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) 2 FeCl 4 , 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 0.017 a * - 0.010 b * + 0.002 c * 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.