Preparation, crystal structure and thermoelectric properties of la _2-2xCa_1+2xMn₂O₇

We have prepared polycrystalline samples of the Ruddlesden-Popper La _2-2xCa_1+2xMn₂O₇ phase and determined their crystal structure and thermoelectric properties above room temperature. Single-phase samples with tetragonal I4/mmm space group are obtained for the samples with 0.50 ≤ x ≤ 1.00; the corresponding valence state of Mn is from 3.5 to 4.0. With increasing x, the a-axis length gradually decreases from 3.8268(4) Å (x = 0.5) to 3.7078(2) Å (x = 1.0), while the c-axis length increases from 19.197(3) Å (x = 0.5) to 19.438(1) Å (x = 1.0). As expected, all the samples exhibit negative Seebeck coefficients, S. The absolute value of S increases with increasing x, and the phases become electrically conductive at high temperatures. For example, the sample with x = 0.50 exhibits the electrical conductivity, σ = 35 S/cm and S = -40μV/K at 1123 K, compared with those of the sample with x = 0.95 of σ= 18 S/cm and S = -175 μV/K. The highest ZT ∼0.03 with the power factor of 5.5 × 10^-5 W/mK², has been obtained for the x = 0.95 sample at 1123 K.