X-ray Investigation for α′-(BEDT-TTF)2BrICl at Low Temperature

The organic semiconductor α′-(BEDT-TTF)₂BrICl which exhibits an abrupt decrease of static magnetic susceptivity at Tc (approximately 200 K) with decreasing temperature was investigated by X-ray diffraction experiments. The drastic first-order structural phase transition at Tc was found. The unitcell volume became approximately half of that above Tc. The result of crystal structure analysis at 25 K indicated that the donor molecular arrangement was no longer “α′-type” but “β″-type” below Tc.