Crystal Structure and Thermoelectric Properties of the Composite Crystal [(Ca_(1-x)Sr_x)_2CoO_3]_rhoCoO_2

摘要

Polycrystalline samples of [(Ca_(1-x)Sr_x)_2CoO_3]_rhoCoO_2 with O<=x<=0.20 have been synthesized at 1193 K in flowing oxygen gas. The modulated crystal structure of the samples has been determined in accordance with a four-dimensional formalism from powder neutron diffraction data. With increasing x, the modulation amplitudes in Co-Co conduction paths become less marked. The non-doped sample (x=0) exhibits the resistivity rho=150 mu OMEGA m and the thermoelectric power S= 130 muV/K at 300K. Both the rho and S values decrease with x but the decrease in rho values is steeper than the at of the S values. The power factor S~2/rho increases with x from 110muW/m/K~23 (x=0) to 140 muW/m/K~3 (X=0.20) AT 300K.This improvement in thermoelectric performance can be explained in terms of the reduction of displacive modulation of the Co-Co conduction paths in the CoO_2 sheets.