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

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 $^{57}$Fe 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. ... mehrThe 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.