J. Moser

Mössbauer spectroscopy with actinide elements

Although formally equivalent to the lanthanide (4f) elements, the light actinides show a much more varied behaviour due to the larger spatial extent and ionizability of the 5f electrons. The application of Mössbauer spectroscopy for the determination of electronic properties of the actinides is outlined. Emphasis is put on high pressure Mössbauer experiments using the 60 keV transition in237Np to study questions of delocalization of 5f electrons.

Mössbauer spectroscopy of actinide intermetallics

Abstract Due to their wider radial extent the 5f electrons may form bands of different width and hybridization in metallic compounds of the light actinides. This leads to a broad spectrum of magnetic properties ranging from the localized magnetism of the lanthanides to the itinerant electron magnetism often found in transition metal compounds. Also, the influence of the crystalline electric field tends to be more pronounced than in rare earth compounds, but is usually not as dominant as in the 3d series. Magnetic structures and the question of 5f electron delocalization will be reviewed with respect to actinide Mossbauer data and new results will be presented. In particular the influence of applying external pressure will be discussed.

High-pressure Mössbauer study of the Curie temperatures and transferred hyperfine fields at 151Eu and substitutional 119Sn in EuS and EuSe

Abstract Changes with pressures up to 55 kbar of the Curie temperatures and the 151 Eu and 119 Sn hyperfine parameters were investigated in the europium monochalcogenides EuS and EuSe doped with small amounts of substitutional Sn 2+ . In both (Eu 1- x Sn x ) S and (Eu 1- x Sn x ) Se with 0.01 ≤ x ≤ 0.05, the observed strong increase of T c is correlated w ith a strong increase of the transferred hyperfine fields. EuSe is found to order ferromagnetically above 14 kbar.

High pressure studies of actinide intermetallics

Magnetic properties of NpOs2,NpAl2. NpCo2Si2, NpAs, and NpSn3 are investigated by high pressure Mössbauer spectroscopy using the 60 keV transition in237Np. A wide spectrum between itinerant (band-like) electron behaviour and highly localized 5f electron magnetism is observed. Delocalization of 5f electrons may be determined by direct 5f-5f overlap between neighbouring Np ions or may be mediated by the valence electrons of the alloying partner.

MöSSBAUER STUDY OF HYPERFINE INTERACTIONS IN NpAs AT HIGH PRESSURE

Using the 60 keV Mossbauer transition in 237Np we have investigated NpAs at pressures up to 85 kbar in the temperature range between 4.2 and 200 K. At high pressures two magnetically split subspectra are observed which suggest a longitudinal-wave modulation of the two ordered moments.

High pressure Mössbauer spectrometer for the high-resolution 93.3 keV resonance in 67Zn

Abstract A high pressure, low temperature Mossbauer spectrometer for the high-resolution 93.3 keV resonance in 67 Zn is described. The pressure is generated by applying the opposed anvil technique. Using B 4 C anvils and a sandwich gasket quasihydrostatic pressures up to 6 GPa were obtained for the required large samples of 7 mm diameter and 2 mm thickness. The piezoelectric Doppler drive is mounted on top of the pressure clamp. The whole system can be cooled to liquid He temperatures. The spectrometer was used to investigate at 4.2 K the pressure dependence of the Mossbauer parameters of Zn metal.

Changes of valence state of Yb in YbCuAl at high pressure: A Mössbauer study

The 84.3 keV Mössbauer transition in170Yb was used to investigate the intermediate valent intermetallic compound YbCuAl at pressures up to 130 kbar and in the temperature range between 1.6 K and 77 K. The electric quadrupole interaction parameters |eQVzz| and η, as well as their temperature variation are drastically dependent on pressure: |eQVzz| is at 4.2 K larger by almost a factor of three at 130 kbar when compared to ambient pressure. On the other hand, |eQVzz| drops by more than a factor of two at 130 kbar when the temperature is raised from 1.6 K to 77 K. A much smaller temperature dependence of quadrupole coupling is present at ambient pressure. Both these effects give strong evidence for a valence transition towards the 4f13 (Yb3+) configuration under applied pressure. At 4.2 K this transition is completed near ∼50 kbar. No magnetic order could be detected down to 1.6 K over the whole pressure range. Some line broadening is observed at high pressures and low temperatures which most probably is related to paramagnetic relaxation phenomena and supports the claim that the Yb ion is in a magnetic state under these conditions.

High pressure mössbauer studies of magnetic actinide systems

Due to the wider radial extent of 5f electrons when compared to their 4f counterparts, intermetallics of the light actinides show a broad spectrum of magnetic properties ranging from the localized magnetism of the lanthanides to the itinerant magnetism often found in transition metal compounds. One parameter which strongly influences the magnetic character of the compound is the actinide-actinide separation which can experimentally be varied by the application of high pressure. The question of 5f electron delocalization will be reviewed with respect to Moesshauer high pressure data on NpCo2si2, NpAl2, NpOs2 and new results will be presented of NpAs. The connection of hyperfine parameters with results of X-ray diffraction studies will be discussed.

Volume dependence of magnetic properties in DyAl2

Abstract We have studied the volume dependence of the electronic structure and the magnetic properties of DyAl 2 by high pressure Mossbauer experiments up to 83 kbar. The found increase of the Curie temperature with pressure can be explained by using a modified RKKY-model. The magnetic hyperfine field B hf and the electric field gradient (EFG) are insensitive to a reduction in volume. This result demonstrates that the localized character of the 4f-electrons remains unaffected. From the temperature variation of B hf and EFG we estimate the exchange field by involving a calculation of the crystal field level scheme.

High pressure Mössbauer investigation of 67ZnS

Abstract The 67 Zn (93 keV) Mossbauer spectrum of ZnS has been investigated in the pressure range up to 33 kbar at 4.2 K. A linear increase with pressure of the s-electron density at the Zn nucleus is seen.