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.
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