Structural and thermoelectric properties of BaRCo4O7 (R=Dy, Ho, Er, Tm,Yb, and Lu)W. Wong-Ng, W. Xie, Y. Yan, G. Liu, J. Kaduk et al. Citation: J. Appl. Phys. 110, 113706 (2011); doi: 10.1063/1.3663526 View online: http://dx.doi.org/10.1063/1.3663526 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v110/i11 Published by the American Institute of Physics. Related ArticlesGiant stability of substituent Co chains in ZnO:Co dilute magnetic oxides AIP Advances 2, 042155 (2012) Polymorphic phases of sp3-hybridized superhard CN J. Chem. Phys. 137, 184506 (2012) Structure and physical properties of K0.63RhO2 single crystals AIP Advances 2, 042140 (2012) An unusual variation of stability and hardness in molybdenum borides Appl. Phys. Lett. 101, 181908 (2012) Structural, magnetic, and optical properties of Pr and Zr codoped BiFeO3 multiferroic ceramics J. Appl. Phys. 112, 094102 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Structural and thermoelectric properties of BaRCo4O7 (R 5 Dy, Ho, Er, Tm,Yb, and Lu)

W. Wong-Ng,1,a) W. Xie,2 Y. Yan,1 G. Liu,1 J. Kaduk,3 E. Thomas,4 and T. Tritt21Ceramics Division, NIST, Gaithersburg, Maryland 20899, USA2Department of Physics, Clemson University, Greensville, South Carolina 29634, USA3Illinois Institute of Technology, Chicago, Illinois 60616, USA4Air Force Research Laboratory, Wright Pattersen, Ohio 45433, USA

(Received 12 September 2011; accepted 22 October 2011; published online 6 December 2011)

The structure and thermoelectric properties of a series of barium lanthanide cobaltites, BaRCo4O7

(R¼Dy, Ho, Er, Tm, Yb, and Lu), which were prepared using the spark plasma synthesis

technique, have been investigated. The space group of these compounds was re-determined and

confirmed to be P31c instead of the reported P63mc. The lattice parameters a and c range from

6.26279(2) A to 6.31181(6) A, and from 10.22468(6) A to 10.24446(15) A for R¼Lu to Dy,

respectively. The crystal structure of BaRCo4O7 is built up from Kagome sheets of CoO4

tetrahedra, linked by triangular layers of CoO4 tetrahedra. The values of figure of merit (ZT) of the

BaRCo4O7 samples were determined to be around 0.02 at 800 K. X-ray diffraction patterns of these

samples have been determined and submitted to the Powder Diffraction File. VC 2011 AmericanInstitute of Physics. [doi:10.1063/1.3663526]

INTRODUCTION

The continuing demand for environmentally friendly

alterative energy technologies has led to increased activities

in the area of thermoelectric research. For high temperature

waste heat conversion applications, low-dimensional layered

oxides have been found to have relatively high efficiency.

The efficiency and performance of thermoelectric energy

conversion or cooling is related to the dimensionless figure

of merit (ZT) of the thermoelectric (TE) materials, given’ by

ZT¼ S2T/qj, where T is the absolute temperature, S is

the Seebeck coefficient or thermoelectric power, q is the

electrical resistivity, and j is the thermal conductivity.1

Examples of these oxides include NaCoOx,2 Ca2Co3O6,3,4

and Ca3Co4O9.5–8 Among these materials, the most efficient

material, Ca3Co4O9, is a misfit layered oxide that has two

monoclinic subsystems with identical a, c, b, but different

b.6 The 1st subsystem consists of triple rock-salt layers of

Ca2CoO3 in the ab plane while the second subsystem con-

sists of a single CoO2 layer, which has the CdI2-type struc-

ture. This phase exhibits strong anisotropic thermoelectric

properties in the ab-plane. The ZT value of single crystal

Ca3Co4O9 was reported to be 0.83 at 1000 K.9 However, in

order to have materials with high enough efficiency for

large-scale industrial applications, ZT of 2 or higher is a

requirement.

The search for low-dimensional cobaltite compounds

with superior thermoelectric properties continues. Layered

BaRCo4O7 compounds have been investigated for their

structure,10–12 oxygen nonstoichiometry,13 and electrical and

magnetic properties.14 Although the electrical properties of

selected BaRCo4O7 have been studied,15 neither thermal

conductivity data nor the ZT values have been reported for

these compounds. The structure for BaRCo4O7 was

reported to be P63mc for R¼Y, Dy, Ho, and Er (Ref. 15) by

assuming they are isostructural with BaRAlZn3O7 and

Ba2R2Zn8O13.16,17 However, a recent report concerning the

structure for the R¼Yb analog using neutron diffraction has

confirmed the space group to be P31c instead of P63mc.18

The 1st goal of this study was to resolve this discrepancy by

reinvestigating the structure series of BaRCo4O7 to confirm

the space group. Since x-ray diffraction is a non-destructive

technique for phase identification, X-ray diffraction patterns

are especially important for phase characterization. There-

fore, the 2nd goal of this investigation was to determine the

experimental patterns for BaRCo4O7 (R¼Dy, Ho, Er, Tm,

Yb, and Lu), and to make them widely available through

submission to the Powder Diffraction File (PDF).19 The 3rd

goal of this study was to determine the thermoelectric

properties and to estimate the resulting ZT values for the

BaRCo4O7 (R¼Dy, Ho, Er, Tm, Yb, and Lu) samples.

EXPERIMENTAL

Sample preparation

All samples were prepared by heating a stoichiometric

mixture of BaCO3, R2O3 (R¼Dy, Ho, Er, Tm, Yb, and Lu),

and Co3O4 in air. R2O3 were first heat-treated at 550 �Covernight prior to use to ensure the absence of carbonates

and hydroxides. Samples were weighed out, well-mixed and

calcined overnight at 850 �C, 950 �C for 50 h, 1050 �C for

40 h, and finally at 1140 �C for another 40 h, with intermedi-

ate grindings. During each heat treatment, the samples were

a)Author to whom correspondence should be addressed. Electronic mail:

winnie.wong-ng@nist.gov.

0021-8979/2011/110(11)/113706/8/$30.00 VC 2011 American Institute of Physics110, 113706-1

JOURNAL OF APPLIED PHYSICS 110, 113706 (2011)

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furnace cooled. The phase purity of the samples was estab-

lished by powder x-ray diffraction.

Spark plasma synthesis preparation of bulk materials

The raw powder materials were consolidated into a den-

sified pellet using a spark plasma synthesis (SPS) method20

at 1000 �C for 5 min under a pressure of 30 MPa. All sam-

ples were determined to have 90% or higher relative

densities.

Thermoelectric properties measurement

To determine electrical conductivity and Seebeck coeffi-

cient, the bulk materials were cut into rectangular bars with

the approximate dimensions of (2� 2� 10) mm3. Commer-

cial equipment (ZEM-1, Ulvac Riko, Inc.21) was employed

under an inert helium gas atmosphere, and the Seebeck coef-

ficient and electrical conductivity were measured from room

temperature to 800 K. Thermal diffusivity was measured by

the laser flash method using the Netzsch LFA45721 system.

Specific heat was determined by differential scanning calo-

rimetry using a Netzsch DSC-404C.21 The density was deter-

mined by the Archimedes method. The resulting high

temperature thermal conductivity, j, was calculated from the

measured thermal diffusivity, d, specific heat, CP, and den-

sity of the sample, qD, using the relationship: j¼ dCPqD.

Uncertainties in the electrical conductivity and thermal con-

ductivity measurements are on the order of 5% to 7% due in

large part to the determination of the sample dimensions.

The thermopower is known to within about 2% and thermal

diffusivity to within about 5%.

X-ray rietveld refinements and powder referencepatterns

The BaRCo4O7 powders were mounted in 0.4 mm deep

corundum sample holders in an Anton Paar HTK1200

furnace mounted on a PANalytical X’Pert Pro MPD21 dif-

fractometer equipped with a PIXcel position-sensitive detec-

tor and a diffracted beam monochromator. Patterns were

measured (Cu Ka radiation, 45 kV, 40 mA, 0.5� divergence

slit, 0.02 rad Soller slits) from 5� to 140� 2h in 0.0130� steps.

The Rietveld refinement technique22 with the software suite

GSAS23 was used to determine the structure of BaRCo4O7.

Reference patterns were obtained with a Rietveld pattern

decomposition technique. Using this technique, the reported

peak positions were derived from the extracted integrated

intensities, and positions calculated from the lattice parame-

ters. When peaks are not resolved at the resolution function,

the intensities are summed, and an intensity-weighted d-

spacing is reported. They are also corrected for systematic

errors both in d-spacing and intensity. In summary, these pat-

terns represent ideal specimen patterns.

Bond valence sum (Vb) calculations

The bond valence sum values, Vb, for Ba, R, and Co

were calculated using the Brown-Altermatt empirical expres-

sion,24,25 and the results are also listed in Table III. The Vb

of an atom i is defined as the sum of the bond valences vij of

all the bonds from atoms i to atoms j. The most commonly

adopted empirical expression for the bond valence vij as a

function of the interatomic distance dij is vij¼ exp[(R0� dij)/B].

The parameter, B, is commonly taken to be a “universal” con-

stant equal to 0.37 A. The values for the reference distance R0

for Ba–O, Co2þ–O, Co3þ-O, Dy–O, Ho-O, Er-O, Tm-O, Yb-O,

and Lu-O are 2.29, 1.692, 1.70, 2.036, 2.023, 2.010, 2.000,

1.985, and 1.971 A, respectively.24,25

RESULTS AND DISCUSSION

Structure of BaRCO4O7

Figure 1 gives the Rietveld refinement results for BaR-

Co4O7. The observed (crosses), calculated (solid line), and

FIG. 1. (Color online) Observed (crosses),

calculated (solid line), and difference

XRD pattern (bottom) for BaRCo4O7 by

Rietveld analysis technique. The differ-

ence pattern is plotted at same scale as the

other calculated peak positions.

113706-2 Wong-Ng et al. J. Appl. Phys. 110, 113706 (2011)

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difference XRD patterns (bottom) for BaRCo4O7, as deter-

mined by the Rietveld analysis technique, are shown. The

difference pattern is plotted at the same scale as the other

patterns up to 70� 2h. At higher angles, the scale has been

magnified five times. The row of tick marks indicates the cal-

culated peak positions. Table I gives the lattice parameters

and calculated density Dx. The Dx values increase as the size

of R decreases, as expected. The atomic coordinates and dis-

placement parameters are given in Table II. Table III gives

the bond distances and bond valence sum values.

The structure of BaRCo4O7 has been reported to be of

hexagonal P63mc symmetry.15 The space group P63mc was

originally suggested by Rabbow and Mueller-Buschbaum16

for LuBaZn3AlO7 and later adopted by Valldor and Ander-

son.12 However, our Rietveld refinement results showed that

the correct space group should be P31c as reported by Huq

et al.8 (except for BaDyCo4O7, refinements in P31c yielded

lower residuals than P63mc). The relation between the two

space groups is that P31c is the maximal non-isomorphic

subgroup of P63mc. A structural/low temperature magnetic

phase transition between 225 K and 150 K from hexagonal

P31c to orthorhombic Pbn21 has also been reported for

BaYbCo4O6.95 by Huq et al.,18 and for BaRCo4O7 (R¼Lu,

Yb, and Tm) by Nakayama et al.26

The lattice parameters for BaRCo4O7 are plotted in Fig.

2. The unit cell volume increases monotonically with the

size of the Shannon ionic radius27 of the metal ion at the

octahedral site. After inspecting Table II, it is obvious that

this volume increase is mainly due to the increase in the

a-axis while the c-parameter is kept relatively constant for

most of the BaRCo4O7 phases.

The structure of BaRCo4O7 (Fig. 3), although deter-

mined to be crystallized in the space group P31c instead of

P63mc,11,12 is qualitatively consistent with both space

groups. The crystal structure is built up of Kagome sheets of

CoO4 tetrahedra, linked by triangular layers of CoO4 tetrahe-

dra. In other words, there are two symmetry-independent

CoO4 tetrahedra in the structure: with Co4-tetrahedra form-

ing the Kagome sheets and Co3-tetrahedra linking these

sheets along the c-axis.28–33 These tetrahedra are in an ap-

proximate ratio of Co3:Co4¼ 1:3. To obtain charge balance

in BaRCo13þCo3

2þO7, the possible formal oxidation state of

Co3 in the trigonal layers can be assigned as 3þ and that of

Co4 in the Kagome sheets as 2þ. If this is the case, then we

would expect the bond distances of Co3-O and Co4-O to be

substantially different,18 as the Co3þ-O distance in a tetrahe-

dral environment is 1.79 A.34 However, our data agree with

Huq’s data that the Co3-O and Co4-O distances are not sig-

nificantly different from each other for one to assume charge

ordering, but more of a result of disordered distribution of

Co2þ-O and Co3þ-O distances. In Table III, we found that

TABLE I. Unit cell parameters of BaRCo4O7 (P31c (No. 159), Z¼ 2, Dx

refers to calculated density). Uncertainties refer to standard deviation.

Compounds a (A) c (A) V(A3) Dx (g cm�3)

BaDyCo4O7 6.31181(6) 10.24440 (15) 353.45 6.082

BaHoCo4O7 6.30584(3) 10.23209 (7) 352.35 6.122

BaErCo4O7 6.29039(3) 10.24466 (13) 351.06 6.171

BaTmCo4O7 6.27834(3) 10.24256 (9) 349.65 6.212

BaYbCo4O7 6.26978(3) 10.23338 (8) 348.38 6.274

BaLuCo4O7 6.26279(2) 10.22468 (4) 347.31 6.311

TABLE II. Atomic coordinates and isotropic displacement factors for BaR-

Co4O7; M stands for site multiplicity. Uncertainties refer to standard

deviation.

Atom x y z Occ. Uiso M

(i) R¼Dy

Dy1 0.66667 0.33333 0.8689(3) 1.0 0.024(2) 2

Ba2 0.66667 0.33333 0.5 1.0 0.012(2) 2

Co3 0.0 0.0 0.4424(6) 1.0 0.0037(9) 2

Co4 0.1843(9) 0.8476(9) 0.6829(5) 1.0 0.0037(9) 6

O5 0.44819 0.50658 0.76253 1.0 0.026(4) 6

O6 0.0 0.0 0.25528 1.0 0.026(4) 2

O7 0.13851 0.80298 0.49584 1.0 0.026(4) 6

(ii) R¼Ho

Ho1 0.66667 0.33333 0.8730 (2) 1.0 0.002 2

Ba2 0.66667 0.33333 0.5 1.0 0.0085(5) 2

Co3 0.0 0.0 0.4427(9) 1.0 0.002 2

Co4 0.1765(10) 0.8368(10) 0.6867(7) 1.0 0.002 6

O5 0.4913(21) 0.484(2) 0.7546(8) 1.0 0.010(2) 6

O6 0.0 0.0 0.2533(11) 1.0 0.010(2) 2

O7 0.1316(11) 0.7912(11) 0.5001(8) 1.0 0.010(2) 6

(iii) R¼Er

Er

Er1 0.66667 0.33333 0.8746 (2) 1.0 0.0145(5) 2

Ba2 0.66667 0.33333 0.5 1.0 0.002 2

Co3 0.0 0.0 0.4290(11) 1.0 0.0056(6) 2

Co4 0.1819(11) 0.8384(11) 0.6831(7) 1.0 0.0056(6) 6

O5 0.464(2) 0.472 (2) 0.7582(7) 1.0 0.066(3) 6

O6 0.0 0.0 0.2389(14) 1.0 0.066(3) 2

O7 0.128(2) 0.797 (2) 0.4962(9) 1.0 0.066(3) 6

(iv) R¼Tm

Tm1 0.66667 0.33333 0.87648(19) 1.0 0.0142(12) 2

Ba2 0.66667 0.33333 0.5 1.0 0.0020(9) 2

Co3 0.0 0.0 0.4394(7) 1.0 0.0100(6) 2

Co4 0.1778(11) 0.8372(12) 0.6871(6) 1.0 0.0100(6) 6

O5 0.48627 0.50408 0.76422 1.0 0.046 (3) 6

O6 0.0 0.0 0.24931 1.0 0.046 (3) 2

O7 0.12349 0.79158 0.49902 1.0 0.046 (3) 6

(v) R¼Yb

Yb1 0.66667 0.33333 0.8771 (2) 1.0 0.002 2

Ba2 0.66667 0.33333 0.5 1.0 0.0080(6) 2

Co3 0.0 0.0 0.4405(7) 1.0 0.0035(7) 2

Co4 0.1855(10) 0.8448(10) 0.6852(7) 1.0 0.0035(7) 6

O5 0.48758 0.49674 0.76141 1.0 0.030(3) 6

O6 0.0 0.0 0.2498 1.0 0.030(3) 2

O7 0.11233 0.77586 0.50073 1.0 0.030(3) 6

(vi) R¼Lu

Lu1 0.66667 0.33333 0.87546(11) 1.0 0.0015(3) 2

Ba2 0.66667 0.33333 0.5 1.0 0.0131(5) 2

Co3 0.0 0.0 0.4387(6) 1.0 0.0057(4) 2

Co4 0.1815(8) 0.8402(8) 0.6838(5) 1.0 0.0057(4) 6

O5 0.4738(12) 0.4793(11) 0.7573(5) 1.0 0.0169(13) 6

O6 0.0 0.0 0.2442(9) 1.0 0.0169(13) 2

O7 0.1337(11) 0.7986(11) 0.4994(5) 1.0 0.0169(13) 6

113706-3 Wong-Ng et al. J. Appl. Phys. 110, 113706 (2011)

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the Co3-O/Co4-O distances in BaRCo4O7 are 1.921/1.914

(R¼Dy), 1.958/1.953 (R¼Ho), 1.945/1.944 (R¼Er),

1.929/1.935 (R¼Tm), 1.932/1.942 (R¼Yb), and

1.946/1.928 (R¼Lu), respectively. No trend in the Co-O

bond distance was observed. The lanthanide ions have a

6-coordination environment with O2�. The RO6 environment

is also shown in Fig. 3 in the ruled surface pattern. Figure 4

gives the polyhedral environment of Ba with respect to 12

O2� sites (anticuboctahedraon).

FIG. 2. Plot of unit cell volume BaRCo4O7 vs. r(R3þ) (where r is the Shan-

non Ionic Radii).26

FIG. 3. Crystal structure of BaRCo4O7 at room temperature based on X-ray

powder data, showing the unit cell outline, labeling of various atoms, and

the two different layers of [CoO4] tetrahedral (Co(1) and Co(2)). The RO6

octahedra were shown in ruled surface pattern.

TABLE III. Bond distances and bond valence sum values (Vb) for BaR-

Co4O7. Uncertainties refer to standard deviation. The symbol x3 means three

bonds are with the indicated distance.

Atom Atom Distances Vb

(i) R¼Dy

Dy1 O5 2.4070(14) �3 2.847

Dy1 O7 2.236 (2) �3

Ba2 O5 3.44073 (3) �3 1.422

Ba2 O5 2.86307 (3) �3

Ba2 O7 3.34084 (3) �3

Ba2 O7 2.97158 (3) �3

Co3 O6 1.917(7) 2.198

Co3 O7 1.923(2) �3

Co4 O5 1.915(5) 2.227

Co4 O5 1.816(5)

Co4 O6 1.987(4)

Co4 O7 1.938(6)

(i) R¼Ho

Ho1 O5 2.154(7) �3 3.872

Ho1 O7 2.219(6) �3

Ba2 O5 3.156(6) �3 1.313

Ba2 O5 3.127(6) �3

Ba2 O7 3.397(6) �3

Ba2 O7 2.910(6) �3

Co3 O6 1.938(8) 1.991

Co3 O7 1.965(6) �3

Co4 O5 1.981(12) 1.977

Co4 O5 1.926(12)

Co4 O6 1.977(4)

Co4 O7 1.930(7)

(ii) R¼Er

Er1 O5 2.219(7) �3 3.388

Er1 O7 2.224(5) �3

Ba2 O5 3.241(6) �3 1.330

Ba2 O5 3.051(6) �3

Ba2 O7 3.379(10) �3

Ba2 O7 2.912(10) �3

Co3 O6 1.948(12) 2.063

Co3 O7 1.944(9) �3

Co4 O5 2.039(11) 2.058

Co4 O5 1.824(11)

Co4 O6 1.958(5)

Co4 O7 1.939(10)

(iii) R¼Tm

Tm1 O5 2.2290(10) �3 3.240

Tm1 O7 2.2270(11) �3

Ba2 O5 3.31208 (2) �3 1.445

Ba2 O5 2.96251 (2) �3

Ba2 O7 3.40581 (2) �3

Ba2 O7 2.87259 (2) �3

Co3 O6 1.946(7) 2.152

Co3 O7 1.924(2) �3

Co4 O5 1.904(6) 2.073

Co4 O5 1.928(6)

Co4 O6 1.959(3)

Co4 O7 1.952(6)

(iv) R¼Yb

Yb1 O5 2.2051(12) �3 3.236

Yb1 O7 2.2221(12) �3

Ba2 O5 3.25835 (2) �3 1.554

Ba2 O5 3.01034 (2) �3

113706-4 Wong-Ng et al. J. Appl. Phys. 110, 113706 (2011)

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In Table I, the anisotropic trend of increase in the cell

parameters of BaRCo4O7 as the ionic radius of R increases

can be explained by the structure. The cell volume increase

is mainly due to the increase in the a-axis while the c-param-

eter kept relatively constant. This implies that the

6-coordinated site, RO6, has a relatively large influence on

the in-plane distances. In the layered BaRCo4O7 structure,

apparently the robust tetrahedral Co-O layer has prevented

the unit cell from a significant expansion along the c-axis.

The bond valence (Table III) for Ba-O (ranging from

values between 1.330 and 1.554) indicates that all Ba2þ are

under tensile stress or underbonding (in an over-sized cage

environment) as Vb values are all much smaller than the ideal

valence of 2þ. Note that most of the Vb values for R-O in

BaRCo4O7 are significantly greater than the ideal value of

3.0, representing a large compressive stress or overbonding

for the R sites except for the phase with R¼Dy. No charge

ordering was confirmed for the Co sites. All Vb values for

the Co3/Co4 sites have very similar values that range from

1.99/1.98 for the Ho compound to 2.198/2.227 in the Dy-

compound. Therefore these sites represent mixed Co2þ val-

ues with a small extent of Co3þ.

Thermoelectric properties

High temperature measurement results of the Seebeck

coefficients, electrical resistivity, and thermal conductivity

for BaRCo4O7 from 300 K to 850 K are shown in Figs. 5, 6,

and 8, respectively.

FIG. 4. The polyhedral environment of Ba (12-coordination) in BaRCo4O7.

FIG. 5. (Color online) Seebeck coefficient, S, measurement of BaRCo4O7 as

a function of T.

TABLE III. Continued

Atom Atom Distances Vb

Ba2 O7 3.48547 (2) �3

Ba2 O7 2.78443 (2) �3

Co3 O6 1.951(8) 2.149

Co3 O7 1.923(2) �3

Co4 O5 1.895(5) 2.040

Co4 O5 1.967(6)

Co4 O6 1.967(4)

Co4 O7 1.940(7)

(v) R¼Lu

Lu1 O5 2.204(5) �3 3.171

Lu1 O7 2.210(4) �3

Ba2 O5 3.212(4) �3 1.349

Ba2 O5 3.056(4) �3

Ba2 O7 3.343(7) �3

Ba2 O7 2.919(7) �3

Co3 O6 1.988(8) 2.101

Co3 O7 1.932(5) �3

Co4 O5 1.989(7) 2.128

Co4 O5 1.865(8)

Co4 O6 1.953(3)

Co4 O7 1.907(6)

FIG. 6. (Color online) Electrical resistivity, q, measurement of BaRCo4O7

as a function of T.

113706-5 Wong-Ng et al. J. Appl. Phys. 110, 113706 (2011)

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In Fig. 5, there appears to be a trend of Seebeck coef-

ficient, namely, the smaller the ionic radius of the lantha-

nide ion, R3þ (or the heavier the lanthanide), the lower the

Seebeck coefficient. The Seebeck coefficients are mostly

between 130 lV/K and 140 lV/K. For R¼Dy and Ho, the

Seebeck coefficient values are much higher at 300 K. At

higher temperature, there is a small decrease of S (except

for the Yb-analog). As all these values are positive in the

measured temperature range, they belong to the family of

p-type semiconductors. The electrical resistivity values, q,

in general decrease as f(T) (Fig. 6), and they also follow a

trend as a function of the size of the ionic radius. Among

the series, the resistivity of the Yb-analog is an anomaly.

For example, in the 300 K to 850 K range, while the resis-

tivity values for the other BaRCo4O7 members are similar

to each other, the values for the Yb-analog are significantly

higher. This may be due to the fact that Yb can adopt both

2þ and 3þ valence states whereas the rest of the

R-analogs in this study possess a stable 3þ valence state.

A sample with a mixed 2þ and 3þ valence states results

in lesser oxygen content because the 2þ valence state

requires lesser amount of oxygen for charge balance. Oxy-

gen non-stoichiometry of BaRCo4O7, which may give rise

to a decrease in carrier concentrations and eventually influ-

ence the electrical resistivity and Seebeck coefficient, has

been reported by Karppinin et al.13 The power factor val-

ues, P (P¼ S2/q), of the BaRCo4O7 series were estimated

to increase with the increase of temperature (Fig. 7). These

P values at high temperature, however, are lower than that

of other polycrystalline layered cobaltites such as

Ca3Co4O9 and Ca3Co2O6.35,36 The thermal conductivity, j,

appears to be increasing slightly as f(T) (Fig. 8). The fluc-

tuation of the first data point in thermal conductivity (at

about 300 K) for BaTmCo4O7 is most likely due to the

fact that the InSb detector has a higher uncertainty around

room temperature.37

The ZT curves of the BaRCo4O7 compounds as a func-

tion of temperature are shown in Fig. 9. These plots were

calculated with a curve fitting technique using data of S, q,

and j (as these data were not determined at the same corre-

sponding temperatures). It was found that although these lay-

ered BaRCo4O7 compounds have reasonably high S and

relatively lowj, the high q values render them with lower ZTvalues as compared to that of the misfit-layered cobaltite,

Ca3Co4O9. The ZT values of BaRCo4O7 are similar to each

other and it increases to a value of 0.021 at 850 K for BaDy-

Co4O7 (Fig. 9).

Reference x-ray diffraction pattern

An example of the reference pattern of BaRCo4O7

(R¼Tm) is shown in Table IV. In this pattern, the symbols

“M” and “þ” refer to peaks containing contributions from

two and more than two reflections, respectively. The symbol

* indicates that the particular peak has the strongest intensity

of the entire pattern and has been designated a value of

“999.” The intensity values reported are integrated intensities

rather than peak heights. All patterns have been submitted

for inclusion in the PDF.

FIG. 9. (Color online) ZT plot of BaRCo4O7 as a function of T (calculated).

These ZT values were obtained using a curve fitting technique.

FIG. 8. (Color online) Thermal conductivity, j, measurement of BaRCo4O7

as a function of T.

FIG. 7. (Color online) Dependence of power factors of BaRCo4O7 on T.

113706-6 Wong-Ng et al. J. Appl. Phys. 110, 113706 (2011)

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SUMMARY

Crystal structure, reference patterns, and thermoelectric

properties of the BaRCo4O7 (R¼Dy, Ho, Er, Tm, Yb, and

Lu) compounds have been determined. Bond valence sum

calculations indicated that charge ordering of Co2þ and

Co3þ was absent in these compounds. While the RO6 cages

are overbonded, which leads to compressive stress, all BaO12

cages are underbonded. The Seebeck coefficients for all six

compounds are positive, therefore BaRCo4O7 are p-type

semiconductors. Among the compounds that we studied,

TABLE IV. X-ray powder pattern for BaTmCo4O7 (P31c (No. 159), 6.27834 (3) A, c¼ 10.24256 (9) A, V¼ 349.65 A3, Dx¼ 6.21 g cm�3). The symbols “M”

and “þ” refer to peaks containing contributions from two and more than two reflections, respectively. The symbol * indicates the particular peak has the

strongest intensity of the entire pattern and is designated a value of “999.”

d I h k l d I h k l d I h k l

5.4372 71 1 0 0 5.1213 65 0 0 2 4.8025 95 1 0 1

3.7280 110 1 0 2 3.1392 621 1 1 0 2.8914 968 1 0 3

2.7186 107 2 0 0 2.6764 999* 1 1 �2M 2.6764 999* 1 1 2M

2.6276 766 2 0 1 2.5606 138 0 0 4 2.4012 30 2 0 2

2.3166 52 1 0 4 2.1267 72 2 0 3 2.0551 12 2 1 0

2.0149 37 2 1 �1M 2.0149 37 2 1 1M 1.9842 6 1 1 �4M

1.9842 6 1 1 4M 1.9170 122 1 0 5 1.9072 57 2 1 �2M

1.9072 57 2 1 2M 1.8640 12 2 0 4 1.8124 124 3 0 0

1.7607 423 2 1 �3M 1.7607 423 2 1 3M 1.7086 300 3 0 2

1.7071 40 0 0 6 1.6360 403 2 0 5 1.6287 40 1 0 6

1.6027 49 2 1 �4M 1.6027 49 2 1 4M 1.5696 296 2 2 0

1.5080 6 3 1 0 1.5002 27 1 1 6þ 1.4919 8 3 1 �1þ1.4508 92 2 1 �5M 1.4508 92 2 1 5M 1.4458 126 2 0 6þ1.4130 23 1 0 7 1.3794 190 3 1 �3M 1.3794 190 3 1 3M

1.35930 11 4 0 0 1.3475 72 4 0 1 1.3382 44 2 2 �4M

1.33819 44 2 2 4M 1.3131 28 2 1 �6M 1.3131 28 2 1 6M

1.29942 9 3 1 �4M 1.2994 9 3 1 4M 1.2803 5 0 0 8

1.2629 13 4 0 3 1.2474 6 3 2 0 1.2462 39 1 0 8

1.2427 9 3 0 6 1.2144 60 3 1 �5M 1.2144 60 3 1 5

1.2120 5 3 2 �2M 1.2120 5 3 2 2M 1.1920 31 2 1 �7M

1.1920 31 2 1 7M 1.1865 64 4 1 0 1.1855 58 1 1 �8M

1.1855 58 1 1 8M 1.1716 123 3 2 �3M 1.1716 123 3 2 3M

1.1583 8 2 0 8 1.1559 129 4 1 �2M 1.1559 129 4 1 2M

1.1554 54 2 2 �6M 1.1554 54 2 2 6M 1.1326 87 4 0 5

1.1302 17 3 1 �6M 1.1302 17 3 1 6M 1.1139 6 1 0 9

1.0867 54 2 1 �8M 1.0867 54 2 1 8M 1.0654 34 3 2 �5M

1.0654 34 3 2 5M 1.0634 43 5 0 2M 1.0634 43 4 0 6M

1.0501 25 3 1 �7M 1.0501 25 3 1 7M 1.0464 23 3 3 0

1.0457 46 3 0 8 1.0362 34 5 0 3 1.0275 9 4 2 0

1.0252 34 3 3 �2M 1.0252 34 3 3 2M 1.0243 5 0 0 10

1.0224 42 4 2 �1M 1.0224 42 4 2 1M 1.0072 16 3 2 �6M

1.0072 16 3 2 6M 1.0066 9 1 0 10 0.9956 12 2 1 �9M

0.9956 12 2 1 9M 0.9921 5 2 2 �8M 0.9921 5 2 2 8M

0.9839 14 4 2 �3M 0.9840 14 4 2 3M 0.9760 32 3 1 �8M

0.9760 32 3 1 8M 0.9743 10 4 1 6þ 0.9737 5 1 1 �10M

0.9737 5 1 1 10M 0.9605 20 5 0 5 0.9593 6 5 1 �2M

0.9593 6 5 1 2M 0.9585 24 2 0 10 0.9493 20 3 2 �7M

0.9493 20 3 2 7M 0.9389 59 5 1 �3M 0.9389 59 5 1 3M

0.9185 112 4 2 �5M 0.9185 112 4 2 5M 0.9178 14 1 0 11

0.9167 13 2 1 �10M 0.9167 13 2 1 10M 0.9124 6 5 1 �4M

0.9125 6 5 1 4M 0.9084 17 3 1 �9M 0.9084 17 3 1 9M

0.9062 58 6 0 0 0.8934 32 3 2 �8M 0.8934 32 3 2 8M

0.8923 6 6 0 2 0.8917 5 3 0 10 0.8815 38 5 1 �5M

0.8815 38 5 1 5M 0.8809 65 2 0 11 0.8804 60 4 2 6þ0.8728 8 5 0 7 0.8707 43 5 2 0 0.8703 74 4 1 �8M

0.8703 74 4 1 8M 0.8647 72 4 3 �3M 0.8647 72 4 3 3M

0.8583 79 5 2 �2M 0.8583 79 5 2 2M 0.8578 39 2 2 �10M

0.8578 39 2 2 10M 0.8543 14 6 0 4 0.8481 37 2 1 �11M

0.8481 37 2 1 11M 0.8474 31 3 1 10þ 0.8439 8 4 3 �4M

0.8439 8 4 3 4M 0.8407 24 3 2 �9M 0.8407 24 3 2 9M

0.8289 40 6 1 0M 0.8289 40 5 0 8M

113706-7 Wong-Ng et al. J. Appl. Phys. 110, 113706 (2011)

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the Yb- analog appears to exhibit anomalous electrical resis-

tivity. For example, from 350 K to 850 K range, while the

electrical resistivity values for the other members are similar

to each other, the values for the Yb-analog are much higher.

One possible reason is that the oxygen content of this sample

is less compared to the other members of the analog series.

This could be due to the mixed 2þ and 3þ valence state of

Yb (instead of 3þ valence adopted by the rest of the lantha-

nide analogs), resulting in a lesser amount of oxygen

required for charge balance. This would give rise to a

decrease in carrier concentrations and eventually increase

the electrical resistivity.

In summary, despite the relatively high Seebeck coeffi-

cient and moderate thermal conductivity values for BaR-

Co4O7, the electrical resistivities are all relatively large,

leading to low ZT values as compared to that of the misfit

Ca3Co4O9 compound. Grain texturing and dopant incorpora-

tions may be some of the strategies to improve electrical

conductivity, and thereby improve the ZT values of these

materials.

1G. S. Nolas, J. Sharp, and H. J. Goldsmid, Thermoelectric: Basic Princi-ples and New Materials Developments (Springer, New York, 2001).

2I. Terasaki, Y. Sasago, and K. Uchinokura, Phys. Rev. B 56, 12685

(1997).3M. Mikami, R. Funashashi, M. Yoshimura, Y. Mori, and T. Sasaki,

J. Appl. Phys. 94(10), 6579 (2003).4M. Mikami and R. Funahashi, J. Solid State Chem. 178, 1670 (2005).5D. Grebille, S. Lambert, F. Bouree, and V. Petricek, J. Appl. Crystallogr.

37, 823 (2004).6A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F.

Studer, and B. Raveau, Phys. Rev. B 62, 166 (2000).7H. Minami, K. Itaka, H. Kawaji, Q. J. Wang, H. Koinuma, and M. Lipp-

maa, Appl. Surface Sci. 197, 442 (2002).8Y. F. Hu, W. D. Si, E. Sutter, and Q. Li, Appl. Phys. Lett. 86, 082103

(2005).9M. Shikano and R. Funahashi, Appl. Phys. Lett. 82(12), 1851 (2003).

10M. Valldor, Solid State Sci. 6, 251 (2004).11M. Valldor, J. Phys.: Condens. Matter 16, 9209 (2004).12M. Valldor and M. Anderson, Solid State Sci. 4, 923 (2002).13M. Karppinen, H. Yamauchi, S. Otani, T. Fujita, T. Motohashi, Y. H.

Huang, M. Valkeapaa, and H. Fjellvag, Chem. Mater. 18, 490 (2006).

14E. V. Tsipis, D. D. Khalyavin, S. V. Shiryaev, K. S. Redkina, and P.

Nunez, Mater. Chem. Phys. 92, 33 (2005).15H. Hao, C. Chen, L. Pan, J. Gao, and X. Hu, Physica B 387, 98

(2007).16Ch. Rabbow and Hk. Muller-Buschbaum, Z. Naturforsch. B51, 343 (1996).17Ch. Rabbow, S. Panzer, and Hk. Muller-Buschbaum, Z. Naturforsch. B52,

546 (1997).18A. Huq, J. F. Mitchell, H. Zheng, L. C. Chapon, P. G. Radaelli,

K. S. Knight, and P. W. Stehpens, J. Solid State Chem. 179, 1126

(2006).19PDF, Powder Diffraction File, produced by International Centre for Dif-

fraction Data, 12 Campus Blvd., Newtown Squares, PA 19073–3273,

USA.20Z. A. Munir, U. Anselmi-Tamburini, and M. Ohyanagi, J. Mater. Sci.

41(3), 763 (2004).21Certain trade names and company products are mentioned in the text or

identified in illustrations in order to adequately specify the experimental

procedures and equipment used. In no case does such identification imply

recommendation or endorsement by the National Institute of Standards

and Technology.22H. M. Reitveld, J Appl. Cryst. 2, 65 (1969).23A. C. Larson and R. B. von Dreele, General Structure Analysis System

(GSAS), Los Alamos National Laboratory Report LAUR 86-748, Los Ala-

mos, USA (2004).24N. E. Brese and M. O’Keeffe, Acta Crystallogr. B 47, 192 (1991).25I. D. Brown and D. Altermatt, Acta Crystallogr. B 41, 244 (1985).26N. Nakayama, T. Mizota, Y. Ueda, A. N. Sokolov, and A. N. Vasiliev.

J. Magnet. Magnet. Mater. 300, 98 (2006).27R. D. Shannon, Acta Crystallogr. A32, 751 (1976).28J. Greedan, J. Mater. Chem. 11, 37 (2001).29C. Broholm, G. Aeppli, S.-H. Lee, W. Bao, and J. F. DiTusa, J. Appl.

Phys. 79, 5023 (1996).30S.-H. Lee, C. Broholm, G. Aeppli, T. G. Perring, B. Hessen, and A. Taylor,

Phys. Rev. Lett. 76, 4424 (1996).31X. Obradors, A. Labarta, A. Isalgue, J. Tejada, J. Rodriguez, and M. Pernet,

Solid State Commun. 65, 189 (1988).32P. Schiffer and A. P. Ramirez, Comments Condens. Matter Phys. 18, 21

(1996).33P. Schiffer, A. P. Ramirez, K. N. Franklin, and S.-W. Cheong, Phys. Rev.

Lett. 77, 2085 (1996).34G. Muncaster, G. Sankar, C. R A. Catlow, J. M. Thomas, S. J. Coles, and

M. Hursthouse, Chem. Mater. 12, 16–18 (2000).35S. Li, R. Funahashi, I. Matsubara, H. Yamada, K. Ueno, and S. Sodeoka,

Ceram. Int. 27, 321 (2001).36K. Iwasaki, H. Yamane, S. Kubota, J. Takahashi, and M. Shimada,

J. Alloy. Compd. 358, 210 (2003).37Private communication, NETZSCH Scientific Instrument Trading (Shang-

hai) Co. Ltd.

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