Oxidation states and thermoelectric properties of BiCuSeO bulks fabricated under Bi or
Se deficiencies in the nominal composition
Mamoru Ishizawa, Yuki Yasuzato, Hiroyuki Fujishiro, Tomoyuki Naito, Hirokazu Katsui, and Takashi Goto

Citation: Journal of Applied Physics 123, 245104 (2018); doi: 10.1063/1.5034499
View online: https://doi.org/10.1063/1.5034499
View Table of Contents: http://aip.scitation.org/toc/jap/123/24
Published by the American Institute of Physics

JOURNAL OF APPLIED PHYSICS 123, 245104 (2018)

Oxidation states and thermoelectric properties of BiCuSeO bulks fabricatedunder Bi or Se deficiencies in the nominal composition
Mamoru Ishizawa,1 Yuki Yasuzato,1,a) Hiroyuki Fujishiro,1,b) Tomoyuki Naito,1
Hirokazu Katsui,2 and Takashi Goto2
1

Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

2(Received 12 April 2018; accepted 5 June 2018; published online 25 June 2018)
We have fabricated the BiCuSeO bulks using raw materials with Bi or Se deficiencies in thenominal composition and investigated crystallographic, chemical compositional, andthermoelectric properties. Owing to the Bi or Se deficiencies in the starting composition, excessive
elements and related compounds were deposited as impurity phases and the matrix phase is nearly
the stoichiometric BiCuSeO phase. The electrical resistivity, q(T), of the bulks decreases andthermoelectric power, S(T), also decreases with increasing the contents of Bi or Se deficiencies inthe starting composition in spite of the stoichiometric matrix phase. These results strongly suggest
that, from the X-ray photoelectron spectroscopy measurements, the actual oxidation states of Bi
and Cu deviate from the formal valences of stoichiometric Bi3þCu1þSe2O2. The introduction ofa small amount of Bi and Se vacancies is also suggested. As a result, mobile carriers are introduced
and the q and S values are changed. The maximum thermoelectric dimensionless figure of merit of
ZT ¼ 0.60 was achieved at 773 K for the Bi1-xCuSeO samples (x ¼ 0.025 and 0.05) in the starting
composition. These results are in clear contrast with the reported results for the Cu deficiency bulks.
Using these results, we propose charge valence equations and the origin of the carriers in the present
BiCuSeO bulks and discuss the influence of created carriers on the thermoelectric properties.
Published by AIP Publishing. https://doi.org/10.1063/1.5034499

I. INTRODUCTION

Oxide thermoelectric materials could be potential candidates in practical applications on the basis of their advantages over heavy metallic alloys in terms of chemical andthermal robustness, compared with conventional thermoelectric materials, such as Bi2Te3 and PbTe. The performance ofthermoelectric materials can be written using the dimensionless figure of merit, ZT (¼S2T/qj), and the thermoelectric
power factor P (¼S2/q), where S is the Seebeck coefficient,
T is the absolute temperature, q is the electrical resistivity,and j is the thermal conductivity. However, the present
potential of the oxide materials is relatively low.
Recently, n-type oxyselenides, BiCuSeO, have been
paid considerable attention for a relatively higher thermoelectric performance from 700 to 1000 K.1–13 This material
crystallizes in a layered crystal structure that is composed of
conductive [Cu2Se2]2 layers constituting a conduction pathway for carrier transport alternately stacked with insulating[Bi2O2]2þ layers acting as a charge reservoir along the c-axisof the tetragonal unit cell, which is similar to layered cobaltites and iron-based superconductors.2,3 For the enhancementof the thermoelectric properties, BiCuSeO showed an excellent charge transport behavior owing to the introduction ofhole carriers by replacing Bi3þ with divalent alkaline-earth
ions such as Sr2þ, Ba2þ, Ca2þ, and Pb2þ in the insulatinga)Present address: Department of Applied Physics, Graduate School ofEngineering, Tohoku University, Sendai 980-8579, Japan.
b)Author to whom correspondence should be addressed: fujishiro@iwate-u.
ac.jp0021-8979/2018/123(24)/245104/9/$30.00[Bi2O2]2þ layer.8,9,13 In the [Cu2Se2]2 layer, the replacementof the Se2 site with the isovalent Te2 also enhances thethermoelectric properties owing to the opening of a small gap
in the electronic structure.14 The hole doping also induces
point defect scattering, which reduces the thermal conductivity and results in the ZT enhancement. The highest ZT of 1.4
was achieved at 923 K for the Bi0.875Ba0.125CuSeO sample todate,15 which is higher than that for other n-type thermoelectric oxides such as SrTi0.8Nb0.2O3 (ZT ¼ 0.37 at 1000 K),16
Zn0.98Al0.02O (ZT ¼ 0.30 at 1273 K),17 and In2-xGexO3 (ZT¼ 0.45 at 1273 K).18
The deficiency of Cu1þ was also investigated by the introduction of hole carriers. BiCu1xSeO,19,20 LaCu1xSO,21,22
and BaCuChF (Ch: chalcogen) systems23 introduce hole carriers into the charge conductive selenide (Cu2Se2)2 layers anddecrease the resistivity. In these samples, the Cu deficiencies
exist actually and the lattice parameter monotonically changes
depending on the Cu deficiencies.19 However, there are noreports for the thermoelectric properties of the BiCuSeO samples with Bi or Se deficiencies.
In this study, we investigated the influence of Bi or Se
deficiencies in the starting composition on thermoelectric
properties in the BiCuSeO system. The impurity phases wereidentified, and the lattice parameters and the chemical composition in the matrix phase were in clear contrast with thereported Cu deficient samples.19 Using the results of X-ray
photoelectron spectroscopy (XPS) and standard defect theory, we propose charge valence equations and the origin of
the carriers in the present BiCuSeO bulks and discuss theinfluence of created carriers on the thermoelectric properties.

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II. EXPERIMENTAL PROCEDURE

The BiCuSeO materials were fabricated using Bi (99.9%)
and Bi2O3 (99.9%), Cu (99.9%), and Se (99.9%) fine powders
as raw materials. For the Bi deficient samples for the nominal
composition, these were weighed with the ratio of Bi:Cu:Se¼ 1–x:1:1 and mixed in an Ar atmosphere. For the Se deficient
samples for the nominal composition, these were weighedwith the ratio of Bi:Cu:Se ¼ 1:1:1–y and mixed in an Ar
atmosphere. After the mixed raw materials were evacuatedand sealed in a quartz tube, they were heated at 973 K for 12 h
and then furnace-cooled. The obtained precursor was pulverized and mixed in air for 0.5 h by an automatic mortar. The
powders were sintered using the spark plasma sintering (SPS)
apparatus (NJS, LABOX-110C) under a uniaxial pressure of
40 MPa at about 973 K for 10 min in vacuum. The mass density of the obtained pellets (10 mm in diameter and 9 mm inthickness) was higher than 93% of the ideal density. We
denoted the nominal composition of the fabricated materials,
as Bi1-xCuSeO and BiCuSe1-yO, respectively, which are not
the measured composition for the bulk.
Powder X-ray diffraction (XRD) measurements were performed (Rigaku, Multi Flex) at room temperature using Cu
Ka radiation to confirm the impurity phase in the sinteredbulks and to decide the lattice constants using Rietveld refinement.24 The microstructure and chemical composition of sintered bulks were analyzed by scanning electron microscopy(SEM) and an electron probe micro-analyzer with a fieldemission gun (FE-EPMA). The sintered pellet fabricated by
SPS was sliced perpendicular to the uniaxial pressure direction and rectangular-shaped bars were prepared and the thermoelectric properties were measured. We confirmed that thethermoelectric properties were independent of the cutting
direction of the bar, although the crystal structure of BiCuSeO
is anisotropic. The electrical resistivity, q(T), and Seebeckcoefficient, S(T), were simultaneously measured in the temperature range from 300 to 773 K using an automated measuring system (Ozawa Science RZ2001i) and the thermoelectric
power factor, P ¼ S2/q, was calculated. The thermal conductivity j(T) was estimated from 300 to 773 K using the relationship j ¼ DaC, where a is the thermal diffusivity, which
was measured by a laser flash method (Ulvac-Riko TC-7000),
C is the specific heat of the x ¼ y ¼ 0 sample, which was measured by the laser flash method, and D is the mass density,
which was measured by the Archimedes method. A dimensionless figure of merit, ZT ¼ S2T/qj, was estimated using
these values, in which the involved uncertainty is about 20%.
X-ray photoelectron spectroscopy (XPS; Ulvac Phi PHI5600)
was measured for the samples after the Ar sputtering by
20 lm in depth. Hall effect experiments were carried out by
the five-terminal method at room temperature to determine
the carrier concentration, p, and Hall mobility, l, which was
calculated by l ¼ RH/q (RH ¼ Hall coefficient).

J. Appl. Phys. 123, 245104 (2018)the main peak of (101) diffraction, respectively. Note againthat the notation of “Bi1-xCuSeO” and “BiCuSe1-yO” is not
the measured composition, but the nominal composition of
the fabricated materials. For both systems, the matrix phase
was confirmed to be a BiCuSeO phase by XRD measurements, in which the peak angle of the (101) diffraction does
not shift for the x or y values. However, for the Bi deficient
samples shown in Fig. 1(a), the CuSe impurity phase can beseen and increases with increasing x in the nominal composition. For the Se deficient system, as shown in Fig. 1(b), thepeaks of Bi and Bi2O3, both of which are raw materials,
increase with increasing y in the nominal composition, which
result from the shortage of the Se atoms in the starting materials. Although the precipitation of the Cu2Se phase is
expected in this case, the Cu2Se phase was not detected by
XRD. These results suggest that a stoichiometric BiCuSeO
phase was obtained as a matrix phase and, as a result, theexpected impurity phases were detected, which increase withincreasing Bi or Se deficiencies in the nominal composition.
The details of the issue are discussed later.

III. RESULTS AND DISCUSSION
A. Crystallographic properties

Figures 1(a) and 1(b) show the magnification of XRD
patterns of the Bi1-xCuSeO and BiCuSe1-yO samples around

FIG. 1. XRD patterns of the (a) Bi1-xCuSeO and (b) BiCuSe1-yO samples
around the (101) diffraction. The notation of Bi1-xCuSeO and BiCuSe1-yO is
not the realistic composition, but the nominal composition of the fabricatedmaterials.

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FIG. 2. The lattice parameters along the a-axis and c-axis of the (a) Bi1-xCuSeO
and (b) BiCuSe1-yO samples, as a function of Bi or Se deficiency in the nominal composition.

J. Appl. Phys. 123, 245104 (2018)

Figures 2(a) and 2(b) show the estimated lattice parameters of the Bi1-xCuSeO and BiCuSe1-yO samples, respectively, as a function of Bi or Se deficiency in the nominal
composition. For both systems, the BiCuSeO matrix phasecan be fitted by the RIETAN program24 within Rwp ¼ 5%
and s ¼ 1.5. The lattice parameter in the ab-plane is nearly
the constant, independent of the x or y value. The lattice
parameter along the c-axis is the constant until x ¼ y ¼ 0.05,
but is seemed to decrease slightly for the x ¼ y ¼ 0.1 samples. It should be noticed that the results of the lattice parameter are in clear contrast with those for the reported Cu
deficient samples,19 in which the a-axis and c-axis lattice
parameters of the BiCu0.9SeO sample shrink about 0.076%
and 0.22%, respectively. The difference of the results withthis study may essentially come from the difference in the
deficiency site or may come from the difference of the fabrication process.
Figure 3(a) shows the backscattered electron composition (BEC) images of the BiCuSeO sample without any deficiency in the nominal composition. Black regions are voids
and a small amount of dark gray regions shown by blue
arrows are Bi2O3. Figures 3(b) and 3(c) show the BEC
images of the Bi1-xCuSeO samples (x ¼ 0.05 and 0.1) in thenominal composition. Tables I and II show the ratio of eachelement for the matrix phase in the Bi1-xCuSeO and
BiCuSe1-yO samples, respectively, in which the total of theratios is normalized to be 4.00. The EPMA measurements
were performed at 3 points for each sample and each phaseand then averaged. The element composition ratio in the

FIG. 3. Backscattered electron composition (BEC) images of the (a) BiCuSeO sample without any deficiency in the nominal composition, Bi1-xCuSeO samples[(b) x ¼ 0.05 and (c) x ¼ 0.1] and BiCuSe1-yO samples [(d) y ¼ 0.05 and (e) y ¼ 0.1]. In (b) and (c), the black regions are confirmed to be CuSe. In (a), (d), and(e), the bright regions (red arrow) and dark gray regions (blue arrow) are, respectively, Bi and Bi2O3 raw materials.

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TABLE I. Estimated atomic ratios of Bi:Cu:Se:O for the matrix phase in the
Bi1-xCuSeO samples by EPMA, in which the total of the ratios is normalized
to be 4.00.

x¼0
x ¼ 0.025
x ¼ 0.05
x ¼ 0.1

Bi

Cu

Se

O

Total

1.02
1.04
1.04
1.04

1.11
1.07
1.08
1.09

0.98
0.99
0.98
0.99

0.88
0.90
0.89
0.88

4.00
4.00
4.00
4.00

matrix phase is nearly Bi: Cu:Se:O ¼ 1:1:1:1, which is independent of the x or y value. These results suggest that the Bi1xCuSeO materials were fabricated according to the following
equation, in which the matrix phase is nearly stoichiometric inspite of the introduction of the Bi deficiency into the nominal
composition and support the results of Fig. 2(a)11ð1  3xÞBi þ Bi2 O3 þ Cu þ Se
3
3
! ð1  xÞBiCuSeO þ xCuSe þ xO:(1)

In Figs. 3(b) and 3(c) for the Bi1-xCuSeO samples, thecontents and size of the black region marked in a red circle,
which was confirmed to be the CuSe phase by EPMA,
increase with increasing x. These results support the XRD
results shown in Fig. 1(a).
Figures 3(d) and 3(e), respectively, show the BEC imagesof the BiCuSe1-yO samples (y ¼ 0.05 and 0.1) in the nominal
composition. The contents of the bright regions shown by red
arrows and the dark grey regions shown by blue arrows
increase with increasing y. These phases were identified as Bi
and Bi2O3 raw materials, respectively. The black region inthese figures was identified as Cu2Se by EPMA, but cannot be
detected by XRD. These results suggest that the BiCuSe1-yO
materials were fabricated according to the following equation,in which the matrix phase is nearly stoichiometric in spite of
the introduction of the Se deficiency into the nominal composition and support the results of Fig. 2(b)1
1Bi þ Bi2 O3 þ Cu þ ð1  yÞSe3
3
2
2
! ð1  2yÞBiCuSeO þ yCu2 Se þ yBi2 O3 þ yBi:
33(2)
B. Thermoelectric properties

Figures 4(a) and 4(b) show the temperature dependenceof the electrical resistivity, q(T), of the Bi1-xCuSeO and
BiCuSe1-yO samples, respectively. In Fig. 4(a), the q(T) of the
TABLE II. Estimated atomic ratios for the matrix phase in the BiCuSe1-yO
samples by EPMA, in which the total of the ratios is normalized to be 4.00.

y¼0
y ¼ 0.025
y ¼ 0.05
y ¼ 0.1

Bi

Cu

Se

O

Total

1.02
1.02
1.01
1.02

1.11
1.11
1.13
1.12

0.98
0.99
1.00
0.98

0.88
0.88
0.87
0.88

4.00
4.00
4.00
4.00

FIG. 4. Temperature dependence of the electrical resistivity, q(T), of the (a)Bi1-xCuSeO and (b) BiCuSe1-yO samples in the nominal composition.

Bi1-xCuSeO samples decreases with increasing x. The reduction in q(T) is simply understood from the introduction ofhole carriers. On the other hand, the behavior of the q(T) in
the BiCuSe1-yO samples as a function of the y value shown in
Fig. 4(b) is fairly different with that of Bi1-xCuSeO samples;the q(T) value decreased for the y value up to 0.05 and thenslightly increased for the y ¼ 0.1 sample, and the q(T) value athigh temperatures is independent of the y value. The detailed
discussion about the different behaviors between both systems
and the origin of the created carriers are shown in 3.3.
Figures 5(a) and 5(b) show the temperature dependenceof the Seebeck coefficient, S(T), for the Bi1-xCuSeO and
BiCuSe1-yO samples above 300 K, respectively. In Fig. 5(a),the S value for the x ¼ 0 sample exceeds þ400 lV/K, whichshows a p-type material, and decreases with increasing x.
For the BiCuSe1-yO samples shown in Fig. 5(b), the S valueslightly decreases with increasing y and the S(T) value athigh temperatures is independent of the y value. These
results strongly suggested that the S(T) value is closely
related to the q(T) value.
Figures 6(a) and 6(b) show the temperature dependenceof the thermoelectric power factor, P(T) ¼ S2/q, for the
Bi1-xCuSeO and BiCuSe1-yO samples above 300 K, respectively. In the Bi1-xCuSeO samples, the P(T) value increases

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FIG. 5. Temperature dependence of the Seebeck coefficient, S(T), of the (a)Bi1-xCuSeO and (b) BiCuSe1-yO samples in the nominal composition.

with increasing T and that at high temperatures takes a maximum for the x ¼ 0.025 and 0.05 samples and then slightly
decreases for the x ¼ 0.1 sample. The highest P value of 5.0
 104 W m1 K2 was achieved for the Bi0.95CuSeO sample at 773 K, which is about five times larger than that forthe x ¼ 0 sample. On the other hand, for the BiCuSe1-yO
samples shown in Fig. 6(b), the P(T) value is fairly low andindependent of the y value, which comes from the small ydependence of q(T) and S(T) values.
Figures 7(a) and 7(b) show the temperature dependenceof the thermal conductivity, j(T), for the Bi1-xCuSeO and
BiCuSe1-yO samples, respectively. j(T) hardly depends onthe x and y values at the entire temperature range. This result
originates from the dominant phonon scattering due to theintrinsic lattice vibration and suggests that the matrix phaseis nearly the stoichiometric composition. In the BiCu1-xSeO
and Bi1-xCaxCuSeO systems, j(T) decreases with increasingx, which indicates that the phonons are mainly scattered by
the crystal defects and inhomogeneity.13,19
Figures 8(a) and 8(b) show the temperature dependenceof the thermoelectric figure of merit, ZT(T), for the
Bi1-xCuSeO and BiCuSe1-yO samples, respectively. The ZT
values of all the samples increased with increasing temperature. In Fig. 8(a), the ZT(T) value takes a maximum for the
Bi1-xCuSeO samples at x ¼ 0.025 and 0.05 and then

J. Appl. Phys. 123, 245104 (2018)

FIG. 6. Temperature dependence of the thermoelectric power factor, P(T)
¼ S2/q, of the (a) Bi1-xCuSeO and (b) BiCuSe1-yO samples in the nominal
composition.

decreased with increasing x, which is closely related to thethermoelectric power factor P(T), because the j(T) is independent of x. The maximum ZT of 0.60 was achieved at
773 K for the Bi1-xSrCuSeO bulks (x ¼ 0.025 and 0.05).
On the other hand, for the BiCuSe1-yO samples, shown in
Fig. 8(b), the ZT(T) values are quite small and are independent of the y value, which is in clear contrast with those of
the Bi1-xCuSeO samples.
Figure 9(a) shows the carrier concentration, p, and carrier mobility, l, of the Bi1-xCuSeO samples, as a function of
the Bi deficiency, x, in the nominal composition, both ofwhich were estimated by the Hall effect measurement. In the
figure, the error bars of the p values of 18% come from thedifference of positive and negative Hall voltages and thoseof the l values of 30% come from both the measurement
errors of the sample size and the resistivity. The carrier concentration, p, increases and the carrier mobility, l, monotonically decreases with increasing x. These results originate fromthe introduction of the hole carriers and the phonon scatteringowing to the increase in the impurity phases and introduced
carriers, both of which are reasonable characteristics. In general, the electrical resistivity, q, and carrier mobility, l, in amaterial can be represented as q ¼ m*/pe2s and l ¼ es/m*,

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FIG. 7. Temperature dependence of the thermal conductivity, j(T), of the(a) Bi1-xCuSeO and (b) BiCuSe1-yO samples in the nominal composition.

respectively (m*: effective mass of carriers, s: mean freetime, e: unit charge). In these equations, the q value isinversely proportional to both p and s values, and the lvalue is proportional to the s value. Since the s value
decreases with increasing the contents of impurities andcarriers, in general, the q value shown in Fig. 4(a) and the lvalue shown in Fig. 9(a) of the Bi1-xCuSeO samples are reasonable. On the other hand, these characteristics are in clear
contrast to those for the BiCuSe1-yO samples. Figure 9(b)shows the p and l values of the BiCuSe1-yO samples, as afunction of the Se deficiency, y, in the nominal composition.
The p values slightly increase up to y ¼ 0.05, but decrease
for the y ¼ 0.1 sample. The l values seem to show the similar y dependence to the p values. However, these resultscannot be systematically explained as a function of the yvalue, which may result from the complicated contribution
from p and s values.
C. XPS results and proposed charge valence
equations

To explain these experimental results, we carried out
XPS spectroscopy. We proposed the charge valence equations and the origin of the carriers in the present BiCuSeO

J. Appl. Phys. 123, 245104 (2018)

FIG. 8. Temperature dependence of the thermoelectric figure of merit,
ZT(T), of the (a) Bi1-xCuSeO and (b) BiCuSe1-yO samples in the nominal
composition.

bulks fabricated from starting raw materials with intentionally introduced Bi or Se deficiencies.
Figures 10(a)–10(c), respectively, show the binding
energy spectrum of Bi, Cu, and Se measured by XPS for the
non-doped BiCuSeO sample at room temperature. Figures10(d)–10(f), respectively, show the same binding energy
spectrum for the Bi0.9CuSeO sample. Figures 10(g)–10(i),respectively, also show the same binding energy spectrum
for the BiCuSe0.9O sample. In the figures, the energy separation curves, which were obtained using Gaussian functions,are also shown. The binding energies of these valence states
for each element shown in Fig. 10 are fairly consistent withthose of the reported values.1,25 For the non-doped BiCuSeO
sample, the oxidation state of Bi is mainly Bi3þ and a small
amount of Bi2þ and Bi4þ co-exist [Fig. 10(a)]. The oxidation
state of Cu is mainly Cu1þ and a small amount of Cu2þ coexists [Fig. 10(b)]. The oxidation state of Se, in which thebinding energy difference between the spin-orbit doublets of
3d3/2 and 3d5/2 in Se is known to be small, is confirmed to be
Se2 [Fig. 10(c)]. The peak can be separated by two
Gaussian peaks, which may come from the difference of theelement bonded to Se.25,26 The ideal oxidation states of theelements in BiCuSeO have been assumed to be Bi3þ, Cu1þ,
Se2, and O2, based on the requirement of the overall

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FIG. 9. Carrier concentration, p, and carrier mobility, l, of the (a) Bi1-xCuSeO
and (b) BiCuSe1-yO samples, as a function of the Bi deficiency, x, and the Se
deficiency, y, in the nominal composition, respectively. These values were estimated by the Hall effect measurement.

charge neutrality.19,27 Apparently, BiCuSeO is not a typicalionic compound and the oxidation state of each element
must not be so explicitly defined. In an ideal stoichiometriccomposition and standard oxidation state of each element,
non-doped BiCuSeO should be an insulator without any
charge carriers. However, the actual non-doped BiCuSeO
has moderate p-type conductivity. This may come from theintrinsic defects such as ion vacancies, interstitials, and antisites, which are the main contributions to the carriers byionizations.
For the Bi0.9CuSeO sample, the oxidation state of Bi
was entirely the Bi3þ [Fig. 10(d)], which is in clear contrast
with that of the non-doped BiCuSeO sample. Based on thestandard defect theory, hole carriers are introduced into thematrix phase by the increase in the Bi vacancy (VBi), which
can accept electrons at most from the valence band maximum in principles.28,29 In this case, the following charge
valence equation is assumed and hole carriers are introduced
into the [Bi2O2]2þ insulating layers:2ð13xÞþ
2
2Bi1x CuSeO ¼ Bi3þO
2ð1xÞ 22 2
þþ Cu1þ
þ 2xV
2 Se2
Bi þ 8xh ;(3)where VBi is a Bi vacancy and hþ is a hole. It is likely to
say that the reduction of q(T) of the Bi1-xCuSeO withincreasing x, as shown in Fig. 4(a), comes from the increasein the Bi vacancies in the matrix phase. In Fig. 2(a), we
found the very small shrink of the c-axis lattice constant for
the Bi1-xCuSeO samples (x > 0.05), which may closely relate
to the introduction of Bi vacancies into the matrix phase. In
the mixed valence of Cu, the ratio of Cu2þ/Cu1þ slightly
increased from 0.073 for the non-doped BiCuSeO to 0.149
for the Bi0.9CuSeO [Fig. 10(e)]. The increase in the Cu2þ
was also confirmed for the Ca-doped BiCuSeO, in whichhole carriers were introduced and the q(T) decreased withincreasing Ca doping.26 The change of the Cu oxidation statefrom þ1 to þ2 introduces donors and donates electrons tothe conduction band minimum, which may compensate thehole carriers. However, the decrease in q(T) in Fig. 2(a) andthe increase in the p value in Fig. 9(a) strongly suggest thatthe introduction of acceptors due to the Bi vacancies fairly
overcomes the donors due to the Cu oxidation with theincreasing x.
For the BiCuSe1-yO samples, the XPS spectra of Bi,
Cu, and Se, as shown in Figs. 10(g) to 10(i), hardly change
largely, compared with those of the non-doped BiCuSeO
sample, which are in clear contrast to those of theBi1-xCuSeO sample, although the electrical characteristics
change moderately. Figure 11 shows the ratios of Bi2þ/
Bi3þ and Bi4þ/Bi3þ in the XPS spectra for the BiCuSe1-yO
samples, as a function of the Se deficiency, y, in the nominal composition, which are extracted from Figs. 10(a) and10(g). Both ratios took a minimum for the y ¼ 0.05 sample
and then increase for the y ¼ 0.1 sample. These y dependences show a similar trend to the electrical resistivity
shown in Fig. 4(b) and the reversal trend as the hole concentration shown in Fig. 9(b). These results suggest thatthe decrease in the Bi2þ and Bi4þ contents is closely
related to the carrier concentration. These tendencies arealso consistent with the case of Bi1-xCuSeO samples, inwhich the Bi2þ and Bi4þ vanish in the Bi0.9CuSeO sample
and electrical resistivity is fairly reduced due to the carrier
doping.
In Fig. 2(b), a very small shrink in the c-axis lattice constant can be also seen for the BiCuSe1-yO samples (y > 0.05).
This may result from the introduction of a very small amountof actual Se vacancies into the matrix phase, in which thefollowing charge valance equation is assumed and electrons
are introduced into the [Cu2Se2]2 conducting layers
2 2þ
2BiCuSe1y O ¼ Bi3þ
2 O2
2ð12yÞ2

þ Cu1þ
þ 2yVþ
2 Se2ð1yÞSe þ 6ye ;(4)where VSeþ is a Se vacancy and e is an electron. In thiscase, the created electrons are compensated by the intrinsic
hole carriers and the electrical resistivity should beenhanced. However, a small amount of Bi vacancies, which
introduce the acceptor, may also be created in the Se deficient samples at the same time. At present, we cannot exactly
explain the relationship between the defect formation andthe carrier creation for the BiCuSe1-yO sample. We limit topresent the experimental results and a simple discussion

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FIG. 10. XPS spectra at the binding energy for Bi, Cu, and Se of non-doped BiCuSeO [(a)–(c)], Bi0.9CuSeO [(d)–(f)], and BiCuSe0.9O [(g)–(i)] samples. The
Gaussian fitting curves for each oxidation state are also shown for each spectrum.

using a standard defect theory. A further discussion of thechemical shift is expected to fully understand the changes ofthe valence state and carrier number in this oxide thermoelectric compound.

FIG. 11. The ratios of Bi2þ/Bi3þ and Bi4þ/Bi3þ in the XPS spectra for the
BiCuSe1-yO samples, as a function of the Se deficiency, y, in the nominal
composition.

IV. CONCLUSIONS

We have fabricated the BiCuSeO bulks using raw materials with Bi or Se deficiencies in the starting composition
and investigated crystallographic properties, chemical composition, and oxidation state of each element and thermoelectric properties. The important results and conclusions
obtained from this study are summarized as follows.
(1) Owing to the Bi or Se deficiencies in the starting composition, excessive raw materials and related compounds
were deposited as impurity phases. However, the matrix
phase is nearly the stoichiometric BiCuSeO phase. These
results are in clear contrast with the previous reports of
the BiCu1-xSeO samples fabricated by Cu deficiencies inthe starting composition.
(2) For Bi1-xCuSeO samples, the electrical resistivity, q(T),decreases and thermoelectric power, S(T), also decreases
with increasing the contents of Bi deficiencies in thestarting composition in spite of nearly the stoichiometricmatrix phase.
(3) On the other hand, for BiCuSe1-yO samples, the q(T)value decreased for the y value up to 0.05 and thenslightly increased for the y ¼ 0.1 sample, and the q(T)

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Ishizawa et al.

value at high temperatures is independent of the y value.
The S(T) value changes closely depending on the q(T)value.
(4) Using the XPS measurements, the actual oxidation statesof Bi and Cu deviate from the formal valences of stoichiometric Bi3þCu1þSe2O2. Based on the standard defecttheory, in the Bi1-xCuSeO system, the hole carriers originating from the Bi vacancies are introduced into conductive [Cu2Se2]2 layers and then, the electrical resistivity
decreases with increasing x. In the BiCuSe1-yO system,
the Bi2þ/Bi3þ and Bi4þ/Bi3þ ratios in the XPS spectra
took a minimum for the y ¼ 0.05 sample and then increase
for the y ¼ 0.1 sample. These y dependences show a similar trend to the electrical resistivity and the hole concentration. We proposed charge valance equations and theorigin of the carriers in the present BiCuSeO samples.
(5) The maximum thermoelectric dimensionless figure of
merit, ZT ¼ 0.60, was achieved at 773 K for the Bi1xCuSeO samples (x ¼ 0.025 and 0.05). These results are
in clear contrast with the reported results for the Cu deficiency bulks.
ACKNOWLEDGMENTS

This work was performed under the Inter-university
Cooperative Research Program of the Institute for Materials
Research, Tohoku University (Nos. 15K0119, 16K0039, and
17K0065). The authors thank Dr. K. Nonaka, Mr. T. Itoh,
Mr. K. Sasaki, and Mr. K. Kobayashi of Iwate University for
their technical supports of EPMA, SEM, and XPS
measurements.
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