Relaxor-like behavior in λ-(BETS)₂FeCl₄ studied by SR X-ray diffraction

We performed precise X-ray diffraction measurements on an organic conductor, λ-(BETS)₂FeCl₄, with synchrotron radiation, where BETS is bis(ethylendithio)tetraselenafulvalene. Around 70 K (=T_FM), the width of the Bragg reflection increased, and its peak was split into two with decreasing temperature. The direction of splitting was 0.017a*-0.010b*+0.002c* in the reciprocal space. At 8.3 K (=T_MI), the discontinuous change in c* was observed, and the peak of the Bragg reflection became single again, but remained broad. In the temperature region of T_FM > T > T_MI, it is considered that the system has a heterogeneous structure, and the coexistence of the high- and low-temperature phases below T_FM is presumably related to the anomalous behavior of dielectric constant in the metallic phase. The size of regions/domains was estimated to be about 0.4 μm.