A. Wojakowski

Anisotropies of the electrical resistivity and Hall effect in UAsSe

Abstract We show that anisotropy of the resistivity in UAsSe proves that its total resistivity contains both an isotropic spin disorder component (controlled by RKKY exchange interaction) and an anisotropic single impurity Kondo-like ( T K = 50 K) component. They are present in both paramagnetic and ordered states. This uniaxial and collinear ferromagnet (below 113 K) exhibits high magnetocrystalline anisotropy. Examination of the Hall effect in the paramagnetic phase (up to 430 K) showed that the isotropic normal Hall coefficient R 0 = −8.8×10 −4 cm 3 C −1 (0.47 e − fu −1 in the single band model) is accompanied by a highly anisotropic spontaneous Hall coefficient. This is equal to 0.78 and 3.0 cm 3 C −1 for R s ac and R s ab (magnetic field along the c or b axis) respectively.

Crystallochemistry and Kondo-like behaviour of the thorium and uranium arsenoselenides

Abstract We have grown single crystals of uranium and thorium arsenoselenide and arsenosulphide by the chemical vapour transport method. The crystals show a Kondo-like effect and its contribution to the resistivity was estimated by the determination of the reciprocal residual resistivity ratio (RRRR) ρ (4,2)/ ρ (300). The ratio varied from 0.70 (for UAsS) to 2.10 (for ThAsSe). A high resolution transmission electron microscopy study of ThAsSe crystals reveals a perfect ordering of cations. The X-ray diffraction examination showed that the unit cells of the examined crystals belong to a tetragonal system with the PbFCl-type structure (space group P4/nmm (no. 129)). Anomalously large anisotropic displacement factors have been observed for all the atoms composing the compounds examined. An interdependence between the displacement factor and RRRR has allowed to ascribe both phenomena to the same source. It indicates that the two-level system Kondo model is most suitable for the theoretical description of the systems.

Magnetic behaviour of NpAs2 from Mössbauer spectroscopy

Abstract The Mossbauer hyperfine spectra of the 60 keV resonance of 237 Np in powder and single crystal absorbers of NpAs 2 were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs 2 . The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μ B at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.

Rare-earth doping of the Mo3Se4 superconductor

Abstract Superconducting properties of the binary Chevrel phase Mo3Se4 have been modified by rare-earth doping. Very small quantities of lanthanides make the Tc of this compound to increase by almost 1 K (from 6.45 to 7.2 K) for the bigger ions, while no effects are seen for heavy rare earths (smaller ions). A clear correlation has been established between Tc and the ionic radius of the dopant. The domain of existence of the doped phase (denoted herein as ( Mo 3 Se 4 )REx) was determined from X-ray diffraction and χAC susceptibility techniques. It is bounded by a Mo/Se ratio equal to or slightly larger than 0.75 and by a maximum content of rare earth of about 1–1.5 at% RE (compared to 6–7 at% RE in the ternary phase RExMo6Se8). Crystallographic, superconducting and some magnetic data are given for several members of the series (REY, La, Ce, Nd, Sm, Eu, Gd, Dy, Ho, Er and Yb). Three interesting cases are outlined since they behave completely different as in the ternary-phase counterparts: cerium and divalent europium show some of the largest increase of Tc, while ytterbium (in a trivalent oxidation state) does not affect the critical temperature of the matrix.

A neutron study of the magnetic ordering in a NpAs2 single crystal

Abstract A neutron diffraction study on a small NpAs 2 single crystal is reported. NpAs 2 orders at T N = 52 K with a pure sine wave modulation propagating along a ≤100≥ direction of the tetragonal unit cell. The value of the wave vector is incommensurate with the lattice ( k = ≤0.141,0,0≥) and is temperature independent down to T IC = 18.5 K where a first-order transition towards a commensurate ferromagnetic state occurs. At all temperatures the magnetic moments are aligned along the c -axis with a value of (1.45±0.1)μ B at T = 5 K .

Magnetic properties of neptunium diarsenide

Abstract The magnetic properties of a collection of single crystals of NpAs2 have been investigated in the temperature range 4.2-300 K and in applied fields up to 100 kOe. For small magnetic fields (3 kOe), NpAs2 is ferromagnetic up to TIC=18 K, antiferromagnetic from 28 K to TN=52 K, then paramagnetic. Both transitions are first order. When the applied field increases TIC is shifted towards TN. The antiferromagnetic phase disappears for H#62;30 kOe. The ferromagnetic range is characterized by a very large anisotropy. In the paramagnetic state, NpAs2 has an effective moment 1.88 μB.

Low-temperature specific heats of some uranium ternary compounds UAsY (Y = S, Se, Te)

The specific heats of the UAsY (Y = S, Se, Te) ternary compounds have been studied in the temperature range 1–300K. λ-type anomalies are reported at 125.8, 108.8, and 62.8 K, respectively, corresponding to ferromagnetic ordering. The experimental results are analyzed in terms of the Debye model and electronic contributions. Debye temperatures, electronic specific heat coefficients, and magnetic entropies are derived. A comparison is made with the isostructural binary compounds UX2 (X = P, As, Sb, Bi).

Mössbauer spectroscopy of Np3As4. Interpretation of the spectra and of the magnetic behaviour in a crystal field model

Abstract Np 3 As 4 (Th 3 P 4 -type structure) is ferromagnetic below T c =81 K. In this work, we present results of Mossbauer experiments performed between 4.2 and 150 K. The isomer shift with respect to NpAl 2 is 5.9 mm/s, suggesting a Np 4+ valence state. The hyperfine field at 4.2 K is 377 T, corresponding to an ordered moment μ ord =1.75μ B . The quadrupolar interaction is very small both in the ferromagnetic and in the paramagnetic state ( e 2 qQ The low value of μ ord (free ion value 3.2μ B ) as well as the absence of a significant quadrupolar component are discussed in terms of crystal field effects. In particular, the contributions to the electric field gradient of both the lattice and the 5f electrons have been considered, taking into account the distortion of the 5f charge cloud in presence of a molecular field below T c . We show that the reduction of μ ord is linked to a rotation of the moment towards the [111] direction as in the case of the isostructural compound U 3 P 4 . The crystal field ground state which accounts for this effect leads to a vanishingly small quadrupolar interaction both in the ordered and in the paramagnetic states. As a consequence, the lattice contribution to the electric field gradient, itself, is likely to be very small. Two mechanisms are proposed for this lattice term reduction: either a compensation by the conduction electrons or a reduction of B 0 2 due to the competing effects of the ligands surrounding Np 4+ . Lastly, the model gives a good agreement with the temperature variation of the hyperfine field.

Low-temperature specific heat for off- and near-stoichiometric UAsSe

Abstract Two UAsSe single crystals with slight variations of the As/Se content ratio have been investigated. The development of the low-T upturn in the resistivity is accompanied by smooth decrease of the Curie temperature. The sample with T C =116.9 K and almost invisible upturn could be regarded as a near-stoichiometric, slightly disordered one, whereas the specimen with T C =101.5 K appears to be an off-stoichiometric, more strongly disordered one. Clear differences between both electronic and lattice contributions to the specific heat for off- and near-stoichiometric UAsSe crystals have also been found. We ascribe the smaller low-T specific heat for the off-stoichiometric sample in comparison to the near-stoichiometric one to a weaker hybridization between 5f and conduction-electron states accompanied by the relative shift of 5f band with regard to EF.

Phase equilibria in the La - Mo - Se system at in the vicinity of and

Phase relations prevalent in the La - Mo - Se system at in the vicinity of the ternary (Chevrel) phase and of the binary phase, have been established. Characteristic features of this system are as follows: (i) Two isostructural, but mutually immiscible, solid solutions covering considerable compositional intervals exist. One of them, defined as La-intercalated , spreads over a triangular surface, with its origin at the binary and developing into the ternary system, as far as La. One edge of this triangle points towards the isostructural ternary phase . The second three-component solid solution is based on the Chevrel-type phase and takes the shape of a rectangular surface about 2 at% La wide, with 6.67 at% La as the mean value. The domain runs along the line , from x = 0 (40 at% Mo), characteristic of the exact stoichiometry 1:6:8, to x = 0.75 (45 at% Mo). (ii) The continuous solid solution of the type , expected to occur between the binary and the ternary , is absent. The domains of both solid solutions have been established using x-ray powder diffractometry and the evolution of the critical temperatures characteristic for each of the phases.