Hervé Muguerra

Disordered misfit [Ca2CoO3][CoO2]1.62 structure revisited via a new intrinsic modulation

The structure of the thermoelectric lamellar misfit cobalt oxide [Ca(2)CoO(3)][CoO(2)](1.62) was refined again using single-crystal X-ray diffraction data. A new commensurate intrinsic modulation was observed involving a modulation vector orthogonal to the misfit direction ((2/3),0,-(1/3)). The five-dimensional superspace group is C2/m(1delta0)(alpha0gamma)gm and the structure was solved using a commensurate approximation. A new model is given involving an occupation modulation of the split sites of the [CoO] layer. This [CoO] layer can be described by triple chains running along b. The residual disorder along b can then be explained by the assumption of a local ordering with two types of clusters: CoO(2) and Co(5)O(4). A powder neutron diffraction experiment confirmed the ordering evidenced by the single-crystal X-ray diffraction study, but was not sufficient by itself to deal with this double modulated scheme. The new intrinsic modulation is destroyed by partial metal substitutions in the [CoO] layer. The structural modifications of this layer directly influence the physical properties which are related to the electronic structure of the [CoO(2)] layers.

Modulated Misfit Structure of the Thermoelectric (Bi0.84CaO2)2(CoO2)1.69 Cobalt Oxide

The structure of the thermoelectric lamellar misfit cobalt oxide [Bi0.84CaO2]2[CoO2]1.69 has been refined using single crystal X-ray diffraction data. Using the four dimensional superspace formalism for aperiodic structures, the superspace group is confirmed P2/m(0delta1/2) (a1 = 4.9069(4), b1 = 4.7135(7), b2 = 2.8256(4), c1 = 14.668(5) A, beta1 = 93.32(1) degrees). The modulated displacements and site occupancies have been refined and are both compatible with the misfit character of the structure, and with a longitudinal modulation of the Bi-O layers of the structure. This modulation is similar to the corresponding one in the related Sr phase [Bi0.87SrO2]2[CoO2]1.82, but now oriented in the orthogonal direction. Because its incommensurate wavelength is locked with the aperiodicity of the misfit structure, it is possible to distinguish between the modulation parameters induced by the accommodation of both subsystems and those related to the longitudinal modulation of the Bi-O layers. In this original structure, two independent aperiodic phenomena coexist in an single crystallographic direction. A particular attention has been paid to the structural configuration of the CoO2 layer, in relation with other similar phases. The thermoelectric properties are probably directly related to the specific distortion of the compressed layer, but the different measured values for the Seebeck coefficient cannot be related to a significant modification of the CoO6 octahedra.

Original disorder–order transition related to electronic and magnetic properties in the thermoelectric misfit phase [Ca2CoO3][CoO2]1.62

A structural phase transition is shown around 400 K for the thermoelectric lamellar misfit cobalt oxide [Ca2CoO3][CoO2]1.62. This transition is related to a rearrangement of the central [CoO] layer of the [Ca2CoO3] slab of this structure, characterized by a commensurate intrinsic modulation q2 = 2/3a*-1/3c*. The partial residual disorder related to split Co and O atomic sites along the misfit b direction disappears and one can describe this layer with its triple chains as a modulated configuration with a regular and not distorted periodicity along b. This phase transition is associated with small changes observed in the transport and magnetic properties as a function of temperature.

From quantum criticality to enhanced thermopower in strongly correlated layered cobalt oxide

We report on susceptibility measurements in the strongly correlated layered cobalt oxide [BiBa0.66K0.36O2]CoO2, which demonstrate the existence of a magnetic quantum critical point (QCP) governing the electronic properties. The investigated low frequency susceptibility displays a scaling behavior with both the temperature T and the magnetic field B ranging from the high-T non-Fermi liquid down to the low-T Fermi liquid. Whereas the inferred scaling form can be discussed within the standard framework of the quantum critical phenomena, the determined critical exponents suggest an unconventional magnetic QCP of a potentially generic type. Accordingly, these quantum critical fluctuations account for the anomalous logarithmic temperature dependence of the thermopower. This result allows us to conjecture that quantum criticality can be an efficient source of enhanced thermopower.

Dual electronic states in thermoelectric cobalt oxide [ Bi 1.7 Ca 2 O 4 ] 0.59 CoO 2

We investigate the low-temperature magnetic-field dependence of the resistivity in the thermoelectric misfit cobalt oxide

Magnetic field dependent specific heat and enhanced Wilson ratio in strongly correlated layered cobalt oxide

{[{\text{Bi}}_{1.7}{\text{Ca}}_{2}{\text{O}}_{4}]}_{0.59}{\text{CoO}}_{2}

New electronic orderings observed in cobaltates under the influence of misfit periodicities

from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hunds coupling. The inferred microscopic description implies dual electronic states, which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding light on the electronic states, which lead to a high thermopower, this result likely provides potential way to optimize the thermoelectric properties.

Hydrothermal synthesis and crystal structure of a new three-dimensional aluminum-organic framework MIL-69 with 2,6-naphthalenedicarboxylate (ndc), Al(OH)(ndc)·H2O

We have investigated the low temperature specific heat properties as a function of magnetic field in the strongly correlated layered cobalt oxide [BiBa

Synthesis and structural characterization of a new open-framework zinc terephthalate Zn3(OH)2(bdc)2 2DEF, with infinite Zn-(μ3-OH)-Zn chains

_{0.66}

Hydrothermal synthesis and structural characterization of a gallium pyromellitate Ga(OH)(btec).0.5H2O, with infinite Ga-(μ2-OH)-Ga chains (MIL-61)

K