Phase diagram, crystal chemistry and thermoelectric properties of compounds in the Ca–Co–Zn–O system

Abstract

Listen

Translate article

In the Ca–Co–Zn–O system, we have determined the tie-line relationships and the thermoelectric properties, solid solution limits, and structures of two low-dimensional cobaltite series, Ca 3(Co, Zn) 4O 9− z and Ca 3(Co,Zn) 2O 6− z at 885 °C in air. In Ca 3(Co,Zn) 4O 9− z , which has a misfit layered structure, Zn was found to substitute in the Co site to a limit of Ca 3(Co 3.8Zn 0.2)O 9− z . The compound Ca 3(Co,Zn) 2O 6− z ( n=1 member of the homologous series, Ca n+2 (Co,Z n ) n (Co,Zn)′O 3 n+3− z ) consists of one-dimensional parallel (Co,Zn) 2O 6 6− chains that are built from successive alternating face-sharing (Co,Zn)O 6 trigonal prisms and ‘ n’ units of (Co,Zn)O 6 octahedra along the hexagonal c-axis. Zn substitutes in the Co site of Ca 3Co 2O 6 to a small amount of approximately Ca 3(Co 1.95Zn 0.05)O 6− z . In the ZnO–CoO z system, Zn substitutes in the tetrahedral Co site of Co 3O 4 to the maximum amount of (Co 2.49Zn 0.51)O 4− z and Co substitutes in the Zn site of ZnO to (Zn 0.94Co 0.06)O. The crystal structures of (Co 2.7Zn 0.3)O 4− z , (Zn 0.94Co 0.06)O, and Ca 3(Co 1.95 Zn 0.05)O 6− z are described. Despite the Ca 3(Co, Zn) 2O 6− z series having reasonably high Seebeck coefficients and relatively low thermal conductivity, the electrical resistivity values of its members are too high to achieve high figure of merit, ZT.