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Abstract

The title compounds were synthesized from RE, REI3 (RE = Ce, Gd, Y) and Al or Ga under an Ar atmosphere at 930-950 degrees C. The non-stoichiometric Ce2+xI2Al2+y and Ce2+xI2Ga2+y compounds crystallize in the space group R (3) over barm (No. 166). with lattice constants a = 4.3645(3), c = 35.914(2) angstrom for the Al and a = 4.3009(2), c = 35.680(4) angstrom for the Ga compound. Excess electron density found in the Wyckoff position 3a could be due to a fractional occupation by Ce or M (x = 0.06, y = 0 or x = 0, y = 0.11 in the case of the Ga compound). The stoichiometric Gd2I2Ga2 and Y2I2Ga2 compounds crystallize in the space group P (3) over bar m1 (No. 164) with lattice constants a = 4.1964(1) and 4.1786(7) angstrom, c = 11.4753(4) and 11.434(2) angstrom, respectively. Their structures feature M-centered (M = Al, Ga) RE trigonal prisms condensed via common rectangular faces. The electronic origin of the surplus of metal atoms in the octahedral voids between the I-layers of the Ce compounds was explored via extended Huckel-type calculations. Magnetic susceptibility, electrical resistivity and heat capacity measurements have also been carried out. These reveal a metal-insulator transition of Gd2I2Ga2 at 40 K.