G. H. Lander

Magnetic studies of actinides—evidence for localized 5f electrons

The position of the 5f elements in the periodic table suggests that the electronic properties of these elements (and their compounds) will resemble those of the lanthanide series. However, the extended nature of the 5f wave functions leads to fundamental differences between 4f and 5f systems. In this review the evidence for ’’localized’’ magnetism will be presented. Results of magnetization, Mossbauer, neutron and low‐temperature x‐ray experiments on Np, Pu, and Am compounds will be used to illustrate both the similarities to and differences from lanthanide magnetism.A determination of the ground‐state 5f electron wave functions is, in principle, possible by measuring the neutron magnetic cross section. The interpretation of such experiments on UO2, USb, and PuP requires a knowledge of the radial extent of the 5f electrons, which we obtain from relativistic Dirac‐Fock calculations, and the use of the tensor‐operator formalism to treat the spin‐orbit interaction. This interaction in Pu3+ (a 5f5 configurati...

Magnetic Phase Diagram of the UAs1−xSx System

The magnetic phase diagram of the UAs1−xSx system (0≤ x ≤1) has been investigated by neutron diffraction with powder samples. All compositions are fcc (NaCl type). For x = 0 (UAs), the type‐I anti‐ferromagnetic ordering (TN = 127°K) has ferromagnetic sheets stacked + − + − that change at 0.5 TN to the type‐IA ordering with the sheets stacked + + − −. This transition is accompanied by an increase in the magnetic moment from 1.9 to 2.2 μB per uranium atom.

Magnetoelastic interactions in UO2

Neutron diffraction measurements of the elastic magnetic scattering cross section from antiferromagnetic UO2 show additional nucelar intensity below TN=30.8 K. We have examined the possibility of analyzing the additional scattering in terms of homogeneous distortions, which involve shifts of the oxygen atoms from their fluorite lattice sites. The behavior arising from the presence of these homogeneous distortion modes formed the basis for Allen’s theory of a cooperative Jahn‐Teller effect in UO2. However, an analysis in terms of these homogeneous distortions cannot explain the neutron data. But, by extending Allen’s concepts to include inhomogeneous deformations, corresponding to a zone boundary q= (π/a)(1,0,0) phonon, excellent agreement is obtained between theory and experiment. The oxygen displacement is 0.014(1) A from the fluorite lattice positions, and the inhomogeneous deformation (T2(Q1)−T1g) does not reuire a change in the dimensions of the unit cell. The essential features of Allen’s theory for UO2 can still be maintained.Neutron diffraction measurements of the elastic magnetic scattering cross section from antiferromagnetic UO2 show additional nucelar intensity below TN=30.8 K. We have examined the possibility of analyzing the additional scattering in terms of homogeneous distortions, which involve shifts of the oxygen atoms from their fluorite lattice sites. The behavior arising from the presence of these homogeneous distortion modes formed the basis for Allen’s theory of a cooperative Jahn‐Teller effect in UO2. However, an analysis in terms of these homogeneous distortions cannot explain the neutron data. But, by extending Allen’s concepts to include inhomogeneous deformations, corresponding to a zone boundary q= (π/a)(1,0,0) phonon, excellent agreement is obtained between theory and experiment. The oxygen displacement is 0.014(1) A from the fluorite lattice positions, and the inhomogeneous deformation (T2(Q1)−T1g) does not reuire a change in the dimensions of the unit cell. The essential features of Allen’s theory for ...

Magnetically Induced Lattice Distortions in the Neptunium Monopnictides

The lattice parameters of the neptunium monopnictides (NaCl crystal structure) have been measured between 5 and 300°K by X‐ray diffraction. NpN becomes ferromagnetic at 87°K. At TC a rhombohedral distortion, which indicates that the 〈111〉 is the easy axis, is observed. At 5°K, the rhombohedral angle is 60.46 ± 0.02°. NpP exhibits a tetragonal distortion at 74°K, at which temperature the antiferromagnetic 3+, 3− structure becomes commensurate with the lattice. At 5°K c/a = 0.9958 ± 2. NpAs becomes tetragonal at TN = 175°K when the material has a 4+, 4− structure. However, NpAs returns to being a cubic at 142°K at which temperature the magnetic structure is the simple + − (type I) configuration. The ‘return’ to cubic is first order; the volume of the unit cell expands by 0.23%. NpSb, which orders antiferromagnetically at 207°K with the type I structure remains cubic in the ordered regime.

Magnetic properties of UC--UN solid solutions

Both UC and UN are semi‐metallic materials with the NaCl crystal structure and lattice parameters 4.96 and 4.89A, respectively. UC is nonmagnetic with a susceptibility that is temperature independent, whereas UN becomes antiferromagnetic at 53°K with ordering of the first kind. Solid solutions of the form UC1−xNx exist for 0 ≤ x ≤ 1. Neutron‐diffraction experiments have been performed on five samples with 0.85 ≤ x ≤ 0.95. Over this region of x both TN and the ordered magnetic moment per U atom decrease linearly from TN = 53°K and 0.75 μB for UN (x=1) to TN = 30°K and 0.35 μB for x = 0.88. The composition x = 0.86 shows no ordering in the neutron experiments. The absence of magnetic order for x < 0.88 has been confirmed by magnetization measurements. In the range of x examined the effective paramagnetic moment is 3.25 ± 0.10 μB/U atom, close to the value of ∼ 3.1 μB for UN.