Crystal structure and physical properties of α′-(EOET)2AuBr2

The 2:1 a'-type AuBr 2 salt with ethylenedioxy-ethylenedithio-TTF (EOET) has a quasi two-dimensional (2D) layer, and is allocated on the insulator side of the Mott-Hubbard transition. EPR spectroscopy shows a sharp upturn of the intensity below T ≤ 12 K, afterwards a rapid decreasing below T ≤ 9 K toward zero, accompanied by a remarkable broadening of the EPR signal at temperatures below 12 K. These findings are associated with the development of three-dimensional antiferromagnetic (AF) fluctuations for T ≤ 12 K, leading to a long-range AF transition at 9 K. The observed magnetic ordering is consistent with the extended Huckel calculation, which predicts the 2D square lattice layers based on spin-1/2 EOET dimers.