Crystal structure and thermal analysis of the tetramethylammonium dichromate and trichromate

The structures of the tetramethylammonium dichromate, [(CH3)(4)N](2)(CrO7)- O-2 and trichromate, [(CH3)(4)N](2)Cr3O10, were determined from single-crys tal X-ray diffraction data. These compounds crystallize in the orthorhombic system (space group Pnma, with Z = 4 and a = 17.192(1) Angstrom, b = 8.55( 1) Angstrom, c = 10.637(1) Angstrom, for the dichromate and in the monoclin ic system (space group P2(1)/n, with Z = 4 and a = 11.366(2) Angstrom, b = 8.493(2) Angstrom, c = 20.187(4) Angstrom, beta = 103.98(3)degrees for the trichromate). The structures consist of discrete dichromate anions (Cr2O7)( 2-) or trichromate anions (Cr2O7)(2-), respectively, stabilized by quaterna ry ammonium [(CH3)(4)N](+). Phase transitions in [(CH3)(4)N](2)Cr2O7 have b een evidenced by differential scanning calorimetry as well as a new allotro pic variety of [(CH3)(4)N](2)Cr2O7 which was characterized by X-ray powder diffraction. It crystallizes in an orthorhombic system with the unit cell p arameters a = 24.49(1) Angstrom, b = 8.85(1)Angstrom, c = 8.705(8) Angstrom . (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights res erved.