RESUMO
A combination of experimental and computational techniques has been employed to study doping effects in perovskite CaMnO3. High quality Sr-Mo co-substituted CaMnO3 ceramics were prepared by the conventional mixed oxide route. Crystallographic data from X-ray and electron diffraction showed an orthorhombic to tetragonal symmetry change on increasing the Sr content, suggesting that Sr widens the transition temperature in CaMnO3 preventing phase transformation-cracking on cooling after sintering, enabling the fabrication of high density ceramics. Atomically resolved imaging and analysis showed a random distribution of Sr in the A-site of the perovskite structure and revealed a boundary structure of 90° rotational twin boundaries across {101}orthorhombic; the latter are predominant phonon scattering sources to lower the thermal conductivity as suggested by molecular dynamics calculations. The effect of doping on the thermoelectric properties was evaluated. Increasing Sr substitution reduces the Seebeck coefficient but the power factor remains high due to improved densification by Sr substitution. Mo doping generates additional charge carriers due to the presence of Mn3+ in the Mn4+ matrix, reducing electrical resistivity. The major impact of Sr on thermoelectric behaviour is the reduction of the thermal conductivity as shown experimentally and by modelling. Strontium containing ceramics showed thermoelectric figure of merit (ZT) values higher than 0.1 at temperatures above 850 K. Ca0.7Sr0.3Mn0.96Mo0.04O3 ceramics exhibit enhanced properties with S1000K = -180 µV K-1, ρ1000K = 5 × 10-5 Ωm, k1000K = 1.8 W m-1 K-1 and ZT ≈ 0.11 at 1000 K.
RESUMO
Single crystals of thermoelectric misfit lamellar cobalt oxide phases in the Bi-Ca-Co-O and I-Bi-Ca-Co-O systems were synthesized. They are characterized by aperiodic structures involving two partially independent sublattices: a CdI(2)-type pseudohexagonal CoO(2) layer and a rocksalt-type BiCaO(2) slab allowing the intercalation of iodine. The crystal symmetry of these structures is discussed using the four-dimensional superspace formalism. The superspace Laue classes of the iodine-free and the intercalated compounds are P2/m(0delta(1/2)) (a(1) = 4.901, b(1) = 4.730, b(2) = 2.80, c(1) = 14.66 A, beta = 93.49 degrees ) and A2/m(0delta1) (a'1 = 4.903, b'1 = 4.742, c'1 = 36.51 A, beta = 87.30 degrees ), respectively. A comparison is given with the related Bi-Sr-Co-O misfit compounds. The present structures are compatible with the presence of an intrinsic modulation with a wavelength matching the misfit aperiodicity in the b direction. Preliminary partial structure refinements confirm the layer stacking of the structure and the intercalation of I between the Bi-O layers for the second phase. A comparison with other cobalt oxide phases, as well as symmetry and metric considerations allow us to predict average structures for these new phases and to describe the common structural features assumed for all these lamellar misfit cobalt oxides.
RESUMO
We report on the structural, microstructural, and electronic properties of iodine intercalated [Bi0.82CaO2]2[CoO2]1.69 misfit cobaltite. We first prove through a detailed and careful structural study that the block layer structure can be modified in the desired way. Iodine enters the material between the [BiO] double layers, and the c-cell parameter of the pristine compound is elongated by 3.6 Angstrom. On the basis of this result, we point out the coupling between the block-layer structure and the transport properties. Additionally, we provide in-depth commentary and discussion of some extra results, clarifying some doping effects in the quasi-2D studied phase. Finally, we also propose some expressions that might be useful to material scientists for the tuning of the properties of such compounds.