Calcium-induced cation ordering and large resistivity decrease in Pr0.3CoO2

• We use electron diffraction tomography for mapping of reciprocal space in 3D.
• We observed a complete disorder of stacking of Praseodymium interlayers along [001].
• Calcium doping increased material conductivity by one order of magnitude.
• Calcium doping incited long-range cationic layer stacking order along [001].

Structure of layered cobaltates Pr0.3CoO2 and (PrCa)0.3CoO2 were investigated by electron diffraction tomography and powder X-ray diffraction. The effect of the calcium substitution on thermoelectric properties was evaluated. The structure consists of hexagonal sheets of edge-sharing CoO6 octahedra interleaved by cationic monolayers. The cations form a 2-dimensional a√3×a√3 superstructure in the a–b plane. While Pr0.3CoO2 showed no layer order in the [001] direction, introduction of calcium resulted in the formation of a superstructure spanning over six cationic layers along the [001]. This superstructure model appears to be valid also for the description of the superstructures of CaxCoO2 and SrxCoO2 with x about 1/3. Thanks to the increased number of charge carriers, the substitution of Ca2+ for Pr3+ significantly lowers the electric resistivity, while keeping quite high thermopower around 100 μV K−1, though the character of resistivity remained semiconducting.

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