Elucidation of local structure deformation in κ−(BEDT−TTF)2Cu[N(CN)2]Br by x-ray fluorescence holography

$\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Br} (\ensuremath{\kappa}\text{\ensuremath{-}}\mathrm{Br})$ is an organic superconductor that changes to an Anderson-type insulator due to the random defects introduced by x-ray irradiation. In this study, we directly investigated the effects of irradiation on the local structures in the anion layer of $\ensuremath{\kappa}\text{\ensuremath{-}}\mathrm{Br}$ using x-ray fluorescence holography. The local structures around the Cu atoms in the anion layer were directly reconstructed from holograms. Prior to radiation damage, the reconstruction clearly shows the atomic images of the Br and N atoms that are directly coordinated to Cu. In the irradiated samples, the N images in the reconstructions were strongly suppressed as a result of the positional fluctuations of N introduced by the defects induced by irradiation. Using the previously proposed ``bond-shifted'' model and the thermal vibration amplitudes calculated from molecular dynamics simulations, we simulated the holograms and the atomic reconstructions of the anion layer. The simulations show that the ``bond-shifted'' model, along with thermal vibrations at 100 K, suppresses the N atomic images, as was observed in the experimental reconstructions.