Intermetallic Cerium Compounds with Ordered U3Si2 Type Structure

Z. Naturforsch. 55 b, 155-161 (2000); received November 10, 1999 Intermetallic Cerium Compounds, Crystal Structure New intermetallic cerium compounds Ce2 7iMg (T = Ni, Cu, Pd), CqiTjCd (T Pd, Pt, Au), and CtiTzPb (T = Pt, Au) were prepared by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. Most C e :^ ^ (X = Mg, Cd, Pb) compounds are stable after annealing at about 1000 K, while Ce:Pd2Mg is obtained as single phase only after melting and quenching. A thermal treatment at about 1000 K leads to decomposition in CePdMg and CePd. The eight compounds were investigated by X-ray diffraction both as powders and single crystals, and most structures were refined from single crystal data. They adopt an ordered l^Si? type structure with space group PA/mbm: a = 759.6( 1), c = 376.71 (9) pm, wR2 = 0.0562, 294 F values, 12 parameters for Ce2Ni2Mg, a = 787.41(9), c = 387.23(7) pm, wR2 = 0.0438, 222 F2 values, 12 parameters for Ce2Cu2Mg, a = 777.14(8), c = 400.03(7) pm, wR2 = 0.0276, 221 F2 values, 13 parameters for Ce2Pd2.03Mg0.97, a = 777.90(6), c = 393.28(6) pm, wR2 = 0.0360,317 F2 values, 12 parameters for Ce2Pd2Cd, and a = 779.90(7), c = 389.97(7) pm, wR2 = 0.0453, 315 F2 values, 12 parameters for Ce2Pt2Cd. Refinement of the occupancy parameters revealed full occupancy for most sites. A mixed palladium/magnesium (97 % Mg / 3 % Pd) occupancy was observed only for the 2a site of Ce2Pd2.03Mg0.97, indicating a small homogeneity range for this compound. Ce2Au2Cd (a = 804.93(7), c 393.36(6) pm) and the plumbides Ce2Pt2Pb {a = 794.63(7), c = 381.50(6) pm) and Ce2Au2Pb (a 810.70(7), c = 394.85(7) pm) were characterized through their Guinier powder data. The structures of the C t2 F X compounds can be interpreted as an intergrowth of distorted AIB2 and CsCl related slabs of compositions CzTi and CeX. The course of the lattice parameters and chemical bonding within the series of C^TjX compounds are briefly discussed.