The valence-fluctuating ground state of plutonium

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Science Advances  10 Jul 2015:
Vol. 1, no. 6, e1500188
DOI: 10.1126/sciadv.1500188

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  • RE: The valence-fluctuating ground state of plutonium

    e_letter (2nd) to Science Advances

    A. P. Murani, 1 Chemin de Savoyeres, 38640 Claix, France.

    Over a year ago Janoschek et al. published a response [1] to some observations I made two years ago [2] on their neutron inelastic scattering study on δ-Pu [3]. I was unaware of their response until a chance meeting recently with one of the authors. I consider the response to be erroneous, unjustified and misleading. I thus feel obliged to write this 2nd e_letter in the interest of scientific accuracy and integrity as well as in the hope that it may help in our understanding of the magnetism of δ-Pu.
    My principal observation concerning the work in Ref. [3] is that, besides the broad magnetic scattering, peaking around ~ 90 meV, there is clear evidence of significant hydrogen scattering particularly at higher energies around ~ 150 meV. However, the authors insist that there is negligible hydrogen in their sample. Also the scattering around 150 meV does not follow the Q-dependence of hydrogen scattering observed in PuO2 [4]. They reaffirm that the “signal around 150 meV is indeed of magnetic origin” and the spectrum is in “good agreement with DMFT calculation.”
    One effective way for readers to appreciate the scattering due to hydrogen in the sample is from a close examination of the data in Fig. S4, from the supplementary material, that I take the liberty to reproduce here. In the following the arguments are based in reference to the data in this figure....

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    Competing Interests: None declared.
  • RE: Reply to Amir P Murani eLetter on 'The valence-fluctuating ground state of plutonium'
    • Marc Janoschek, Staff Scientist, Los Alamos National Laboraty
    • Other Contributors:
      • Eric D. Bauer, Staff Scientist, Los Alamos National Laboraty
      • Joe D. Thompson, Staff Scientist, Los Alamos National Laboraty
      • Jon M. Lawrence, Professor Emeritus, Los Alamos National Laboratory
      • Gerard H. Lander, Emeritus, European Commission, Joint Research Centre, Institute for Transuranium Elements

    In the following we reply to comments made by Amir P. Murani in an e-Letter to our recent neutron spectroscopy study on δ-plutonium (Pu) [1]. We will address each of his comments.

    (1) The possible presence of localized hydrogen modes:
    The principle objection listed by Murani is that the presence of hydrogen impurities in the δ-Pu sample may affect the analysis. This is a legitimate concern for Pu samples, as we have discussed in the supplementary material of Ref. [1]. However, based on our significant experience with H-impurities in actinide systems, we show in the following that this is not the case for the sample used in Ref. [1]. In early work on PuO2 [2], a strong signal from a local mode involving a H-impurity was found at ~ 90 meV. The energy of this mode will depend crucially on the lattice, and would be certainly different in Pu metal than in PuO2. However, the momentum (Q) dependence should be very similar, as in both cases it involves localized modes of hydrogen. Ref. [2] shows that this mode has a maximum around Q ~ 9 Å-1 (Q/4 = 0.7 Å-1), which agrees with calculations. In a further publication of 1999 [3], it was demonstrated that after treatment to remove hydrogen at Los Alamos National Laboratory the localized H-mode disappeared, leaving a single peak at 124 meV associated with the crystal-field excitation in PuO2.
    An examination of the Fig. S4 in the supplementary material of Ref. [1] shows clearly that both resonances discussed in the...

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    Competing Interests: None declared.
  • RE: ‘The valence-fluctuating ground state of plutonium’ by Janoscheck et al.

    e_letter to Science Advances

    A. P. Murani, 1 Chemin de Savoyeres, 38640 Claix, France.

    In the article ‘The valence-fluctuating ground state of plutonium’ Janoscheck et al. (1) have reported interesting high-energy neutron scattering data on δ-Pu. This metal, considered non-magnetic (2), now reveals the long-awaited phenomenon of magnetic valence fluctuations within the 5f shell, similar to the 4f shell of α-Ce which was also for many years considered to be a non-magnetic metal (3), except that both δ-Pu and α-Ce have enhanced low temperature constant susceptibility χ0 and enhanced linear specific heat coefficient γ at low temperatures that distinguish them from other classic non-magnetic metals or materials. Valence fluctuations phenomena have been observed in a number of 4f inter-metallic compounds of Ce, Sm, Eu, Tm, Yb …(4).

    Here I wish to share some observations on this work on δ-Pu.

    1. The principal remark is that a significant part of the measured spectrum, particularly at higher energies, does not appear to be all magnetic. Allowing the limits of color contour plots for quantitative assessment, the data in Fig. 1E and, more clearly, Fig S4B indicate that an energy cut centered on the higher energy peak at ~ 150 meV does not follow the 5f5 (δ-Pu ) magnetic form-factor. This contrasts with the energy cut at ~ 90 meV, as shown in Fig. 2 (1), that follows roughly the magnetic form factor of δ-Pu. One therefore infers strong pollution of the...

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    Competing Interests: None declared.