Thermodynamic and structural properties of PrBaCo2O 5+δ

Article abstract of DOI:10.1134/S0020168506020154

The oxygen content of the double cobaltite PrBaCo₂O _5+δ was determined by coulometric titration in broad temperature and oxygen pressure ranges, and the partial thermodynamic functions of oxygen were evaluated. The results on the nonstoichiometry and structural properties of PrBaCo₂O_5+δ demonstrate that, at 5 + δ < 5.3, this cobaltite has a high-temperature phase with lattice parameters a = 11.774 A, b = 11.932 A, and c = 7.609 A. The composition dependences of ΔH(O) and ΔS(O) indicate that the 3a_p x 3a_p x 2a_p orthorhombic structure persists in the range 5.15 ≤ 5 + δ ≤ 5.30. Some of the Co ³⁺ ions are shown to disproportionate according to the scheme 2Co³⁺ = Co²⁺ + Co⁴⁺. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506020154

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 2, pp. 196–201. © Pleiades Publishing, Inc., 2006.
Original Russian Text © I.A. Leonidov, M.V. Patrakeev, E.B. Mitberg, O.N. Leonidova, V.L. Kozhevnikovv, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 2,
pp. 232–237.
Thermodynamic and Structural Properties
of PrBaCo O
2
5 + d
I. A. Leonidov, M. V. Patrakeev, E. B. Mitberg,
O. N. Leonidova, and V. L. Kozhevnikov
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences,
Pervomaiskaya ul. 91, Yekaterinburg, 620219 Russia
e-mail: leonidov@imp.uran.ru
Received April 21, 2005; in final form, August 25, 2005
Abstract—The oxygen content of the double cobaltite PrBaCo O
was determined by coulometric titration
5 + δ
2
in broad temperature and oxygen pressure ranges, and the partial thermodynamic functions of oxygen were
evaluated. The results on the nonstoichiometry and structural properties of PrBaCo O demonstrate that, at
2
5 + δ
5
+ δ < 5.3, this cobaltite has a high-temperature phase with lattice parameters a = 11.774 Å, b = 11.932 Å, and
c = 7.609 Å. The composition dependences of ∆H(O) and ∆S (O) indicate that the 3a × 3‡ × 2‡ orthorhom-
p
p
p
3
+
bic structure persists in the range 5.15 ≤ 5 + δ ≤ 5.30. Some of the Co ions are shown to disproportionate
3
+
2+
4+
according to the scheme 2ëÓ = ëÓ + ëÓ .
DOI: 10.1134/S0020168506020154
INTRODUCTION
properties of PrBaCo O
and the effect of oxygen
2
5 + δ
content on the thermodynamic properties of this
cobaltite.
Partial substitution of alkaline-earth metals for rare-
earth metals in RCoO cobaltites leads to a number of
3
effects associated primarily with the size factor and the
mechanism underlying the charge compensation of
acceptors. These effects include a tendency toward A-
site ordering, changes in the oxidation state of the
EXPERIMENTAL
PrBaCo
O
5 + δ was prepared by solid-state reaction.
2
cobalt ions, and the development of significant oxygen A stoichiometric mixture of Pr
O11, Co
O
, and BaCO
6
3
4
3
nonstoichiometry. For example, substitution of barium was reacted in air at temperatures from 900 to 1150°C
for half of the rare-earth ions leads to the formation of with several intermediate grindings. After firing at
RBaCo O
double perovskites in which δ may vary 1150°C, the sample was slowly cooled to room temper-
2
5 + δ
ature. To prepare samples with reduced oxygen con-
from 0 to 1 [1].
tents, PrBaCo O
synthesized in air was then fired in
a helium atmosphere ( pO2 = 10 Pa) at temperatures
2
5 + δ
This broad range of oxygen stoichiometries in the
double cobaltites is of considerable interest because it
allows one to control the relative amounts of cobalt in
different valence states. Moreover, a change from octa-
hedral to pyramidal coordination influences the elec-
2
from 300 to 900°C for 2 h, followed by cooling to room
temperature.
The phase composition of the resultant materials
tronic configuration of the cobalt ions through changes was determined by x-ray diffraction (XRD) analysis
3
+
in crystal-field parameters [2]. Given that Co ions
tend to disproportionate, resulting in high concentra-
(
Stoe Stadi-P diffractometer, CuKα1 radiation, λ =
.54051 Å, step size of 0.02°, 2θ = 10°–100°) at room
temperature. The XRD data were analyzed using the
Fullprof program [3].
1
2
+
4+
tions of Co and Co ions, which may be in several
3
+
spin states, like Co , the RBaCo O
oxides are con-
2
5 + δ
venient model systems for studying the properties of
materials containing 3d cations in various electronic
configurations. For this reason, double cobaltites are
now receiving ever increasing attention.
The oxygen content of the samples was determined
by thermogravimetry, using a SETARAM TG-92 ther-
moanalytical system. The samples were reduced by
heating in a 5% H + 95% He gas mixture to give BaO,
2
Note that many questions regarding the nature of the Pr O , and cobalt metal. The associated weight loss
2
3
ordered state in the double cobaltites still remain open. was used to evaluate the oxygen content of the as-pre-
In this paper, we report our findings on the structural pared sample.
1
96

THERMODYNAMIC AND STRUCTURAL PROPERTIES OF PrBaCo O
197
2
5 + δ
3
The equilibrium oxygen content of the cobaltite as a
function of temperature and oxygen partial pressure in
the gas phase was determined by coulometric titration.
Measurements were made isothermally in a stabilized
zirconia electrochemical cell in the form of a tube to
which a lid of the same material, with the platinum
electrodes of the oxygen sensor and oxygen pump, was
attached with high-temperature adhesive. The sample
was placed in an alundum container, which was then
mounted in the tube. The temperature was monitored
with a Pt–30% Rh/Pt–6% Rh thermocouple. In electri-
cal measurements, we used a Solartron 7081 precision
voltmeter and a Solartron 1286 electrochemical inter-
face as a controllable current source. The measuring
cell was sealed in flowing oxygen at 1070°C and then
cooled to 950°C. The oxygen content determined by
(
4
b/2 – a) × 10 , Å
0
(a)
3
0
20
1
0
0
4
thermogravimetry at 650°C and pO2 = 2.1 × 10 Pa was
used as a reference point in coulometric titration exper-
iments.
V, Å3
2
2
2
2
36
35
34
33
RESULTS AND DISCUSSION
(b)
Effect of oxygen content on structural parame-
ters. PrBaCo O
synthesized in air had an orthor-
2
5 + δ
hombic structure (sp. gr. Pmmm) with lattice parame-
ters a = 3.8981(1) Å, b = 7.7069(2) Å, and c =
.6354(2) Å. The value of δ was 0.77. In the XRD pat-
7
terns of the samples with other δ values, all of the
reflections could be indexed in the same orthorhombic
cell, a × 2‡ × 2‡ .
p
p
p
The lattice parameters, unit-cell volume V, and δ of
our samples are listed in the table.
Figure 1a shows the difference b/2 – a as a function
of oxygen content. The difference between a and b/2
has a maximum near the composition PrBaCo O ,
2
5.5
where the average charge state of the cobalt ions is 3+.
The maximum in b/2 – a as a function of 5 + δ points to
ordering of oxygen vacancies along the ‡ axis. More-
over, it seems likely that, just as in TbBaCo O [2], a
232
5
.2
5.4
5.6
5.8
5
+ δ
2
5.5
structural analog of PrBaCo O , this difference is par-
2
5.5
Fig. 1. Effect of oxygen content on the structural parameters
of PrBaCo O
tially due to the strong difference between the a- and
.
5 + δ
2
3
+
b-axis Co–O bond lengths in the Co O pyramids and
5
3
+
Co O octahedra, associated with (d
2
2
/d3y
2
2
)
6
3
x
– r
– r
the formation of the larger sized ions Co2+ in Co O
2+
5
orbital ordering. The decrease in unit-cell volume at 5 +
pyramids.
δ > 5.5 (Fig. 1b) is mainly due to the drop in b, which
3
+
is caused by the distortion of the Co O octahedra at
6
Effect of temperature and oxygen pressure on
increased oxidation states of the cobalt ions. The rise in
unit-cell volume for 5 + δ < 5.5 is related primarily to
the increase in ‡ with decreasing δ, which is associated
the oxygen content and structure. Figure 2 shows the
equilibrium oxygen content of PrBaCo O
as a func-
2
5 + δ
tion of oxygen partial pressure in the temperature range
650–950°C. Under these conditions, 5 + δ varies from
3
+
both with the change from Co O octahedra (elongated
6
3
+
along the b axis) to Co O pyramids (elongated along 5.0 to 5.56. The kinks near δ = 1/3 in the range 750–
5
the a axis) and the partial reduction of cobalt, leading to 850°C suggest the formation of a high-temperature
INORGANIC MATERIALS Vol. 42 No. 2 2006

1
98
LEONIDOV et al.
5
5
+ δ
ference b/3 – a/3 = 0.052 Å indicates that the CoO pyr-
5
amids in the high-temperature phase are elongated
along the b axis, just as in PrBaCo O . It should be
.6
.5
.4
.3
.2
.1
.0
2
5.5
5
5
5
5
5
5
emphasized that PrBaCo O
2
with a 3a × 3‡ × 2‡
5 + δ p p p
cell is formed at δ < 0.33 and temperatures above
00°C (Fig. 2) and can be quenched to room tempera-
ture. According to XRD results (table), PrBaCo O
7
2
5 + δ
samples with 0.22 < δ < 0.77 slowly cooled to room
temperature have an a × 2‡ × 2‡ cell.
7
6
5
4
3
2
1
p
p
p
Thermodynamic properties of weakly bound
oxygen. The data presented in Fig. 2 can be used to
evaluate the chemical potential of weakly bound oxy-
gen, ∆µ , in PrBaCo O relative to the standard state
é
2
5 + δ
in the gas phase since, in thermodynamic equilibrium,
the difference in the chemical potential of oxygen
between the gas phase and the cobaltite is constant:
0
1
2
3
4
logpO2 [Pa]
Fig. 2. Oxygen content of PrBaCo O
as a function of
2
5 + δ
oxygen partial pressure in the gas phase at (1) 650, (2) 700,
3) 750, (4) 800, (5) 850, (6) 900, and (7) 950°C.
0
O2
∆
µ ≡ µ (PrBaCo O5 + δ) – 1/2µ
O
O
2
(
(
1)
=
1/2RT ln p .
O
2
phase of PrBaCo O5 + δ, which has not been described
2
previously.
The results indicate that, in both polymorphs, the chem-
ical potential is a linear function of temperature at fixed
δ. In view of this, we determined the partial molar
To confirm this assumption, the cobaltite was fired
at 1000°C in an He atmosphere ( pO2 = 10 Pa) and then
2
enthalpy ∆H (O) and entropy ∆S (O) of oxygen by the
standard formula
quenched in liquid gallium. As would be expected, the
XRD pattern of that sample showed that the high- and
low-temperature phases of PrBaCo O
differed sig-
2
5 + δ
∆
µé = ∆H (O) – í∆S (O).
(2)
nificantly in structural parameters (Fig. 3). The oxygen
ordering near δ = 1/3 give grounds to expect that the lat-
tice parameters of the high-temperature phase are
The results are presented in Fig. 4. At 5 + δ < 5.33,
the enthalpy is a weak function of the oxygen content
related by ‡ ≈ b ≈ 3a . Indeed, all of the reflections in
p
the XRD pattern of the high-temperature phase can be of the high-temperature phase with the 3a × 3a × 2a
p
p
p
indexed in a 3a × 3‡ × 2‡ orthorhombic cell with a =
p
p
p
cell. In contrast, the entropy varies rapidly in this com-
1
1.774 Å, b = 11.932 Å, and c = 7.609 Å. The large dif- position range, which is associated with the reduction
in the concentration of vacancies accessible to oxygen.
The complex shape of the ∆H (O) and ∆S (O) curves at
Oxygen content and lattice parameters of PrBaCo O5 + δ
2
5
+ δ > 5.33 indicates that oxygen is incorporated in at
V, Å3
least two types of vacant sites, differing in energy. The
5
+ δ
a, Å
b, Å
c, Å
increase in ∆H (O) and ∆S (O) with δ at 5 + δ > 5.33
indicates that oxygen begins to occupy energetically
less favorable sites. One possible reason for this behav-
5
5
5
5
5
5
5
5
5
.765
.673
.629
.595
.538
.441
.371
.294
.22
3.8981
3.9036
3.9040
3.9038
3.9058
3.9075
3.9177
3.9304
3.9342
7.8069
7.8195
7.8332
7.8534
7.8690
7.8786
7.8830
7.8758
7.8818
7.6345
7.6291
7.6296
7.6214
7.6157
7.6068
7.6171
7.6074
7.5999
232.3
232.9
233.3
233.7
234.1
234.2
235.2
235.5
235.7
ior of ∆H (O) and ∆S (O) is that, as the oxygen stoichi-
ometry approaches 5 + δ = 5.5, some of the oxygen ions
undergo local ordering, as, e.g., in the low-temperature
orthorhombic cell a × 2‡ × 2‡ of PrBaCo O .
p
p
p
2
5.5
The incorporation of oxygen into vacancies V in
O
2
+
3+
PrBaCo Co O
can be represented by the reaction
+ δ
5
scheme
2
+
2–
3+
1
/2O + V + 2Co = O + 2Co .
(3)
2
é
INORGANIC MATERIALS Vol. 42 No. 2 2006

THERMODYNAMIC AND STRUCTURAL PROPERTIES OF PrBaCo O
199
2
5 + δ
(a)
(b)
1
0
20
30
40
50
60
70
θ, deg
2
2
Fig. 3. XRD patterns of PrBaCo O5 + δ samples fired in a helium atmosphere ( pO2 = 10 Pa) and then (a) quenched from 1000°C
2
or (b) cooled from 600°C.
The equilibrium constant of reaction (3),
can be expressed through the standard enthalpy and
entropy:
2
–
3+
2
[
O ][Co ]
–1/2
2
K = ------------------------------ -- pO
,
(4)
p
2
+
2
0
0
[
V ][Co ]
–RTlnK = ∆ H – T∆ S .
(5)
O
p
r
r
INORGANIC MATERIALS Vol. 42 No. 2 2006

2
00
LEONIDOV et al.
∆
S(O), J/(mol K)
close to [V ] = 0.33 – δ, Eq. (7) for entropy takes the
O
–
40
60
80
form
(
a)
2
(
0.33 – δ)(1 – 2δ)
0
∆
S(O) = Rln-------------------------------------------- -- + ∆ S .
(8)
r
2
δ(1 + 2δ)
–
–
As seen in Fig. 4, the partial molar entropy of oxy-
gen in PrBaCo O
0
calculated by Eq. (8) with ∆ S =
2
5 + δ
r
–
51 J/(mol K) agrees with experimental data only in the
narrow composition range 0.17 < δ < 0.25. The discrep-
ancy between the calculation results and experimental
∆
S (O) data can be reduced by taking into account that
3
+
some of the Co ions (2α) disproportionate according
to the reaction
–
100
∆
H(O), kJ/mol
3+
2+
4+
2
ëÓ = ëÓ + ëÓ .
(9)
–
40
60
80
With allowance made for this process, the crystal-
chemical formula of the cobaltite can be represented as
(
b)
–
–
2
+
3+
4+
α
PrBaCo1
Co
Co O5 + δ.
– 2δ + α
1 + 2δ – 2α
Note that Co3+ disproportionation accounts for the
high hole conductivity of PrBaCo O5 + δ, which is
2
1
00 S/cm at δ = 0 and 400 S/cm at δ = 0.5 (Fig. 5).
–
100
Moreover, since conductivity is proportional to the
3
+
4+
product of the Co and Co concentrations [4], the
increase in this product by a factor of 4 as δ increases
from 0 to 0.5 indicates that the disproportionation
parameter is a function of composition. Clearly, for
–
–
120
140
δ
0 this function approaches some constant. Given
5
.05
5.15
5.25
5.35
5.45
5.55
+ δ
this, one can show by simple calculations that the rela-
2
5
tion α = 0.15 + δ provides a good fit to the conductivity
versus oxygen content data. The partial entropy of the
cobaltite can then be expressed in the form
Fig. 4. Partial molar (a) entropy and (b) enthalpy as func-
tions of oxygen content; the dashed and solid lines represent
calculations by Eqs. (8) and (10), respectively.
2
(
0.33 – δ)(1 – 2δ + α )
0
∆
S(O) = Rln------------------------------------------------------ -- + ∆ S . (10)
r
2
δ(1 + 2δ – 2α )
Combining (1), (2), (4), and (5), we obtain
0
Figure 4 demonstrates that Eq. (10) with ∆ S =
0
r
∆
H(O) – T∆S(O) = ∆ H
r
−62 J/(mol K) gives a good fit to the experimental
∆
S(O) data over most of the composition range of the
2
+
2
(6)
⎛
[V ][Co ] ⎞
O
0
high-temperature phase of PrBaCo O
with the 3a ×
5 + δ p
2
–
T⎜ ∆ S + Rln------------------------------ -- ⎟ .
r
2– 3+
2
3‡ × 2‡ cell. The discrepancy between the experimen-
⎝
[O ][Co ] ⎠
p p
tal ∆S (O) data and calculation results at δ < 0.15 is
attributable to the possible formation of small
PrBaCo O -type a × ‡ × 2‡ domains (clusters) as δ
Therefore,
2
5
p
p
p
0
∆
H(O) = ∆ H
approaches zero. The number of vacancies accessible to
r
oxygen in such domains is obviously smaller than that
2
+
2
and
(7)
[
V ][Co ]
in the 3a × 3‡ × 2‡ cell. For 5 + δ > 5, such vacancies
p p p
O
0
∆
S(O) = Rln------------------------------ -- + ∆ S .
r
2– 3+
2
are rapidly filled with oxygen and, according to Eq. (7),
the entropy decreases.
[
O ][Co ]
In the study by Maignan et al. [1], the formation of
For 5 + δ < 5.5, the composition of the cobaltite can be
a domain structure in LnBaCo O nonstoichiometric
2
+
3+
2
5 + δ
written in the form PrBaCo – 2δCo
O5 + δ. Under
1
1 + 2δ
cobaltites with different subcell dimensions was
the assumption that the concentration of vacancies revealed by electron diffraction and high-resolution
accessible to oxygen in the high-temperature phase is microscopy. HoBaCo O was shown to consist of
2
5.3
INORGANIC MATERIALS Vol. 42 No. 2 2006

THERMODYNAMIC AND STRUCTURAL PROPERTIES OF PrBaCo O
201
2
5 + δ
logσ [S/cm]
Thermodynamic analysis indicates that the 3a × 3‡
p p
×
2‡ long-range order persists in the range 5.15 ≤ 5 +
2
2
2
2
1
.7
.5
.3
.1
.9
p
δ ≤ 5.30. These stability limits of the high-temperature
phase are close to δ = 1/9 and 1/3, corresponding,
respectively, to one and three occupied oxygen sites in
the ab plane of the 3a × 3‡ × 2‡ cell. For δ < 0.11 and
p
p
p
δ > 0.33, this cobaltite seems to contain domains with
another structure, PrBaCo O and PrBaCo O ,
2
5
2
5.5
respectively, even at high temperatures.
7
6
5
4
3
2
1
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, project no. 04-03-32948.
–
1
0
1
2
3
4
REFERENCES
logpO2 [Pa]
1. Maignan, A., Martin, C., Pelloquin, D., et al., Structural
and Magnetic Properties of Ordered Oxygen-Deficient
Fig. 5. Log–log plots of hole conductivity vs. oxygen partial
pressure in the gas phase for PrBaCo O5 + δ at (1) 650,
2) 700, (3) 750, (4) 800, (5) 850, (6) 900, and (7) 950°C.
Perovskites LnBaCo O5 + δ, Closely Related to the
2
2
(
“112” Structure, J. Solid State Chem., 1999, vol. 142,
pp. 247−260.
2
3
4
. Moritomo, Y., Akimoto, T., Takeo, M., et al., Metal–
3
a × 3‡ × 2‡ domains. At the same time, the XRD
p p p
Insulator Transition Induced by a Spin-State Transition
pattern of this cobaltite can be indexed with high accu-
racy in a homogeneous orthorhombic structure with an
a × 2‡ × 2‡ cell.
in TbBaCo O
(δ = 0.5), Phys. Rev. B: Condens. Mat-
2
5 + δ
ter, 2000, vol. 61, no. 20, pp. 13325–13328.
p
p
p
. Rodriguez-Carvajal, J., Fullprof:A Program for Rietveld
Refinement and Pattern Matching Analysis, Satellite
Meet. on Powder Diffraction of the XV Congr. of the
IUCr, Toulouse, 1990.
CONCLUSIONS
The present results on the nonstoichiometry and
structural properties of PrBaCo O demonstrate that,
. Mitberg, E.B., Patrakeev, M.V., Leonidov, I.A., et al.,
2
5 + δ
High-Temperature Electrical Conductivity and Ther-
at 5 + δ < 5.3, this cobaltite has a high-temperature
mopower in Nonstoichiometric La1 – xSr CoO
(x =
x
3 – δ
phase with lattice parameters a = 11.774 Å, b =
0.6), Solid State Ionics, 2000, vol. 130, no. 3/4,
pp. 325−330.
1
1.932 Å, and c = 7.609 Å.
INORGANIC MATERIALS Vol. 42 No. 2 2006