Tuning Thermoelectric Properties of Misfit Layered Cobaltites by

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Tuning Thermoelectric Properties of Misfit Layered Cobaltites by Chemically Induced Strain Jakub D Baran, Marco Molinari, Nuth Kulwongwit, Feridoon Azough, Robert Freer, Demie Kepaptsoglou, Quentin M. Ramasse, and Stephen C. Parker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05583 • Publication Date (Web): 27 Aug 2015 Downloaded from http://pubs.acs.org on September 4, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Tuning Thermoelectric Properties of Misfit Layered Cobaltites by Chemically Induced Strain. J. D. Baran1, M. Molinari1, N. Kulwongwit2, F. Azough2 R. Freer2, D. Kepaptsoglou3 Q. M. Ramasse3 and S. C. Parker1* 1 2

Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK

Material Science Centre, School of Materials, University of Manchester M1 7HS, UK 3

SuperSTEM Laboratory, STFC Daresbury Campus, Daresbury WA4 4AD, U.K.

Corresponding Author Details S.C. Parker Department of Chemistry University of Bath Bath BA2 7AY United Kingdom Tel 044 (0)1225 386505 Fax 044 (0)1225 386231 Email: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 36

Abstract

We have applied density functional theory and high-resolution transmission electron microscopy to investigate the relationship between chemically induced strain and charge transfer on the structural, electronic, vibrational and thermoelectric properties of misfit layered cobaltites (M2CoO3)0.6CoO2 (M=Mg, Ca, Sr, Ba). The electron and phonon density of states are analysed and rationalized by accounting for the effects of internal strain and charge transfer, and lay the foundations to disentangle these effects on a promising thermoelectric oxide material. We found that the choice of different interlayer cations has little effect on the magnetic properties, but it generates internal strain between the rocksalt M2CoO3 and hexagonal CoO2 subsystems, changing the hybridization of the cations with the environment. Increasing the mass of the cation leads to decoupling of the vibrations between the rocksalt and CoO2 subsystems so that heavier cations are predicted to enhance phonon scattering. On the other hand, applying compressive strain to the system, which corresponds to doping with smaller interlayer cations is shown to enhance the Seebeck coefficient. The calculations suggest that thermoelectric efficiency of misfit layered cobaltites may be tuned by co-doping the rock-salt layer with isovalent alkali earth cations. Keywords: Thermoelectric oxide materials, Thermoelectric layered materials, Doping, Boltzmann Transport Theory, Vibrational Properties, Seebeck coefficient, Electronic structure.


Page 3 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I.

Introduction

Thermoelectric materials (TEMs) interconvert heat and electricity directly and thus provide an alternative technology to reduce consumption of fossil fuels. Low efficiency, high cost, poor stability and often hazardous toxicity of current TEMs prevent them from being widely exploited as an effective renewable and green energy technology. The technological challenge is therefore to fabricate TEMs that have high efficiency, are stable, non-toxic and contain earth abundant elements. Although transition metal oxides, due to poor electrical conductivity and low charge carrier mobility, are generally not considered as TEMs, the discovery of large thermoelectric power and low thermal conductivity in NaxCoO21-2 has attracted increasing attention to oxides. Moreover, metal oxides possess several advantages over traditional TEMs including chemical and thermal stability at high temperature, low price and chemical diversity for a given crystal structure, makes the TE properties highly tunable. The efficiency of TEMs is measured by the dimensionless figure of merit ZT,

=

,

(1)

where S is the Seebeck coefficient, σ is the electrical conductivity, ke and kl are the thermal conductivities due to electronic carriers and phonons respectively, and T is the operating device temperature. The numerator is called power factor (PF). Therefore, to maximize material efficiency, PF should be maximized, whereas the total thermal conductivity ( + ) should be minimized. This is a challenge, as with the exception of kl, all other properties are governed by the electronic structure of the system and therefore cannot be controlled independently. To date, many routes have been proposed to remove interdependence of the electrical and thermal conductivities. A particularly interesting idea was proposed by Slack et al.3, namely the ”phonon-glass electron-crystal“ (PGEC) system. In 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 36

PGEC, electrons and phonons follow different paths, so that the material can have very low thermal conductivity associated with glassy like amorphous materials, and high electrical conductivity associated with semiconducting crystals. Within PGEC materials such as filled skutterudites, antimonides, inorganic clathrates, the layered misfit cobalt oxides have attracted considerable attention in the last decade.4 In layered misfit cobalt oxides, their structure, consisting of alternating CoO2 and rock-salt (RS) layers, can be thought as formed by building blocks with different electronic and thermal properties. Such a modular structure opens up possibilities for manipulating the atomistic configuration within each layer by modifying selectively individual components, resulting in highly tuneable properties. The best example of such material is calcium cobaltite [Ca2CoO3]0.62CoO2 (CCO)5-8 (Fig.1), which is characterized by high chemical stability up to 1000K and high Seebeck coefficient (S) of ~125 μVK-1 at 300 K.5 It is suggested that its large measured ZT value, of 0.83 at 973K9, is related to strong electron correlation and spin-entropy relation10-11; however, the detailed mechanism behind it is still unclear. In CCO, highly crystalline hexagonal sheets of CoO2 (electron crystal) and rock-salt layers Ca2CoO3 (phonon glass) are alternately stacked along the c-axis (Fig.1). The incommensurate nature of the stacking is related to the different lattice constants along the b axis of the two subsystems, whose ratio corresponds to the “golden ratio” of ~1.618. The lattice mismatch between these two subsystems imposes an internal stress in the system, which together with the charge reorganization between the CoO2 and RS layer is believed to enhance desirable electronic properties of the material from the thermoelectric perspective. Moreover, due to the possibility of substituting different cations in the RS layers, these materials possess an extraordinary chemical richness. Therefore, an understanding of the interaction between the different subsystems in CCO, as well as a systematic study on the relationship between 4 ACS Paragon Plus Environment

Page 5 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the properties induced by cation substitution in the RS layer (i.e. size and valency) on the structural, electronic and thermoelectric properties (TEP) of the material is of high technological interest. In this work, we use a combination of density functional theory (DFT), Boltzmann Transport Equations (BTE) in the rigid band approximation, DFT lattice dynamics calculations and high-resolution transmission electron microscopy (HRTEM), to study CCO and a number of isostructural systems, where the alkaline earth cations (Ca2+) are replaced by isovalent cations, i.e. Mg2+, Sr2+ and Ba2+, to form MCO, SCO and BCO, respectively. We predict and analyse the changes in the structural, electronic, magnetic, and thermal properties upon substitution of the alkaline earth cations in the RS layer disentangling the complex correlation between them. Moreover, lattice dynamics calculations reveal the vibrational interrelation between the different subsystems and the role of the interlayer cations in the phonon-scattering process. The paper is structured as follow: firstly, the theoretical and experimental approaches are presented along with the approximation used to model the crystallographic structure of the systems; secondly, we discuss the calculated electronic properties and the changes of vibrational properties upon cation exchange; finally, the thermoelectric transport properties (Seebeck coefficient, electronic conductivity and PF) are discussed as a function of chemical potential. II.

Methodology

Computational details The structural and electronic properties of the materials are calculated using the DFT+U formalism as implemented in Quantum Espresso (QE).12 The ultrasoft13 PBE14 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 36

pseudopotentials are used to describe electronic exchange and correlation. The Hubbard U parameter is used within the methodology as describe by Cococcini and de Gironcoli.15 The selected value of Ueff was 4 eV. The choice of DFT+U is dictated by the improved description of the Co-d states near the Fermi level as compared to the photoemission experiment16-19, than for example by pure LDA/GGA functional20 (see “Magnetic and Electronic Properties” in “the Results and Discussion section” for more details). The energy cutoff for the wavefunction and charge density are 45 and 400 Ry, respectively. Methfessel–Paxton21 8x6x4 k-point grids for the electronic relaxation is used. Convergence to the ground state is tested by imposing different starting magnetizations, and hence the configuration with the lowest total energy is selected for further study. For the charged cells a compensating jellium background is inserted to remove Coulomb divergences. The electronic transport properties are calculated using the Boltzmann transport equation within the constant relaxation time approximation as implemented in the BoltzTraP code.22 For this purpose the Kohn-Sham eigenvalues () are calculated on a denser 24x18x18 kpoint grid, which results in 3888 irreducible k-points. As the electronic conductivity is calculated with respect to relaxation time τ, τ is introduced as a parameter23 to reproduce the experimental in-plane (parallel to the CoO2 layer) conductivity at 300K for an undoped CCO.5 The relaxation time employed is τ=1.64x10-16 s for all the systems. Note that the relaxation time introduced here is an approximation independent of the temperature. The choice of the same τ for all the systems is dictated by the lack of the experimental details for MCO, SCO and BCO and validity of such approximation cannot be fully verified until the


Page 7 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


corresponding experimental data is available. Thus the electronic conductivity/PF values calculated for these systems should serve as a guide only. For phonon calculations, force constant matrixes are constructed based on the forces on atoms calculated using linear response PBE+U calculations as implemented in the VASP package.24-25 The pseudopotentials are treated within the framework of Bloch’s PAW method.26 The Hubbard U parameter27 and k-point grid are the same as for QE calculations whereas the energy cutoff is set to 550 eV and the electronic states are converged to 10-8 eV. The Phonopy28-29 package is then used in order to obtain phonon density of states (PHDOS). For postprocessing, sampling of the phonon frequencies on a 18 x 8 x 12 q mesh was used to ensure convergence of the vibrational density of states. Experimental details Ceramic samples of CCO were prepared by the conventional mixed oxide route. Starting materials were high purity powders of CaCO3 (Solvay, 99.9%) and Co3O4 (Sigma – Aldridge, 99.9 %). The powders were weighed in batches according to the required formulations and wet milled for 24 hours in a vibratory mill using zirconia balls and propan2-ol. The powders were then dried and calcined at 850oC for 12 hours. Powders were uniaxially compacted into pellets of 20mm diameter and 5mm thickness at a pressure of 50 MPa prior to sintering at 930oC for 24 hours in air. The heating and cooling rates were 180oC/hr. CCO samples for transmission electron microscopy were prepared by crushing techniques. The sintered disks were crushed to powder using an agate mortar and pestle. Grains of individual powders were dispersed in chloroform, dropped onto a copper grid with holy carbon film, and then dried. Structures were initially investigated using selected area 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 36

electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) techniques using a FEI FEGTEM (Tecnai G2, Hillsboro, OR) operating at 300 kV. Subsequently, STEM observation was performed at the SuperSTEM facility using the aberration-corrected Nion UltraSTEM (Nion Co., Kirkland, WA) instrument equipped with a Gatan Enfina spectrometer (Gatan Inc., Pleasanton, CA). The microscope was operated at 100kV, with a probe convergence angle (α) of ~30 mrad. In these operating conditions the size of the electron probe is ~0.9 Å. High angle annular dark field (HAADF) imaging was performed using an annular detector with an inner semi-angle of 100 mrad and an outer semi-angle of 185 mrad. III.

Results and discussion

The incommensurate nature of the misfit-cobaltites crystals required their structure to be approximated.30 Rebola et al.16 have studied structural and electronic properties of CCO increasing the numbers of unit cells of CoO2 and CaCoO3 with lattice constant b1 and b2 respectively. The ratio b1/b2 forms two consecutive Fibonacci numbers, i.e. ( / = ( + 1)/() = 3/2, 5/3, 8/5, 13/8, … ),

which

gives

composition

of

the

unit

cell

(Ca2CoO3)2F(n)(CoO2)2F(n+1). They concluded that a 5/3 ratio was sufficient to model accurately the electronic and vibrational properties of CCO within the PBE+U approach.16-18 Therefore, in the present work, we have approximated the CCO structure by using a b1/b2 ratio of 5/3 as shown in Fig. 1., with (Ca2CoO3)6(CoO2)10 composition. This choice is supported by our Zcontrast HAADF image along the [010] zone axis as shown in Fig. 1.


Page 9 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: a) Simulated unit cell of CCO seen along [001] direction resulted from PBE+U calculations. b) Comparison between the atomically resolved experimental HAADF Z – contrast images (right) and the lattice positions as evaluated from DFT+U calculations (left) along the [010] crystallographic direction.

The Z- contrast image provides direct evidence of the atomic positions for the Ca and Co in the structure and the relative positions of the RS and CoO2 layers with respect to each other, and shows good match with the ab-initio calculated atomic positions for the cations, see Fig. 1b. The Ca columns in the RS layer and the Co columns in both RS and CoO2 layer are shown by arrows in Fig. 1 to aid visualization. In Fig. 1a, the unit cell along the b direction shows formation of Ca2CoO3 clusters. Clustering in the RS layer has been found in low temperature experiments, whereas around 400K disorder-to-order transition occurs and a more homogenous RS layer is formed.31 Table I lists lattice parameters, magnetic moments for Co atoms in the CoO2 and RS layers and the average magnetic moment per Co atom for the whole systems. For an ideal RS layer, it is expected that the lattice parameters a and b2 would be almost identical. The average b2 is shorter than a by 0.22 Å (Table I), which implies that an in-plane tensile stress is applied to the RS in the a direction, in agreement with both our and previous experimental data.11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 36

Table 1: Lattice parameters from DFT+U in Å and angles in degrees (α=γ=90˚). Average magnetic moments per Co in the RS layer (μRS/Co), in the CoO2 layer (μCoO2/Co) and in the whole structure (μAVG/Co) in Bohr. *The experimental value of 1.3/1.42 is the effective magnetic moment determined experimentally, from a fit to a modified Curie-Weiss law. Volume in Å3. a

b (CoO )

b (RS) 2

b2/b1

c

β

μRS/Co

μCoO2/Co

μAVG/Co

V

MCO

4.88

2.71

4.52

1.67

10.10

99.9

2.63

0.20

1.5

657.1

CCO

4.91

2.82

4.71

1.67

10.73

98.3

2.64

0.15

1.5

745.1

System

1

2

SCO

5.00

2.90

4.84

1.67

11.32

98.0

2.63

0.20

1.5

813.2

BCO

5.14

2.99

4.99

1.67

11.92

97.5

2.61

0.20

1.5

906.9

4.83

2.82

4.56

1.62

10.84

98.1

-

-

Exp (CCO)

5

5

*1.3 /1.42

10

745.6

As shown in Table 1, with increasing the ionic radius of the alkaline earth cation, rMg2+(0.72)< rCa2+(1.00)< rSr2+(1.18)< rBa2+(1.35Å)32, b1 and b2 lattice parameters increase, although the ratio b1/b2 remains constant at 1.67. The increase of the lattice parameters with increasing the size of the interlayer cation is in line with the experimental measurements when Ca was partially substituted by Mg and Sr.11

Magnetic and Electronic Properties The computational challenge in describing the physics of misfit layered cobaltites is related to their complex structure, their magnetic properties, which result in several magnetic phase transitions over a wide range of temperatures5, and the difficulty in describing the highly-correlated nature of the wavefunction by theoretical methods.33 Although, there is consensus on the structural approximation for CCO, the electronic and magnetic properties are still controversial. To date, the properties of the CCO have been investigated experimentally and theoretically by various groups. Takeuchi et al.19 observed that, using high-resolution photoemission spectroscopy measurements, the transport properties of CCO arise from the CoO2 layer. This was related to the Co-d population of the CoO2 layer in the vicinity of the Fermi level. However, local density functional (LDA)


Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


calculations by Asahi et al.20 showed that the electronic transport originates from the RS and not from the CoO2 layer. Subsequently, Rebola et al.16 showed that if Hubbard like correlation is included (DFT+U), calculations were able to better reproduce Takeuchi’s experiment. The inclusion of the Hubbard U correction results in the states around the vicinity of the Fermi level to be dominated by Co states of the CoO2 layer, whereas in case of LDA/GGA calculations by both Co states of the CoO2 and RS layer,16,20 thus implying that electronic conduction arises from both subsystems. Consequently, Wu at al.18 applied DFT+U predicting that the electronic conductivity in the CCO is confined to the CoO2 layer, in agreement with the experiment of Takeuchi et al.. An interesting study on CCO has been performed by Soret and Lepetit33 using embedded cluster quantum chemical methods that explicitly treat the strong correlation at the Fermi level. They found that, due to the deformation of the CoO6 octahedra in the CoO2 layer, the t2g orbitals split into one a1g and e’g orbitals with the former 240 meV higher. Such splitting results in the Co atoms being always in the low-spin state regardless of their oxidation state. Moreover, they assigned metal-semiconductor transition at 396K with a structural rearrangement within the RS layer that induced a modulation within the CoO2 layer. The Co-3d ions may have low spin (LS), intermediate spin (IS) and high spin (HS) configuration depending on both the environment and crystal field splitting strength. As shown in Table 1, the cation substitution has a marginal effect on the magnetic properties of the systems under study. The calculated average spin magnetic moment per Co of 1.50μB is close to those calculated by others.16, 18 The effective moment determined experimentally from a fit to a modified Curie-Weiss law has been reported as 1.305/1.4210. We should emphasise that our calculated magnetic moment is the spin-only component, whereas the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 36

experimental value includes both the orbital and spin components. However, we can use the spin-only magnetic moments to distinguish between the Co atoms of CoO2 and RS layers. For example the average magnetic moments per Co atom in the CoO2 layer (μCoO2) and in the RS layer (μRS) are approximately 0.20 and 2.63μB, respectively. The breakdown of the magnetization on the individual Co atoms reveals that out of 10 Co of the CoO2 layer, 6 have a magnetization of ~0.05, whereas other 4 are ~0.4μB. In the RS layer, it is found that out of 6 Co, 4 have a value of ~3.0 μB whereas the other two are 2.2μB. Although, it is not possible to assign unambiguously the calculated spin state on the individual ions to their formal oxidation state, it is clear that the Co ions separate into species of different spin/oxidation state depending on the layer in which they are located. Cobalt species in the CoO2 layer are mainly in low spin configurations while those in the RS layer are mainly high spin configurations. Furthermore, the majority of the species in the CoO2 layer have spinonly magnetic moment of approximately 0μB, and as Co3+ in low spin configuration is the only Cobalt species with spin-only magnetic moment of 0μB, we infer that the majority of the Co ions in the CoO2 layer are Co3+. This is also in line with the experimental evidence of Co3+ species in low and intermediate spin in NaxCoO2.In the following, we discuss the main features of the density of states (DOS). Fig. 2 shows spin polarized DOS for all four systems, as well as projected contributions from Co-d of RS and CoO2, oxygen and alkaline earth cations. In case of MCO (Fig. 2a) and BCO (Fig. 2d), there are small but finite DOS features at the Fermi level for the spin up channel, whereas there are missing in case of CCO (Fig. 2b) and SCO (Fig. 2c). As this may suggest metallic character of MCO and BCO compared to halfmetallic of CCO and SCO phases, may also be an artefact of the electronic smearing used in the calculation. The overall shape of the DOS is similar for all four systems, with the vicinity of the Fermi level dominated by O and Co-d states. Co-d states of the CoO2 layer dominate 12 ACS Paragon Plus Environment

Page 13 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


below the Fermi energy, while Co-d states of the RS dominate above the Fermi Energy. Despite a small shift of the Co-d (of CoO2 layer) states at higher energy, relatively small changes are seen in the spin up channel around the Fermi level for all compounds. This largely holds also for the valence band of the spin down channel, but the conduction band (CB) of the DOS shows a more pronounced Co-d character of the RS layer. However, the most striking change in the spin down channel is seen around the Fermi level where the well-defined pseudo-gap for MCO, is small in CCO, and disappears in SCO and BCO, see Fig. 2. This is in line with an increase of the metallicity of the system with an increase of the cation mass observed experimentally.11,

34

Thus, whether this change in the spin down

population arises from the internal stress caused by the cation substitution or from the charge transfer, needs to be investigated further.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 36

Figure 2: Total and partial density of states projected on Co-d levels of the CoO2 and the RS as well as on the O and cations for M/C/S/BCO. The DOSs are aligned with respect to the Fermi level which is set up at 0.

The Co in CoO2 layer tends to be reduced formally from Co+4 to Co3+ and the CoO2 layer can be hole-doped giving to the system p-type behaviour, in agreement with the experiment of Takeuchi et al..19 In order to disentangle the contributions from the internal stress and charge transfer between the two subsystems, we removed the RS layer from all the systems, fixed the volume, optimized the positions of the atoms, and evaluated the DOS of the CoO2 layer (Fig. 3a). On the other hand, to account for the charge transfer from the RS to the CoO2 layer, an electron per Co atom is added, which should reduce all Co atoms from the formal Co+4 to Co+3. For comparison, we have also performed a constant pressure calculation of layered CoO2 with two Co per formula unit (CoO2)2, where the volume was allowed to relax (black line Fig. 3). We will refer to it as the unstrained system. The aim of 14 ACS Paragon Plus Environment

Page 15 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


this is to provide insights into the effect of the expansion and contraction of the CoO2 layer on the electronic distribution and density around the Fermi energy, as it controls the electronic contribution to the thermoelectric properties of the material.

Figure 3: DOS of the CoO2 layer for a) charge neutral layer, b) after insertion of 10 e into the layer. The DOSs are aligned with respect to the Fermi level which is set up at 0. The DOS of the CoO2 is multiplied 5 times.

The results show that there are substantial differences in the DOS of the CoO2 systems both when only strain (Fig. 3a) and when strain and added electrons are considered (Fig. 3b). This is more appreciable at the Fermi energy. Comparing the DOS of the unstrained CoO2 structure and the CoO2 layer of CCO for the neutral system, little difference can be seen, suggesting that there is only a small electronic structure perturbation of this layer by the internal stress when Ca is the alkaline earth ion. However, when the lattice is expanded, due to the presence of larger Sr and Ba with ionic radii of 1.18 and 1.35 Å, the top of the VB and the bottom of the CB for the spin up channel are closer. The opposite behaviour is seen in the spin down channel, where it is a contraction of the lattice, due to Mg with smaller ionic



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 36

radii of 0.72 as compared to 1.00 Å for Ca,32 to shift the VB and CB closer. There are further changes to the CoO2 DOS after the extra electrons are added to the system (Fig. 3b). When an electron per Co of the unstrained CoO2 layer is added the Co become LS Co+3 resulting in a diamagnetic system. However, when an electron is added per Co in the CoO2 layer for the strained systems, the Co separate into different species as shown by the differences in the spin-up and down channels (see Fig. 3). In the spin up channel, the top of the VB is the same for all systems indicating that strain effects have little influence for these states in charged systems, while the bottom of the CB shows similar behaviour of the CB of the spin up channel of neutral systems. The most unexpected change can be seen in the spin-down channel, where there is a considerable part of the DOS extending above the Fermi level. There are however qualitative similarities between spin-down component of the charged unstrained system and that of the charged systems CoO2 DOS (cf. Fig. 2). The magnitude of the spin-down CoO2 DOS that cuts the Fermi level in the later also increases with increase of the unit cell parameters. The increase of the average magnetic moment on the Co ions can be explained as a result of the unit cell expansion that changes the Co-O bond distances in result favouring different spin/oxidation states of the Co. The presence of the Co+3 cations with their larger ionic radii than Co+4 makes the electronic properties of the CoO2 layer more sensitive to the lattice expansion/contraction than when only Co+4 are present. In summary, it is shown that reducing from Co+4 to Co+3 in the bare CoO2 layer recovers behaviour of Co-d states around the Fermi level of the full system Table 2: Magnetization in Bohr per Co for the charge neutral and when an electron per Co is added to the CoO2 layer. CoO2

Magnetization [Bohr/Co] MCO CCO SCO

BCO

CoO2 + e

0.00

0.33

0.37

0.38

0.40

CoO2

1.07

1.08

1.08

1.02

0.92 16

ACS Paragon Plus Environment

Page 17 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


It is therefore clear that, upon reducing the Co ions in the CoO2 layer and applying strain (intrinsically imposed by the presence RS layer), the electronic density increase at the Fermi energy. Indeed, experimentally the Fermi level is dominated by Co-d states of the CoO2 layer,19 implying that the electronic conductivity is attributed to this subsystem and not to the RS. Thus, the RS layer will not just act as a “carrier reservoir” but also will impose a strain on the CoO2 layer altering its electronic properties at the Fermi energy. These subtle but important differences in the DOS caused by the changes in the internal stress significantly affect the transport properties as shall be discussed in the next section.

Electronic Transport Properties The spin polarized nature of the electronic structure of the cobaltites results in spin dependent transport properties and this makes the interpretation somewhat difficult. Note that here the electrical conductivity and thus PF are calculated with respect to the relaxation time which is a fixed parameter independent of temperature, band number or wave vector directions. While this approximation leads in some cases to a reasonable agreement with the experiment35 it should be treated with care. Therefore, the calculated transport quantities should serve only as guide comparing the different systems rather than their quantitative estimation. Figure 4 shows DOS, electrical conductivity, Seebeck coefficient and power factor at 300K for both spin channels for the four systems. The spinup channel DOS (see Fig. 4a) shows similar feature for all systems (as described in the section above). There is a gap between the VB and CB of approximately 0.6 eV. The calculated Seebeck coefficient has positive value indicating p-type conductivity and is 257, 377, 423 and 492 μV/K at the Fermi level for MCO, BCO, SCO and CCO, respectively (Fig. 4c). 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

At the Fermi level the electrical conductivity is 240, 52, 19, 14 (Ωm)-1 (Fig. 4e), whereas the calculated PF is 16, 7, 3, 3 μWK-2m-1 for MCO, BCO, SCO and CCO respectively (Fig. 4g). In the case of MCO, the low value of the Seebeck coefficient is compensated by the large value of the electrical conductivity at the Fermi level, which results in the largest value of the PF among all four systems.


Page 19 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Spin dependent DOS, Seebeck coefficient, electrical conductivity and power factor as a function of chemical potential at 300K.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

The calculated properties of the spin-down channel look drastically different to the spin-up channel. The spin-down channel DOS (see Fig. 4b) indicates metallic character of this spin channel. The value of the Seebeck coefficient at the Fermi level is found to be around 0 for the all systems; however, it has a positive maximum of 902, 537, 235, 252 μV/K at the chemical potential of 0.36, 0.29, 0.35 and 0.44 eV above the Fermi level for MCO, CCO, SCO and BCO, respectively, see Fig. 4d. The largest calculated conductivity at the Fermi level is found for BCO of 3028 (Ωm)-1 followed by CCO (2738(Ωm)-1), SCO (1986(Ωm)-1) and MCO (958(Ωm)-1), see Fig. 4f. The PF calculated at the chemical potential corresponding to the maximum value of the Seebeck coefficient is of 0.23, 2.33, 4.14 and 3.19 μWK-2m-1 for MCO, CCO, SCO and BCO, respectively, see Fig. 4h. The thermoelectric properties of the studied misfit-layered cobaltites will benefit from the different behaviour of the spin channels. This is due to the possibility of combination of a high Seebeck coefficient coming from the semiconducting spin-up, and metallic conductivity that arises from the spin-down channel. Moreover, the contribution of the thermally activated spin-up carriers can compensate the low value of the Seebeck coefficient for spin-down carriers at the Fermi level. We have now presented the transport properties separately for the two channels. However, by weighing the Seebeck coefficients for the two channels by the corresponding electrical conductivities, we can define a single value as in Eq 2. "# $%&' = ("# ∗ "# + $%&' ∗ $%&' )/( "# + $%&' )

(2)

Subscripts up and down refer to the spin up and down components, respectively. Figure 4

shows the plot of Sup-down for all the systems against a shifted chemical potential: the Fermi energy (Ef(shift)) is now shifted at the discontinuity Sup-dn=0 μV/K, which correspond to put the Fermi energy at the top of the VB for the spin-down channel DOS. This discontinuity appears 20 ACS Paragon Plus Environment

Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at 0.43, 0.35, 0.42 and 0.49 eV above the Fermi level for M/C/S/BCO. It is worth noting that at the Ef(shift) the electronic conductivity of the spin-up channel is 0 (see Fig. 5), and therefore Sup+down is defined solely by the spin-down channel which corresponding electronic states are localized on the CoO2 layer (cf. Fig. 2). Note that such shift may suggest that that the experiment and simulations may have slightly different compositions36 but also recognize that there may be difficulties in describing the strongly correlated wave-function of the systems by DFT. Particularly, important in this respect is oxygen non-stoichiometry in the misfit-layered cobaltites, which occurs as deficiency at the RS site.37-38

Figure 5: Seebeck coefficient as sum of spin up and down channel weighted by the corresponding electrical conductivity (Sup+down) plotted versus chemical potential at which Seebeck discontinuity appears Ef(shift).

The magnitude of the Sup+down tends to decrease as the interlayer cation size increases. This trend is in qualitative agreement with the experimentally observed decrees of the Seebeck coefficient upon substitutions of Ca for heavier cations.11 Thus the transport



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

calculations capture the essential features of these materials and indicate that the magnitude of the Seebeck coefficient can be increased by choosing interlayer cations with a smaller ionic radius which is equivalent to apply compressive strain to the system, particularly the CoO2 subsystem. Despite difficulties in direct comparison of the calculated and experimental electronic properties, our calculation revel an important interrelation between them and internal strain/charge transfer and thus can serve as guide in the development of this class of thermoelectric materials. It is therefore clear that the Seebeck coefficient that is related to the size of the VB-CB gap and the nature of the DOS around the Fermi level e.g., its steepnes, can be altered by choosing different interlayer cation or applying strain to the system.

Vibrational Properties The thermal conductivity of the layered metal oxide is related to the nature of the interatomic interactions between the different subsystems. The presence of interlayer alkaline earth cations not only radically changes the electronic DOS, but it also triggers phonon scattering that reduces the lattice thermal conductivity of the compound. An insight into the role of the different subsystems on lowering the thermal conductivity of the material can be gained analysing phonon density of states (PHDOS). In order to lower thermal conductivity, the low frequency phonons need to be scattered. PHDOS for all systems are shown in Fig. 6. The two subsystems, RS and CoO2, contribute over the entire spectrum of the PHDOS indicating strong hybridization between the two subsystems and delocalized character. This behaviour is particularly visible in MCO (Fig. 6a), whereas the shape of PHDOS changes dramatically when Mg is replaced by heavier Ca (Fig. 6b), Sr (Fig.


Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6c) and Ba (Fig. 6d). In MCO, contributions from all the compound components reach through a wide range of the frequencies. The shape of the PHDOS is flat and the contributions from Mg, RS and CoO2, in the range up to 12 THz are very similar, which suggests a strong hybridization between the subsystems. The region from around 12 to 20 THz originates from contributions from oxygen atoms of CoO2, while the contribution from oxygens of the RS layer is spread along the whole spectrum of frequencies.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

Figure 6: Total and projected onto cation, CoO2 and RS contributions DFT+U phonon density of states of MCO, CCO, SCO and BCO

A clear result is that with increasing mass of the cation, its contribution shifts towards lower frequency and becomes significantly more localized while the overlap with the surrounding environment is weakened suggesting that heavier cations are less bonded to the environment. Although, many distinct peaks are formed in the RS layer with increasing mass


Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the cation, the CoO2 layer stays relatively unperturbed by the vibrations of atoms in the neighbouring RS layer, which suggest that atom vibrations in the CoO2 and RS layer becomes increasingly independent of each other as the mass of the cation increases.39 Such a significant difference between PHDOS of the cations, indicates that phonons experience increasing scattering as the mass of the cation increases. Therefore, an increase of the rattling modes, which scatter lower frequency vibrations more effectively, is expected with increasing the mass of the cation. This will then result in a reduction of the thermal conductivity of the material.

IV.

Conclusions

We have calculated the relation between the electronic, transport and thermal properties and the interlayer cation mass/size in isostructural misfit layered cobaltite systems. The results show that by choosing the interlayer cations with different size/mass but the same formal oxidation state, the electronic and thermal properties of these compounds can be modified. These modified properties are due to cooperative effects of the charge transfer and the strain imposed to the CoO2 layer by the substituted RS layer. The exchange of Ca ions for other isovalent cations has marginal effect on the magnetic properties of the systems but rather complex effects on the TE properties with the Seebeck coefficient and the electronic conductivity generally increasing and decreasing respectively, with the decrease of the mass of the cation. Larger cations will decrease the lattice thermal conductivity of the material due to the enhanced scattering of the low frequency phonons and weakened hybridization between the two subsystems. Our results therefore lay the foundations for a possible development in the synthesis of mixed cations cobaltites and suggest that optimized TE properties might be achieved by mixing light and heavy interlayer alkaline earth cations. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

Acknowledgements This work was funded by EPSRC Programme grants EP/K016288/1 and EP/I03601X/1. Computations were run on ARCHER through the Materials Chemistry Consortium funded by EPSRC grant number EP/L000202. We also thank the EPSRC grant number of EP/16366230 for providing the microscope time.

V.

References

(1) Terasaki, I.; Sasago, Y.; Uchinokura, K. Large Thermoelectric Power in NaCo2O4 Single Crystals. Phys. Rev. B 1997, 56, R12685-R12687. (2) Singh, D. J. Electronic Structure of NaCo2O4. Phys. Rev B 2000, 61, 13397-13402. (3) Slack, G. A. New Materials and Performance Limits for Thermoelectric Cooling. In CRC Handbook of Thermoelectrics ; Rowe, D. M., Ed.; CRC Press: London, 1995; 407− 440. (4) Chen, Z.-G.; Han, G.; Yang, L.; Cheng, L.; Zou, J. Nanostructured Thermoelectric Materials: Current Research and Future Challenge. Prog. Nat. Sci. 2012, 22, 535-549. (5) Masset, A. C.; Michel, C.; Maignan, A.; Hervieu, M.; Toulemonde, O.; Studer, F.; Raveau, B.; Hejtmanek, J. Misfit-Layered Cobaltite with an Anisotropic Giant Magnetoresistance: Ca3Co4O9. Phys. Rev. B 2000, 62, 166-175. (6) Miyazaki, Y. Crystal Structure and Thermoelectric Properties of the Misfit-Layered Cobalt Oxides. Solid State Ion. 2004, 172, 463-467. (7) Li, S.; Funahashi, R.; Matsubara, I.; Ueno, K.; Sodeoka, S.; Yamada, H. Synthesis and Thermoelectric Properties of the New Oxide Materials Ca3-xBixCo4O9+δ (0.0 < x < 0.75). Chem. Mater. 2000, 12, 2424-2427. (8) Klie, R. F.; Qiao, Q.; Paulauskas, T.; Ramasse, Q.; Oxley, M. P.; Idrobo, J. C. Examining the Structure and Bonding in Complex Oxides Using Aberration-Corrected Imaging and Spectroscopy. Phys. Rev B 2012, 85, 054106. (9) Shikano, M.; Funahashi, R. Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure. Appl. Phys. Lett. 2003, 82, 1851-1853 (10) Altin, S.; Bayri, A.; Demirel, S.; Aksan, M. A. Thermal Conductivity and Magnetic Properties of the B Substituted Ca3Co4O9. Curr. Appl. Phys. 2014, 14, 590-595. (11) Matsubara, I.; Funahashi, R.; Shikano, M.; Sasaki, K.; Enomoto, H. Cation Substituted (Ca2CoO3)XCoO2 Films and Their Thermoelectric Properties. Appl. Phys. Lett. 2002, 80, 4729-4731.


Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Giannozzi, P., et al. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Cond. Matt. 2009, 21, 395502. (13) Vanderbilt, D., Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892-7895. (14) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671-6687. (15) Cococcioni, M.; de Gironcoli, S. Linear Response Approach to the Calculation of the Effective Interaction Parameters in the LDA+U Method. Phys. Rev. B 2005, 71, 035105. (16) Rébola, A.; Klie, R.; Zapol, P.; Öğüt, S. First-Principles Study of the Atomic and Electronic Structures of Misfit-Layered Calcium Cobaltite Using Rational Approximants. Phys. Rev. B 2012, 85, 155132. (17) Rébola, A.; Klie, R. F.; Zapol, P.; Öğüt, S., Phonon and Thermal Transport Properties of the MisfitLayered Oxide Thermoelectric Ca3Co4O9 from First Principles. Appl. Phys. Lett. 2014, 104, 251910251915. (18) Wu, L.; Meng, Q.; Jooss, C.; Zheng, J.-C.; Inada, H.; Su, D.; Li, Q.; Zhu, Y. Origin of Phonon Glass– Electron Crystal Behavior in Thermoelectric Layered Cobaltate. Adv. Funct. Mater. 2013, 23, 57285736. (19) Takeuchi, T.; Kondo, T.; Soda, K.; Mizutani, U.; Funahashi, R.; Shikano, M.; Tsuda, S.; Yokoya, T.; Shin, S.; Muro, T. Electronic Structure and Large Thermoelectric Power in Ca3Co4O9. J. Electron Spectrosc. Relat. Phenom. 2004, 137–140, 595-599. (20) Asahi, R.; Sugiyama, J.; Tani, T. Electronic Structure of Misfit-Layered Calcium Cobaltite. Phys. Rev. B 2002, 66, 155103. (21) Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B 1989, 40, 3616-3621. (22) Madsen, G. K. H.; Singh, D. J., Boltztrap. A Code for Calculating Band-Structure Dependent Quantities. Comput. Phys. Commun. 2006, 175, 67-71. (23) Molinari, M.; Tompsett, D. A.; Parker, S. C.; Azough, F.; Freer, R. Structural, Electronic and Thermoelectric Behaviour of CaMnO3 and CaMnO3-δ. J. Mater. Chem. A 2014, 2, 14109-14117. (24) Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558561. (25) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal–AmorphousSemiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251-14269. (26) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

(27) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 15051509. (28) Togo, A.; Oba, F.; Tanaka, I. First-Principles Calculations of the Ferroelastic Transition between Rutile-Type and CaCl2-Type SiO2 at High Pressures. Phys. Rev. B 2008, 78, 134106. (29) Skelton, J. M.; Parker, S. C.; Togo, A.; Tanaka, I.; Walsh, A. Thermal Physics of the Lead Chalcogenides PbS, PbSe, and PbTe from First Principles. Phys. Rev. B 2014, 89, 205203. (30) Muguerra, H.; Grebille, D.; Guilmeau, E.; Cloots, R. Modulated Misfit Structure of the Thermoelectric [Bi0.84CaO2]2[CoO2]1.69 Cobalt Oxide. Inorg. Chem. 2008, 47, 2464-2471. (31) Muguerra, H.; Grebille, D.; Bourée, F. Disordered Misfit [Ca2CoO3][CoO2]1.62 Structure Revisited Via a New Intrinsic Modulation. Acta Cryst. Sect. B 2008, 64, 144-153. (32) Shannon, R., Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crys. Sect. A 1976, 32, 751-767. (33) Soret, J.; Lepetit, M.-B. Electronic Structure of the Ca Span Class Compound from Ab Initio Local Interactions. Phys. Rev. B 2012, 85, 165145. (34) Maignan, A.; Pelloquin, D.; Hebert, S.; Klein, Y.; Hervieu, M. Thermoelectric Power in Misfit Cobaltites Ceramics: Optimization by Chemical Substitutions. Bol. Soc. Esp. Ceram. Vidr. 2006, 45, 122-125. (35) Xiang, H. J.; Singh, D. J. Suppression of NaxCoO2 by an External Magnetic Field: Boltzmann Transport Combined with Spin-Polarized Density Functional Theory. Phys. Rev. B 2007, 76, 195111. (36) Schrade, M.; Norby, T.; Finstad, T. G. Hall Effect Measurements on Thermoelectric Ca3Co4O9: On How to Determine the Charge Carrier Concentration in Strongly Correlated Misfit Cobaltites. J. Appl. Phys. 2015, 117, 205103. (37) Schrade, M.; Fjeld, H.; Finstad, T. G.; Norby, T. Electronic Transport Properties of [Ca2CoO3−Δ]q[CoO2]. J. Phys. Chem. C 2014, 118, 2908-2918. (38) Schrade, M.; Casolo, S.; Graham, P. J.; Ulrich, C.; Li, S.; Løvvik, O.-M.; Finstad, T. G.; Norby, T. Oxygen Nonstoichiometry in (Ca2CoO3)0.62(CoO2): A Combined Experimental and Computational Study. J. Phys. Chem. C 2014, 118, 18899-18907. (39) Fujii, S.; Yoshiya, M.; Yumura, A.; Miyauchi, Y.; Tada, M.; Yasuda, H., Impact of Dynamic Interlayer Interactions on Thermal Conductivity of Ca3Co4O9. J. Electron. Mater. 2014, 43, 19051915.


Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

TOC: Tailoring thermoelectric properties of p-type layered cobaltites using chemically induced strain effect.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a) Simulated unit cell of CCO seen along [001] direction resulted from PBE+U calculations. b) Comparison between the atomically resolved experimental HAADF Z – contrast images (right) and the lattice positions as evaluated from DFT+U calculations (left) along the [010] crystallographic direction. 100x40mm (300 x 300 DPI)


Page 30 of 36

Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Total and partial density of states projected on Co-d levels of the CoO2 and the RS as well as on the O and cations for M/C/S/BCO. The DOSs are aligned with respect to the Fermi level which is set up at 0. 97x73mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DOS of the CoO2 layer for a) charge neutral layer, b) after insertion of 10 e into the layer. The DOSs are aligned with respect to the Fermi level which is set up at 0. The DOS of the CoO2 is multiplied 5 times. 163x110mm (300 x 300 DPI)


Page 32 of 36

Page 33 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Spin dependent DOS, Seebeck coefficient, electrical conductivity and power factor as a function of chemical potential at 300K. 129x173mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Seebeck coefficient as sum of spin up and down channel weighted by the corresponding electrical conductivity (Sup+down) plotted versus chemical potential at which Seebeck discontinuity appears Ef(shift). 201x141mm (300 x 300 DPI)


Page 34 of 36

Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Total and projected onto cation, CoO2 and RS contributions DFT+U phonon density of states of MCO, CCO, SCO and BCO 214x249mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Tailoring thermoelectric properties of p-type layered cobaltites using chemically induced strain effect. 86x37mm (300 x 300 DPI)


Page 36 of 36

Tuning Thermoelectric Properties of Misfit Layered Cobaltites by

Recommend Documents