NpFeAsO and PuFeAsO - a New Insight into Oxypnictides

The rare-earth iron oxyarsenide compounds have been known for at least a decade. However considerable interest in these and related materials arose in 2008 after the discovery of superconductivity in doped LaFeAsO. The reason for this frenzy of activity is clear; the superconducting transition temperatures are very high, second only to the cuprates. Polycrystalline samples with nominal composition NpFeAsOF and PuFeAsO were synthesized by solid state reaction using Fe3O4 (Alfa Aesar 99.997%), elemental Fe, and high purity crystals of NpAs and PuAs as starting materials. The thoroughly mixed powder was pressed into a pellet, sealed in an evacuated silica ampoule and heated at 900oC for 48 hours. All operations were carried out in a radioprotected glovebox with low oxygen and water concentrations. High resolution X-ray diffraction (XRD) data were measured as a function of temperature on the powder diffraction beamline, ID31 at the ESRF. Figure 1 shows an example of the XRD pattern for the NpFeAsO sample, measured at 300 K. All peaks in the XRD spectra were indexed to the ZrCuSiAs-type structure (P4/nmm, s.g. 129). On cooling to 5 K no orthorhombic distortion was observed within the resolution of our NpFeAsO data (inset (B)), in contrast to all of the RFeAsO compounds such as SmFeAsO (inset (A)). Inset C shows the superconducting transition temperature (TC) as a function of Fe-As-Fe bond angle for a series of LnFeAsO1-xFx compounds and also shows the angle determined for NpFeAsO. The specific heat was measured in a Quantum Design PPMS system via the relaxation method. Two clear transitions can be identified for NpFeAsO (blue circles); a sharp feature at about 60K, which we attribute to the AF ordering of the Np, and a shoulder at 25 K that we attribute to the magnetic ordering of a NpO2 impurity phase. For PuFeAsO (red squares) AF ordering is observed at lower temperature, TN = 50K. Although one may expect the physical properties of Np(Pu)FeAsO to be similar to the lanthanide RFeAsO analogues, in reality this system behaves differently. In particular, we do not observe the structural transition, in agreement with the fact that neither transport properties (resisitivity and Hall effect) nor specific heat measurements reveal high temperature anomalies associated with in-plane magnetic ordering in the Fe-As layer. We suggest that the lack of orthorhombic structural distortion is caused by strong positive unaxial magnetic anisotropy energy. Transport properties (resistivity, Hall effect), magnetic susceptibility, Mössbauer spectroscopy and neutron diffraction results will be presented for NpFeAsO. ACKNOWLEDGMENT Np metal required for the fabrication of the compound was made available through a loan agreement between Lawrence Livermore National Laboratory (LLNL) and ITU, in the framework of a collaboration involving LLNL, Los Alamos National Laboratory, and the US Department of Energy. TK acknowledges the European Commission for financial support.

KLIMCZUK Tomasz;  SPRINGELL R.;  WALKER H.C.;  SHIK Alexander;  HILL A. H.;  GACZYNSKI Piotr;  GOFRYK Krzysztof;  KIMBER S. A. J.;  RITTER C.;  COLINEAU Eric;  GRIVEAU Jean-Christophe;  BOUEXIERE Daniel;  ELOIRDI Rachel;  CACIUFFO Roberto; 

2013-03-18

Plutonium Futures 2012

JRC69395