Modulated structure of misfit-layered cobalt oxide [Ca2(Co0.65Cu0.35)2O4]0.6CoO2

We have determined the crystal structure of novel thermoelectric compound [Ca-2(Co0.65Cu0.35)(2)O-4](0.63)CoO2 by a superspace group approach. Structural parameters have been refined with a superspace group of C2/m(l p 0)s0 using powder neutron diffraction data collected at 293 K. The compound consists of mutually interpenetrating triangular CoO2 layers and distorted four-layered rock salt-type Ca-2(Co0.65Cu0.35)(2)O-4 slabs, which are incommensurate parallel to the b-axis. The CoO2 layer is composed of the CdI2-type edge-shared CoO6 octahedra. Both the subsystems have common a- and c-axes, and beta-angle with a = 4.8212(4) Angstrom, c 12.7671(8) Angstrom and beta = 93.93(1)degrees. The b-axis lengths are b(1) = 2.8103(3) Angstrom for the [CoO2] subsystem and b(2) = 4.4915(3) Angstrom for the [Ca-2(Co0.65Cu0.35)(2)O-4] subsystem. A marked displacive modulation of the (Co, Cu) and O sites in the [Ca-2(Co0.65Cu0.35)(2)O-4] subsystem has been observed, yielding a large fluctuation of the (Co, Cu)-O distances in the range from 1.8 Angstrom to 2.7 Angstrom. Cu2+ ions appear to play an important role in stabilizing such a four-atom thick subsystem in this compound.