Lanthanide Contraction Effect In Magnetic ThermoelectricMaterials Of Rare Earth-doped Bi_(1.5)Pb_(0.5)Ca_2Co_2O_8

摘要

We report in this paper the result of synthesis and crystal structure characterization of magnetic thermoelectric materials of rare-earth-doped Bi_(1.5)Pb_(1.5)Ca_2Co_20_8, namely Bi_(1.5)Pb -(0.5)Ca_(1.9)RE_(0.1)Co_20_8 (RE = La, Pr, Sm, Eu, Gd, Ho). Single phase samples have been prepared by solid state reaction process using precursors of Bi_20_3, PbO, CaCO_3, RE_20_3, and Co_30_4. The precursors were pulverized, calcinated, and sintered in air at various temperatures for several hours. Analysis of XRD data shows that Bi_(1.5)Pb_(0.5)Ca_(1.9)RE_(0.1)1Co_2O_8 compound is a layered system consisting of an alternate stack of CoO_2 layer and Bi_2Sr_20_4 block along the c-axis. The misfit structure along b-direction is revealed from the difference of the b-axis length belonging to two sublattices, namely hexagonal CdI_2-type CoO_2 layer and rock-salt (RS) NaCl-type Bi_2Sr_20_4 block, while they possess the common a- and c-axis lattice parameters and fl angles. The overall crystal structure parameters (a, b, and c) increases with type of doping from La to Ho, namely by decreasing the ionic radii of rare-earth ion. We discuss this phenomenon in terms of the lanthanide contraction, an effect commonly found in the rare-earth compound, results from poor shielding of nuclear charge by 4f electrons. In addition, the values of b-lattice parameters in these rare-earth doped samples are almost the same with those belongs to undoped parent compound (Bi_(1.5)Pb_(o5)Sr_2Co_2O_8) and its related Y-doped (Bi_(1.5)Pb_(0.5)Ca_(1.9)Y_(0.1)Co_2O_8) samples, while the c-values reduced significantly in rare-earth doped samples, with opposite trend with those of variation of a-axis length. Morevover, the misfit degree in rare-earth doped compound is higher in compared to parent compound and Y-doped samples. We argue that these structural changes induced by rare-earth doping may provide information for the variation of electronic structure of Co-ions (Co~(3+) and Co~(4+), in particular their different spin states of low-spin, intermediate-spin, and high-spin. This, in turn, will affect the thermoelectric properties (Seebeck coefficient) of the system.