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Revealing charge anisotropies in metal compounds via high-purity x-ray polarimetry

Lena Scherthan1, Juliusz A. Wolny1, Isabelle Faus1, Olaf Leupold2, Kai S. Schulze3,4,5, Sebastian Höfer3,4, Robert Loetzsch3,4, Berit Marx-Glowna3,4,5, Christopher E. Anson6 et al.

Annie K. Powell6,7, Ingo Uschmann3,4,5, Hans-Christian Wille2, Gerhard G. Paulus3,4,5, Volker Schünemann1, and Ralf Röhlsberger3,5,4,2

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Phys. Rev. Research 8, 023297 – Published 15 June, 2026

DOI: https://doi.org/10.1103/rj8d-86dj

Abstract

Linear polarization analysis of hard x-rays is employed to probe electronic anisotropies in metal-containing complexes with high selectivity. We use polarization-resolved nuclear forward scattering (PR-NFS) of synchrotron radiation at the 14.4 keV nuclear resonance of Fe57 to determine electric field gradients (EFGs) in an iron(II) spin-crossover compound as they evolve during a temperature-dependent high-spin/low-spin transition. A pair of crossed Si(840) channel-cut crystals provides a high-purity x-ray polarimeter that suppresses the nonresonant background by nearly nine orders of magnitude and enables selective detection of nuclear resonant σ→π scattering. Using the spin-crossover complex Fe(PM-BiA)2(NCS)2 as a model system, we determine the orientation of the EFG tensor in the high-spin and low-spin states and follow its reorientation across the gradual thermally driven spin transition. By choosing suitable sample orientations, the polarization selectivity enhances the signal of a high-spin minority fraction of a few percent at low temperature. It reveals that the high-spin EFG at 120 K is not randomly distributed and does not align with the dominant low-spin EFG when these high-spin centers form a minority phase embedded in a low-spin matrix. The combination of PR-NFS with dynamical NFS simulations and density-functional calculations thus provides a sensitive, element- and site-selective method to quantify electronic anisotropies and their changes across phase transitions in Mössbauer-active metal compounds.

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Phys. Rev. Research 8, 023297– Published 15 June, 2026
  • Received 25 September 2017
  • Accepted 4 May 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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