DEUTERIUM SOLUBILITY AND ISOTOPE EFFECT FOR H/D
UPTAKE IN PROTON-CONDUCTING OXIDES
La1-ySrySc1-yMgyO3-α
V.B. Vykhodets1, T.E. Kurennykh1
V.I. Tsidilkovski2, V.P. Gorelov2, A.Yu. Stroeva2
A.Ya. Fishman3
1
Institute for Metal Physics, RAS, Ekaterinburg, Russia
2
Institute of High-Temperature Electrochemistry, RAS, Ekaterinburg,
Russia
3
Institute of Metallurgy, RAS, Ekaterinburg, Russia
Abstract
Solubility of deuterium in proton-conducting oxides La1-ySrySc1yMgyO3-α (y = 0.01 to 0.2) has been studied by the nuclear microanalysis
method based on the nuclear reaction 2H (d, p) 3H. The saturation of
samples with hydrogen isotopes was made in wet atmosphere at PO2 =
10-18 atm and the temperature of 600 OC. It has been shown that for these
conditions deuterium content in oxides is sufficiently small. A new
method for experimental studies of isotope effect for H/D uptake has
been proposed. This method is based on measuring the concentration of
only one of the hydrogen isotopes in oxides.
Introduction
The interest to proton-conducting oxides is due primarily to their
perspective use in various electrochemical devices (fuel cells,
electrolyzers, converters, sensors, etc.) [1, 2]. Oxides of the АIIВIV1III
III
II
III
xR xO3-α and А 1-xC xВ O3-α families are known best among such
materials. Protons appear in these materials when water vapors from the
gaseous phase dissolve. In this case, as they are incorporated in the
oxide, oxygen ions fill oxygen vacancies and protons form hydroxyl
groups (OH). Oxygen vacancies, which are necessary for dissolution of
hydrogen in ABO3 oxides, are induced by embedding acceptor dopants
(RIII, CII). Mechanisms of the transport and defect formation in these
materials are rather complicated and at present they are analyzed using
diverse experimental and theoretical methods [3]. Studies of isotope
effects present a particular interest with respect to proton-conducting
oxides if one considers their possible impact on the understanding of the
1
state and the dynamics of protons. Until recently isotope studies of these
materials were focused mainly on isotope effects in conductivity [4] and
vibration spectra [5].
In a recent study [6] the abnormal behavior and significant
magnitude of the so-called thermodynamic isotope effect (TDIE) were
predicted for proton-conducting oxides. It was shown [6] that under
equivalent external conditions and in nominally identical samples the
difference of the concentrations of H and D ions can be considerable if
samples are exposed to gas phases containing one of these elements, but
not others. TDIE originates from the difference of the chemical
potentials of H and D ions in oxides (because of different vibration
terms) and those of their isotope vapors in a gas. The value of the effect
considerably depends on external conditions and the ratio of the
hydrogen content over the hydration limit in the oxide. The researchers
[7] performed an experimental study of TDIE using thermogravimetry
and confirmed the existence of the effect and its direction. To the best
our knowledge, differences of solubility of hydrogen and its isotopes in
proton-conducting oxides have not been studied by direct methods so
far. However, these studies are important not only for determination of
mechanisms of defect formation and charge compensation, but also for
correct analysis of transport mechanisms.
The present study deals with the effect of double doping of the
LaScO3 compound with strontium and magnesium on hydrogen
solubility and the analysis of the isotope effect in solubility. Perovskites
with two doped sublattices have been studied extensively in recent years
with the view of improving characteristics of these materials as proton
conductors. Solubility of hydrogen in these compounds presents an
independent interest, because the hydration limit for proton-conducting
oxides is often much lower than the concentration of acceptor
impurities.
1. Samples and Experimental Technique
1.1. Synthesis of the samples
The initial materials for synthesis of the ceramic samples were
La(NO3)36H2O, Sc(NO3)34H2O, SrCO3 and MgCO3. The ceramic was
prepared by a semi-chemical method. For this purpose, titrated water
solutions of La(NO3)3 and Sc(NO3)3 were mixed and were precipitated
with ammonia in the form of hydroxides. The sediment of the
2
hydroxides was dried and was calcined in air at 600C for 1 hour. The
calculated amount of magnesium and strontium carbonates was added to
the powder. The samples had the formula La1-ySrySc1-yMgyO3- with y
equal to 0.01, 0.05, 0.10, 0.15 and 0.20. The powders were mixed in a
jasper mortar with ethyl alcohol. Test samples in the form of pellets 14
mm across were compacted from the mixtures, which were calcined at
850 C. The pellets were compacted without a binder in a steel mold
under a pressure of 200 MPa. The samples were sintered in air at 1600
C for 2 hours. The density of the ceramic, which was prepared by this
technology, accounted for at least 98% of the theoretical value. The Xray diffraction analysis demonstrated that the ceramic had one phase and
its structure was of the perovskite type.
1.2. Saturation of the samples with hydrogen isotopes
The samples were saturated with hydrogen isotopes in a tube
furnace. They were placed in a porcelain tube and the gas was circulated
in a closed loop. The gas circuit of the furnace included a bubbler with
water, a circulation pump and an electrochemical oxygen pump. The last
pump was based on an oxygen-conducting solid electrolyte having the
formula Zr0.75Y0.15O2-α. The oxygen pump produced a reducing
atmosphere in the gas circuit. The oxygen pressure was about 1018 atm.
The direction of the gas flow in the circuit was such that the gas was fed
to the bubbler and was saturated with water vapors after it passed the
oxygen pump.
Prior to saturation of the samples, the bubbler in the gas circuit
was replaced by a column with ceolites for drying of the system. It was
kept under these conditions at a preset temperature for about 24 hours.
Then the ceolite column was replaced by the bubbler with water. The
bubbler water temperature was maintained at 20±0.5 C. The saturation
time was 3-4 days. The furnace temperature was measured with a
platinum / platinum-rhodium thermocouple to within ±1 C. The
experiments were performed with both heavy water D2O and a mixture
of heavy and common water. As soon as isothermal annealing was
complete, the furnace was turned off and the samples were cooled to
room temperature in a reducing atmosphere.
3
1.3. Measurements of the deuterium concentration of the
samples
The concentration of dissolved atoms was measured by the
method of nuclear microanalysis using accelerated light ions. The
analysis was performed in a 2 MV Van de Graaf accelerator. The
reaction 2H (d,p) 3H was used. Nuclear reaction products were registered
by means of silicon surface-barrier detectors. The number of deuterons
striking the sample was measured with an accuracy of better than 1%.
Preliminary studies revealed that the deuterium concentration
measurements were complicated due to the release of products of
competing nuclear reactions 16O (d,p0) 17O and 12C (d,p)13C. The last
reaction was due not to the presence of carbon in the test samples, but to
the known deposition of hydrocarbons on the surface of samples
exposed to radiation. The optimal conditions of the nuclear physical
experiment, which provided no-background registration of protons from
the reaction 2H (d,p) 3H, consisted in the following. The energy of
particles in the primary beam was 700 keV and a mylar film 26 m
thick was fitted in front of the detector. Also, the chamber with test
samples was evacuated differentially using a cryogenic pump. As a
result, the rate of deposition of hydrocarbons on the surface of the
irradiated sample decreased to an acceptable level. A special experiment
demonstrated additionally that the deuterium concentration remained
unchanged in the irradiated volume of the sample during measurements.
Figure 1 presents a typical spectrum, which was obtained during
irradiation of the test samples. It is seen that the aforementioned
measures ensured a no-background determination of products of the
reaction 2H (d,p) 3H. The deuterium concentration was calculated by
comparing spectra of the test samples and a standard sample, in which
the concentration of deuterium atoms was constant in depth [8]. The
standard sample was ZrCr2D4 deuteride. The braking capacity of highenergy particles for complex targets was calculated using Bragg's
additive rule. Data on the braking capacity of pure substances were
adopted from Ref. [9].
The described method allowed measuring the deuterium
concentration averaged over the irradiated volume of the sample. The
irradiated volume was determined by the diameter of the incident beam
4
(~1 mm) and the depth, to which the concentration was measured
without failure of the test sample. The depth was not larger than 2 m.
80
16
17
O(d,p0) O
2
3
H(d,p) H
yield
60
40
12
13
C(d,p) C
20
0
250
300
350
400
450
500
550
600
channel number
Fig. 1. Spectrum of nuclear reactions
Considering these local measurements and a high mobility of
hydrogen atoms in solids even at room temperature, one could not
exclude a considerable decrease in the concentration of deuterium in
surface layers of the samples when they were kept in air and a vacuum.
These operations were unavoidable in our experiments since deuteriumsaturated samples had to be transferred to the vacuum chamber. In this
case, measurements of the deuterium concentration began 1.5-2 hours
after the samples were removed from the humid atmosphere. No
changes of the deuterium concentration were observed in repeated
measurements, which were made during several days. It was concluded
therefore that deuterium did not escape from surface layers of the test
samples at room temperature.
Different thermal treatments, which were used for synthesis and
preparation of the samples, could lead to an inhomogeneous distribution
of the metal atoms over the volume of a grain and generally over the
sample. This circumstance should also be taken into account in the case
of a high locality of measurements. Therefore, measurements of the
5
deuterium concentration were accompanied by recording of Rutherford
backscattering spectra. They provided information about the chemical
composition of the metal subsystem of the crystals strictly in the same
microvolume, which was used for measuring the deuterium
concentration. Figure 2 presents the dependence of the lanthanum
concentration, which was measured by the Rutherford backscattering
method, on the average concentration of lanthanum in the sample. Here
and in what follows all the concentrations are given with respect to the
number of atoms in the La1-ySrySc1-yMgyO3- compound. It is seen that
C1 and C2 correlate well in three cases, while the difference is
considerable in two cases. The corresponding corrections are introduced
into the parameter y of these samples in the section 2. This information
was obtained in the present study for the lanthanum concentration only.
The sensitivity of the method proved to be insufficient for the other
metal atoms.
20
C1 , at.%
19
18
17
16
16
17
18
19
20
C2 , at.%
Fig. 2. Ratio between the lanthanum concentration in the studied microvolume
C1 and the concentration averaged over the sample C2
Information about the isotope effect is traditionally presented as the
ratio of hydrogen and deuterium solubility values. Since capabilities of
this approach are limited (nuclear reactions are absent on the light
hydrogen isotope), it was reasonable to use another method. Deuterium
6
concentrations in oxide were measured in two cases. In the first case, the
gaseous phase contained atoms of one hydrogen isotope (deuterium),
while in the second case, it had atoms of two isotopes (hydrogen and
deuterium). The ratio CD(H)/CD was the measure of the isotope effect,
with CD and CD(H) being the deuterium concentrations in the first and
second cases, respectively. This ratio may be used to calculate the value
of the isotope effect in the traditional representation CD(H)/CH(D).
When CD and CD(H) were measured, the total pressure of water vapors
was determined by the pressure of supersaturated water vapors at a
temperature of 20C. CD(H) was measured at the same partial pressures
of H2O and D2O vapors.
2. Results and Discussion
Values of CD and CD(H), which were measured at 600 C, are
given in the table. The mean-square measurement error did not exceed
4%.
Table
Deuterium concentrations of the samples
y
0.01
0.05
0.09
0.13
0.17
CD, at.%
0.039
0.39
0.47
0.57
0.59
CD(Н), at.%
0.025
0.37
0.43
0.39
0.45
These data were analyzed with making use of the results [6] and
of the solution of the isotopes separation problem for proton-conducting
oxides in equilibrium with the gas phase (V.I. Tsidilkovski, to be
published).
The discussion of the last issue is beyond the scope of this study
and we shall restrict ourselves only to analysis results and their brief
discussion.
According to the standard simple defect model, see, e.g., [3] and
Refs therein,
7

1
16 x 
C D  PD2O K D 1 
4
PD2O K D 


1/ 2

 1 ,


(1)
where PD2O is the partial pressure of D2O vapors, KD is the solubility
equilibrium constant, and x is the maximal possible effective
concentration of oxygen vacancies (x equals half of hydration limit).
It was found that the obtained CD values satisfied the expression (1) if it
was assumed that effective concentrations x and nominal dopants
content Z (totally strontium and magnesium) were related as x = 0.3Z. In
other words, when LaScO3 was doped with strontium and magnesium,
only 0.3 of oxygen vacancies was formed per dopant atom. Of course,
only the vacancies, which participated in the water uptake, are meant.
The obtained result is illustrated in Fig. 3. Mean-square errors of the
parameter x were correlated with the data shown in Fig. 2. Thus, it was
postulated that possible inhomogeneities in the distributions of all the
metal atoms had the same values as those for the lanthanum atoms.
0.7
0.6
0.5
CD,at.%
0.4
0.3
0.2
0.1
0.0
0.0
0.5
1.0
1.5
2.0
2.5
X, at.%
Fig. 3. Deuterium solubility CD versus effective concentration of oxygen
vacancies x in the La1-ySrySc1-yMgyO3- oxide. The line denotes values
calculated from the formula (1) at PD2O K D  0.6 at.%
8
Figure 4 presents experimental data on the isotope effect and its
theoretical values.
1.0
CD(H)/CD
0.8
0.6
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
X, at.%
Fig. 4. Dependence of the ratio CD(H)/CD on the effective concentration of
oxygen vacancies x in the La1-ySrySc1-yMgyO3- oxide. The line denotes
a theoretical value equal to 1/2.
It is seen that experimental values of the ratio CD(Н)/CD are in
good agreement with the estimated theoretical ratio 1/2. This value of
CD(Н)/CD is expected for the high ratios 16 x /( PD2O K D ) which occur in
our case. In such a region of diluted proton (deuteron) concentrations the
contribution of the isotope effect (a difference of H and D contents in the
sample) to the CD(Н)/CD magnitude must be small.
3. Conclusion
The effective concentration of oxygen vacancies (i.e. vacancies,
which can be occupied by oxygen when water vapors dissolve), which
was determined in the study, proved to be nearly 3 times smaller than
the concentration of acceptor impurities. The fact that the hydration
limit turned out to be lower, than it followed from the formal valence
ratio, can be explained by a number of circumstances, such as
9
embedding of impurities in a wrong sublattice, blocking of vacancies,
etc.
Some theoretical findings in the hydrogen solubility as well as
our estimations of the isotope effect in H/D solubility in protonconducting oxides were confirmed experimentally. These include the
solubility dependence on the effective concentration of oxygen
vacancies and the values of the ratio CD(Н)/CD. The experimental value
of the latter agrees well with the estimated theoretical ratio 1/2 which
is valid for low H (D) concentrations, such as obtained in the work.
This study was supported by the Russian Foundation for Basic
Research (grant 04-03-32377) and the Program for Basic Research
“Hydrogen Energy” (N 26) of RAS.
1.
2.
3.
4.
5.
6.
7.
8.
9.
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