Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

Abstract

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2 ] hexagonal layer. Both cobaltites exhibit large thermopower values (S300 K ∼ 140 µV K−1), low resistivity values (ρ300 K = 40–60 mΩ cm) and small thermal conductivities (κ300 K ∼ 1 W K−1 m−1). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = ρH − ρH = 0/ρH = 0), reaching, at 2.5 K, − 85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

Transport properties and magnetic-field-induced localization in the misfit cobaltite [Bi2Ba1.3K0.6Co0.1O4]RS[CoO2]1.97single crystal

“…[11] As M =Ba, commensurate modulation along b-axis between RS and hexagonal sublattices with b RS /b H =2.0 was found, [12] being in contrast to the incommensurate modulation in other misfit cobaltites. Metallicity increases with increasing the ionic radii from Ca to Ba, [6,7,12] and the MR at low temperature also changes the sign from negative in Ca and Sr compounds [6,7] to positive in Ba compounds. [12] It has been found by us that there coexist large negative and positive contributions to the nonmonotonic magnetic-field dependent MR in Pb-doped Bi 2 Sr 2 Co 2 O y .…”

“…[1, 2, 3, 4] The complex physical properties include the unconventional superconductivity in water-intercalated Na 0.35 CoO 2 , [5] temperature-dependent Hall effect, [3,6] large negative MR in (Bi,Pb) 2 M 2 Co 2 O y (M=Sr and Ca) and Ca 3 Co 4 O 9 ,[2, 6,7] large thermopower (TEP) with low resistivity,[1, 2] and complicated magnetic structure, [8,9,10] etc. These triangular cobaltites have the common structural unit of CdI 2 -type hexagonal [CoO 2 ] layer, which is composed of edge-shared CoO 6 octahedra.…”

Enhancement of Ca 3 Co 4 O 9 thermoelectric properties by Cr for Co substitution

“…These layers show an alternate stacking of a common conductive CdI 2 -type hexagonal CoO 2 layer with a two-dimensional triangular lattice and a block layer composed of insulating rock-salt-type (RS) layers. The two sublattices (RS block and CdI 2 -type CoO 2 layer) possess common a-and c-axis lattice parameters and β angles, but different b-axis length, causing a misfit along the b-direction [10,11]. Furthermore, it has also been found that the Seebeck coefficient values are governed by the incommensurability ratio and/or the charge of the RS block layer between the CoO 2 ones [9].…”

Mejora de las propiedades termoeléctricas de Bi₂Sr₂Co_1.8O_x por métodos de síntesis en disolución

“…Crystallographically, these cobaltites are composed of two different layers, with an alternate stacking of a common conductive CdI 2 -type CoO 2 layer, with a two-dimensional triangular lattice, and an insulating block layer, with the rocksalt-type (RS) structure. The two sublattices (RS block and CdI 2 -type CoO 2 layer) possess common a-and c-axis lattice parameters and b angles but different b-axis length, causing a misfit along the b-direction [6][7][8] that gives their common name: misfit cobaltites.…”