Anomalous Splitting of ¹H-NMR Spectra in λ-(BEDT-TSF)₂FeCl₄

Abstract

For an organic π-d metal λ-(BEDT-TSF)₂FeCl₄, where BEDT-TSF is an abbreviation of bis(ethylenedithio)tetraselenafulvalene, we have measured ¹H-NMR to clarify the quite anomalous ferroelectric-like metallic states below T_FM = 70 K recently found in the microwave conductivity and specific heat measurements as well as the metal-to-insulator transition at T_MI = 8.3 K associated with antiferromagnetic ordering. Just at the temperature T_FM, the second-order ferroelectric-like phase transition temperature, a single peak at higher temperatures of 100 K and 80 K starts to split into three asymmetric peaks, which are analyzed by Gaussian deconvolution with three components. It is considered that this anomalous splitting of the NMR spectra is attributed to the hyperfine field between the proton nucleus and π electrons. This anomaly at T_FM is discussed in terms of a charge disproportionation.