[en] The magnetic properties of the cubic Laves phases (C 15 structure) NpMn₂, NpFe₂, NpCo₂, and NpNi₂ have been studied from 4 to 300⁰K by means of magnetization, neutron-diffraction, and nuclear-γ-ray resonance (Moessbauer) measurements. The respective ordering temperature (and types of ordering) are 18⁰K (ferro), approximately 500⁰K (ferro), 15⁰K (antiferro) and 32⁰K (ferro). Magnetic moments are present on the Np atom in all compounds and on the Fe atom in NpFe₂. Certain magnetic properties, such as the large anisotropy and the relationship between the magnetic moment and the hyperfine field, are best interpreted in terms of localized 5f electrons, whereas other properties suggest hybridization between the 5f and 3f electrons

[en] A solubility limit of approx. 6 mol % PuO₂ in sodium disilicate (Na₂O.2SiO₂) glass has been determined. Magnetic susceptibility measurements on these glasses yield approximate Curie-Weiss behavior, in contrast to the temperature-independent susceptibility of crystalline PuO₂. This result is interpreted to indicate that the local site symmetry around the Pu ion in the sodium disilicate glass is much different than in crystalline PuO₂. The effective paramagnetic moments determined from the temperature dependence of the susceptibility are found to be consistent with calculated free-ion values based on the most likely 5f electron configurations

[en] X-ray diffractometry measurements were made as a function of temperature on a series of polycrystalline LaV/sub x/Nb/sub 1-x/O₄ compounds (0 < x < 0.3) to determine the effect of V substitutions on the temperature of the monoclinic/tetragonal transformation. The purpose was to provide basic information relating to crystal lattice stability in ABO₄ compounds that are either candidates or are appropriate models for candidate materials for hosting nuclear-waste ions. Partial substitution of V⁵⁺ on the Nb⁵⁺ site significantly lowers the tetragonal scheelite (I4₁/a) to monoclinic fergusonite (I2/c) transformation, from 770⁰K in LaNbO₄ to approximately 215⁰K for LaV₀₂₅Nb₀₇₅O₄ (the solubility limit is close to x = 0.35). The transformation is displacive, of second order, involving two coupled order parameters. Heat capacity measurements on LaV₀₂₅Nb₀₇₅O₄ showed that the specific heat anamoly at the transformation point is extremely small. It is concluded that the two polymorphic forms of LaV/sub x/Nb/sub 1-x/O₄ have very nearly the same free energies over a substantial range of temperature below the transformation

[en] The magnetization of UMn₂, UFe₂, UCo₂, UNi₂, PuMn₂, PuFe₂, PuCo₂, and PuNi₂ was measured from 4 to 300⁰K in fields up to 14 kOe. The susceptibility of UMn₂ shows a small maximum near 240⁰K (which may indicate an antiferromagnetic transition), in agreement with previous results. Data for UFe₂, UCo₂, and UNi₂ are not in good agreement with earlier work; the ferromagnetic ordering temperature (58⁰K) of a single-crystal sample of UFe₂ is lower than any reported value, UNi₂ orders ferromagnetically at 30⁰K and UCo₂ may order less than 5⁰K. In contrast, PuMn₂, PuCo₂ and PuNi₂ have small weakly temperature-dependent susceptibilities. PuFe₂ is ferromagnetic at room temperature, in agreement with previous Moessbauer results, and has a saturation moment of approximately 2.6 μ/sub B//mole

[en] The magnetization of UMn₂, UFe₂, UCo₂, UNi₂, PuMn₂, PuFe₂, PuCo₂, and PuNi₂ from 4 to 300⁰K in fields up to 14 kOe were measured. The susceptibility of UMn₂ shows a small maximum near 240⁰K (which may indicate an antiferromagnetic transition) in agreement with previous results. The data for UFe₂, UCo₂, and UNi₂ are not in good agreement with earlier work; the ferromagnetic ordering temperature (158⁰K) of a single crystal sample of UFe₂ is lower than any reported value, UNi₂ orders ferromagnetically at 30⁰K, and UCo₂ may order below 5⁰K. In contrast, PuMn₂, PuCo₂, and PuNi₂ have small weakly-temperature-dependent susceptibilities. PuFe₂ is ferromagnetic at room temperature, in agreement with previous Moessbauer results, and has a saturation moment of approximately 2.6 μ/sub B//mole. (U.S.)

[en] The room-temperature lattice parameters of a series of CeNb/sub 1-x/V/sub x/O₄ and NdNbO₄ compounds (0less than or equal to x less than or equal to 1) have been measured. Substitution of vanadium for niobium in both CeNbO₄ and NdNbO₄ drives the structure from monoclinic (fergusonite type) to tetragonal (scheelite type) at x = 0.296 and 0.346, respectively. In both systems, there is an expansion of the long-axis dimension of the fergusonite structure as x increases, despite the fact that vanadium is a smaller ion than niobium. This leads to an anomalous variation of the unit cell volume with concentration. The composition dependence of the strain parameters, which are a measure of the relative distortion of the monoclinic lattice, is found to fit a Landau mean-field model, thus demonstrating a thermodynamic correspondence between composition and temperature. 13 references, 4 figures, 4 tables

[en] To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of A³⁺P/sub 1-x/⁵⁺V/sub x/⁵⁺O₄ compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO₄) have the monoclinic Monazite structure (P2₁/n) for A = La, Ce-Gd, and the tetragonal Zircon structure (I4¹/amd) for A = Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO₄, and the Zircon structure for A = Ce - Lu. When the end members are isostructural, e.g., LaPO₄/LaVO₄, Monazite, YbPO₄/YbVO₄, Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vegard's Law is followed. When the end members are not isostructural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO₄ compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 11 references, 6 figures, 1 table

[en] This paper reviews recent high-field (up to 80 kOe) magnetization, nuclear-gamma-ray resonance, and neutron diffraction measurements on a number of actinide ferromagnets with the C-15 Laves phase crystal structure. NpAl₂ and NpOs₂ are an interesting contrast; NpAl₂ behaves as a localized 5f system whereas NpOs₂ exhibits properties usually associated with itinerant ferromagnets. The second part of the paper reports similar measurements on the series AnFe₂, where An = U, Np, Pu, and Am. The properties of these compounds suggest an increasing localization of 5f electrons as one proceeds from uranium to americium. The Pu ion in PuFe₂ is definitely trivalent 5f⁵, but the observation of a small negative moment on the Am site in AmFe₂ implies a partial occupancy of the Am²⁺ 5f⁷ state in this compound. This is the first indication of a mixed valence configuration in an actinide system

[en] An investigation was made of a series of binary americium alloys with AB₂ stoichiometry (B = Al, Co, Fe, Rh and Ru) prepared from the ²⁴³Am isotope. The C-15 (MgCu₂-type) structure appears in AmAl₂, AmCo₂, AmFe₂, and AmRh₂, whereas AmRu₂ is isostructural with AmOs₂ which has the C-14 (MgZn₂-type) structure. The magnetic properties of the cubic americium laves phases were studied by means of magnetization and nuclear gamna-ray resonance (Moessbauer effect) measurements between 2.5 and 300⁰K. All the cubic Laves phases studied, except AmFe₂, exhibit almost temperature-independent paramagnetism. The AmFe₂ sample is ferromagnetic with an estimated Curie temperature of approximately 400⁰K. The hyperfine field at the Am site in all the compounds is small. The bulk magnetic moment of AmFe₂ (approximately 3 μ/sub B//F.U.) is thus associated only with the Fe atoms. The magnetic properties of the compounds are consistent with the assumption that the Am ion is in the +3 state (5f6)

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