Double Modulation and Microstructure of the Thermoelectric Misfit Compound [Ca2-yLnyCu0.7+yCo1.3-yO4][CoO2]b1/b2 (Ln = Pr, Y and 0 ≤ y ≤ 1/3)

Abstract

In the present paper, we present one route for increasing the copper content in the misfit cobaltites by studying the [Ca 2-y Ln y Cu 0.7+y Co 1.3-y O 4 ][CoO 2 ] b1/b2 (Ln = Pr,Y) series (Ln = magnetic Pr and nonmagnetic Y). We pay special attention to the modulated character of these compounds. The electron diffraction study shows a double modulation of the structure. The parameters of the two systems (rock salt and CdI 2 types) are a 1 = a 2 = 4.805(9) A, b 1 = 4.482(9) A, b 2 = 2.804(9) A, c 1 = c 2 = 12.75(3) A, and β 1 = β 2 = 93.7(2)°, v = b 1 /b 2 = 1.599(4). The first modulation vector q 1 * = 0.63b 1 * is associated with the misfit character of the compound, and the second one, q 2 * = 0.44a 1 * + 0.43c 1 *, is an intrinsic modulation of the rock salt layers. Accounting for this second modulation, the reconstruction of the reciprocal space leads the super-space group C2/m(1q0,α0γ)mm, with q 1 * = 0.63b 1 * and q 2 * = 0.44a* + 0.43c*. The intrinsic modulation in the double [(Co,Cu)O] 2 rock salt-type layer of a misfit cobaltite is shown for the first time by electron microscopy. Different twinning mechanisms are also characterized. The relationships with the strontium ferrites built up from four rock salt-related layers are discussed. An increase of the thermopower is obtained, as well with a magnetic (Pr) as a nonmagnetic (Y) cation, which shows that the enhancement is due to a valence effect rather than to a magnetic effect.