped diamond anvil pressure cell similar to that reported by Merrill and Bassett. Based on analysis of the absorption spectra obtained from the CfBr/sub 3/ sample at several pressures up to 3.4 GPa, it is concluded that the structural transformation of monoclinic CfBr/sub 3/ to orthorhombic CfBr/sub 3/ takes place between 1.7 and 3.4 GPa. 10 references, 1 figure.

Superconductivity of americium.

The metal americium becomes superconducting at temperatures as high as 0.79 K for the room temperature, double-hexagonal-close-packed phase. We also have evidence of a slightly higher transition temperature for the face-centered-cubic phase. This discovery of superconductivity in the midst of nonsuperconducting manmade elements is somewhat surprising.

Study of the phase behavior of Eu2O3 under pressure via luminescence of Eu3

Abstract We have investigated the phase transition of Eu 2 O 3 under pressure from a C-type b.c.c. structure to a B-type monoclinic structure by the luminescence spectra of the Eu 3+ ion. It is suggested that the luminescence from f—f transitions in Eu 3+ is very sensitive to the Eu 3+ ion environment and can be used as a spectral probe to identify the crystal structure. The C-to-B transition occurs at about 8.0 GPa and the B-type structure was retained after the pressure had been released. The transition thermodynamics and kinetics are discussed, and the entropy for the C-to-B transformation was estimated. It was concluded that the C type is the room temperature and pressure stable structure, whereas the B type is metastable at room temperature.

Further examples of the failure of surrogates to properly model the structural and hydrothermal chemistry of transuranium elements: insights provided by uranium and neptunium diphosphonates.

In situ hydrothermal reduction of Np(VI) to Np(IV) in the presence of methylenediphosphonic acid (C1P2) results in the crystallization of Np[CH2(PO3)2](H2O)2 (NpC1P2-1). Similar reactions have been explored with U(VI) resulting in the isolation of the U(IV) diphosphonate U[CH2(PO3)2](H2O) (UC1P2-1), and the two U(VI) diphosphonates (UO2)2[CH2(PO3)2](H2O)3.H2O (UC1P2-2) and UO2[CH2(PO3H)2](H2O) (UC1P2-3). Single crystal diffraction studies of NpC1P2-1 reveal that it consists of eight-coordinate Np(IV) bound by diphosphonate anions and two coordinating water molecules to create a polar three-dimensional framework structure wherein the water molecules reside in channels. The structure of UC1P2-1 is similar to that of NpC1P2-1 in that it also adopts a three-dimensional structure. However, the U(IV) centers are seven-coordinate with only a single bound water molecule. UC1P2-2 and UC1P2-3 both contain U(VI). Nevertheless, their structures are quite distinct with UC1P2-2 being composed of corrugated layers containing UO 6 and UO 7 units bridged by C1P2; whereas, UC1P2-3 is found as a polar three-dimensional network structure containing only pentagonal bipyramidal U(VI). Fluorescence measurements on UC1P2-2 and UC1P2-3 exhibit emission from the uranyl moieties with classical vibronic fine-structure.

Curium(III) borate shows coordination environments of both plutonium(III) and americium(III) borates.

Curium is the heaviest element that is relevant to the nuclear fuel cycle produced by neutron capture of lighter actinides followed by a b decay in nuclear reactors. Separation of curium from americium is desirable during the reprocessing of used nuclear fuel. However, Am and Cm possess extraordinarily similar ionic radii that only differ by 0.005 , making the separation of these two elements challenging. Curium is an underexplored element for a variety of reasons. First, for several decades after its discovery the one isotope available was Cm. Cm has a short half-life of 18 years and radiation damage in its compounds is very rapid. Cm was also available, but is even shorter-lived with a half-life of 163 days. The highly neutron-rich isotope, Cm, became available in small quantities in the late 1970s. This isotope too has serious issues despite its long half-life of 3.48 10 years because 8.3% of its decay is by spontaneous fission, and therefore even milligram amounts of Cm release large fluxes of neutrons. The lack of availability of material combined with the hazards of working with the different isotopes of curium has greatly curtailed the development of a fundamental and applied chemistry of curium. Evidence for this is that CmCl3, [3] Cm(IO3)3, [4] Cm[M(CN)2]3·3 H2O (M = Ag, Au), and [Cm(H2O)9][SO3CF3]3 [6, 7] are the sole inorganic compounds of curium for which single crystal structures are known. Like Gd, Cm has a half-filled f shell with seven unpaired electrons. However, the spin–orbit coupling is much stronger in Cm than in Gd, so its electronic properties are absolutely unique. Curium is the only f-block element, whose magnetic interactions can control its crystal structure. We have recently undertaken the study of the preparation of actinide borates with the aim of developing periodic trends that may aid in predicting the fate of actinides in nuclear waste repositories that are in salt deposits, such as the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico, USA. A similar repository is being considered in Germany. These deposits contain borate in high concentrations in intergranular brines, and landmark work by Reed and coworkers has shown that borate, not carbonate, is the primarily complexant for trivalent cations in these repositories. We recently showed that Pu and Am borates possess substantially different compositions, structures, and local coordination environments at the metal centers. Our prediction was that the chemistry of Cm would closely parallel that of Am, and that the borate compounds would be very similar given their nearly identical ionic radii and lack of redox activity. Herein, we show that this hypothesis is incorrect and that Cm borate simultaneously displays a coordination chemistry of both Pu and Am borates. Ln (Ln = La–Lu), Pu, and Am when reacted with boric acid do not yield a compound with the same composition as the Cm compounds reported herein. Crystals of Cm2[B14O20(OH)7(H2O)2Cl] were isolated from the reaction of CmCl3 with molten boric acid at 240 8C. The crystals take the form of small tablets (around 40 mm) with very pale yellow coloration. Single-crystal X-ray diffraction experiments on Cm2[B14O20(OH)7(H2O)2Cl] pose a number of interesting challenges based on the fact that this compound combines one of the heaviest elements in the periodic table with some of the lightest elements. Clearly curium is responsible for the majority of the X-ray scattering. We have found in layered borates that the heavy elements can be arranged with higher symmetry than the rest of the extended network. For example, in Pu2[B12O18(OH)4Br2(H2O)3]·0.5H2O, which has some structural similarities with Cm2[B14O20(OH)7(H2O)2Cl], the two different Pu III centers appear to be crystallographically related, but in fact differ by one coordinating water molecule. The relationship between the plutonium centers leads to the erroneous conclusion that the structure possesses higher symmetry than it actually does. We find a very similar phenomenon in Cm2[B14O20(OH)7(H2O)2Cl], that is, there seem to be two different curium sites [*] M. J. Polinski, S. Wang, Prof. Dr. T. E. Albrecht-Schmitt Department of Chemistry and Biochemistry and Department of Civil Engineering and Geological Sciences, University of Notre Dame Notre Dame, IN 46556 (USA) E-mail: talbrec1@nd.edu

Actinide compounds under pressure

Abstract An overview of pressure-induced structural phase transitions and compressibility of actinide compounds will be given. Systematic trends in the nature of the high-pressure phases, the transition pressures, the hysteresis to retransformation on pressure release, and the compressibility are observed in the family of AnX compounds of B1 (NaCl) structure type. The dioxides studied up to now form high-pressure phases of PbCl2 type. UX2 compounds of Fe2As type also tend to have PbCl2 type high-pressure phases. The Th3P4 type compounds studied up to now did not transform up to 50 GPa. The same is true for ThOS and UOSe up to about 45 GPa. Comparison with rare earth compounds will be made where possible.

Raman spectra of some actinide dioxides and of EuF2

Abstract Raman spectra have been obtained for the first time from polycrystalline samples of NpO2, PuO2 and EuF2. Raman spectra were also obtained from polycrystalline samples of ThO2, UO2 and CeO2 for comparison with published results. Each of these compounds exhibits the CaF2 (fluorite)-type cubic crystal structure. The observed Raman active phonon (T2g) frequencies were used in conjunction with reported optically active IR phonon (T1u) frequencies to calculate the force constants for both the cation-anion stretching (K) and anion-anion repulsion (F).

Synthesis and characterization of crystalline phosphates of plutonium(III) and plutonium(IV)

Abstract The formation of both PuPO4 and PuP2O7 from reactions of PuO2 with (NH4)2HPO4 or BPO4 was studied by X-ray diffraction and Raman spectroscopy. The oxidation of plutonium(III), in PuPO4, to plutonium(IV), in PuP2O7, by air in the presence of BPO4 and the thermal reduction of PuP2O7 to PuPO4 were demonstrated. In addition plutonium(III) trimetaphosphate Pu(PO3)3 was synthesized and characterized. Attempts to prepare plutonium(IV) orthophosphate, Pu3(PO4)4, by high temperature reactions were unsuccessful; instead, mixtures of PuP2O7 and a new phase identified tentatively as “(PuO)2P2O7” were obtained.

X-ray diffraction of berkelium metal under pressure to 57 GPa

Abstract An investigation of the structural behavior of berkelium metal under pressure has been carried out up to 57 GPa. Evidence for a sudden decrease in volume (“collapse”) of the berkelium metal with increasing pressure up to 22 GPa was not observed. Instead, three different metal phases were observed over the range of applied pressure. The initial double-hexagonal close-packed structure of the berkelium metal first transformed to an f.c.c. structure at 8 GPa, which above 22 GPa converted to a third phase. Diffraction data obtained from this latter phase can be indexed on the basis of the α-U-type (orthorhombic) structure. A bulk modulus of 30(10) GPa was estimated from the relative volume V V 0 and pressure data for berkelium metal below 22 GPa. (The error is given in parentheses.)