Modulated Structure of Misfit-Layered Cobalt Oxide [Ca2(Co0.65Cu0.35)2O4]0.63CoO2

We have determined the crystal structure of novel thermoelectric compound [Ca 2 (Co 0.65 Cu 0.35 ) 2 O 4 ] 0.63 CoO 2 by a superspace group approach. Structural parameters have been refined with a superspace group of C 2/ m (1 p 0) s 0 using powder neutron diffraction data collected at 293 K. The compound consists of mutually interpenetrating triangular CoO 2 layers and distorted four-layered rock salt-type Ca 2 (Co 0.65 Cu 0.35 ) 2 O 4 slabs, which are incommensurate parallel to the b -axis. The CoO 2 layer is composed of the CdI 2 -type edge-shared CoO 6 octahedra. Both the subsystems have common a - and c -axes, and β-angle with a =4.8212(4) Å, c =12.7671(8) Å and β=93.93(1)°. The b -axis lengths are b 1 =2.8103(3) Å for the [CoO 2 ] subsystem and b 2 =4.4915(3) Å for the [Ca 2 (Co 0.65 Cu 0.35 ) 2 O 4 ] subsystem. A marked displacive modulation of the (Co,Cu) and O sites in the [Ca 2 (Co 0.65 Cu 0.35 ) 2 O 4 ] subsystem has been observed, yielding a large fluctuation of the (Co,Cu)–O distances in the range from 1.8 Å to 2.7 Å. Cu 2+ ions appear to play an important role in stabilizing such a four-atom thick subsystem in this compound.