Identification of Vacancy Formation Sites in LaNi5Cu During

Article pubs.acs.org/JPCC

Identification of Vacancy Formation Sites in LaNi5Cu During Hydrogenation Using in Situ Coincidence Doppler Broadening Technique Kouji Sakaki,*,† Yumiko Nakamura,† and Etsuo Akiba†,‡ †

National Institute of Advanced Industrial Science and Technology, AIST Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Department of Mechanical Engineering, Faculty of Engineering and International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

‡

ABSTRACT: To study the formation mechanism of lattice defects introduced during hydrogenation, we developed equipment that can measure in situ positron lifetime and coincidence Doppler broadening (CDB) simultaneously with pressure−composition (P−C) isotherms at room temperature. The equipment, which consisted of a high-pressure sample holder, reduced the intensity of the background signals in the relatively high momentum region of the Doppler broadening spectrum by more than 2 orders of magnitude in comparison to the background shown by conventional Doppler broadening equipment. Further, the quality of data obtained by our instrument was similar to that obtained from ex situ CDB equipment without a high-pressure sample holder. The substitution effect on the positron annihilation behavior was observed from the ratio curves of CDB spectra for LaNi5Cu and LaNi5‑xAlx. In addition, using the equipment we developed, we attempted to identify the vacancy formation sites introduced in LaNi5Cu during hydrogenation. Changes in positron lifetime and the S parameter indicated that the vacancies were introduced above 0.35 H/M during hydrogenation and the introduced vacancies were completely recovered below 0.33 H/M during dehydrogenation. The changes in the ratio curves suggested that vacancies were introduced at Ni sites during hydrogenation.

1. INTRODUCTION

hydrogenation and dehydrogenation were observed in LaNi4.93Sn0.27.17 Measurement of coincidence Doppler broadening (CDB) using two Ge detectors is the only technique that provides information on positron annihilation sites.18,19 The CDB spectrum acquired using two Ge detectors has a better peak to background ratio in the high-momentum region than does the conventional DB spectrum acquired using a single Ge detector. The high-momentum region of the spectrum contains information on the inner electrons, which is specific to individual elements. Therefore, the positron annihilation sites can be identified experimentally using the CDB technique. In this paper, we report the development of an apparatus for the in situ measurement of CDB and positron lifetime, which simultaneously measures pressure−composition (P−C) isotherms at room temperature. Further, using this apparatus, we investigated the formation and recovery of vacancies in LaNi5Cu.

Hydrogen storage alloys have been utilized as negative electrodes in nickel hydride batteries, and LaNi5-based alloys are one of the most suitable materials for this application. One of the important requisites of these electrode materials is their durable reversibility. However, degradation of hydrogen storage capacity is generally observed after a number of hydrogenation and dehydrogenation cycles. Since the lattice of LaNi5 expands and contracts by more than 20% in volume during hydrogenation and dehydrogenation,1 lattice strain and lattice defects such as dislocations and vacancies are introduced in the LaNi5based alloys.2−9 Substituting a part of Ni in LaNi5 by a third element, especially Al or Sn, was reported to reduce the lattice strain and dislocation density generated by hydrogenation4,5,7−9 and aid in preventing the degradation of hydrogen storage capacity during hydrogenation cycles.10,11 In order to clarify the relation between the lattice defects and durable reversibility, we have applied the positron lifetime measurement, which can detect the open-volume defects such as dislocation, vacancies, and vacancy clusters,12−14 to the hydrogen storage materials for more than a decade.15−17 Our previous studies have clearly shown that substituting a part of Ni in LaNi5 by Al or Sn reduces the concentration of the lattice defects introduced by hydrogenation to around one tenth (or lesser) of that found in binary LaNi5.15−17 In addition, reversible formation and recovery of vacancies accompanying © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Experimental Procedure. LaNi5 and LaNi5Cu alloys were prepared by arc melting in an Ar atmosphere. To obtain Received: August 1, 2012 Revised: October 2, 2012 Published: October 2, 2012 22238

dx.doi.org/10.1021/jp307630d | J. Phys. Chem. C 2012, 116, 22238−22244

The Journal of Physical Chemistry C

Article

Figure 1. In situ positron annihilation equipment capable of simultaneously measuring P−C isotherms at room temperature.

homogeneous alloys, annealing of LaNi5 and LaNi5Cu were carried out under vacuum at 1223 K for 3 days and at 1473 K for 1 week, respectively. LaNi4.5Al0.5 and LaNi4Al (from Chuo Denki Kogyo Co. Ltd.) were prepared by high-frequency induction melting. These alloys were annealed at 1373 K for 10 h under an atmosphere of Ar. The annealed ingots were crushed into powders for the positron annihilation experiments. The crushed alloys were again annealed at 1473 K for LaNi5Cu and at 1223 K for the other alloys under vacuum for 5 h in order to remove lattice defects introduced by mechanical crushing. The annealed powders were loaded into the sample holder of an in situ positron annihilation equipment with a positron source, 22Na, as shown in Figure 1. 22NaCl (30 μCi), which was sealed with a Kapton film, was used as a positron source. The equipment consisted of devices for measuring positron lifetime and CDB, in addition to a system for hydrogen supply, which was used to evaluate the hydrogen content using Sieverts’ method (Figure 1). The in situ measurement of positron lifetime and CDB were performed simultaneously with the determination of P−C isotherms at 293 K. For the measurement of positron lifetime, the data collection was carried out with a digital oscilloscope (LeCroy, WaveRunner64Xi) in order to obtain better time resolution than that acquired with the conventional analog-type system. Each positron lifetime spectrum consisted of more than 106 counts, and in order to ensure the measurement reproducibility, several spectra were obtained at each measurement point. The source correction and the resolution functions were evaluated using the code Resolution.20 The positron lifetime spectra were analyzed using the Positronfit Extended program.21 The contributions from the source to the positron lifetime and intensity were approximately 365 ps and 15%, respectively. For positrons undergoing annihilation in a perfect lattice, the positron lifetime spectrum T(t) exhibits a simple exponential form given by eq 1. T (t ) = (1/τ1) exp(−t /τ1)

T (t ) = (I1/τ1) exp( −t /τ1) + (Id /τd) exp( −t /τd)

(2)

Therefore, a model assuming the existence of only a single type of positron annihilation sites (i.e., interstitial sites) cannot explain the obtained positron annihilation profile because two positron annihilation sites (interstitial sites and the lattice defects) are present in the samples. When all the positrons are trapped at the lattice defects, the positron lifetime spectrum exhibits a single exponential form described by eq 3. T (t ) = (1/τd) exp(−t /τd)

(3)

Therefore, for one-component analysis of the positron lifetime spectra, the variance of the fit (χ2/q) indicating the accuracy of the analysis initially increases with the formation of lattice defects because more than two kinds of positron annihilation sites exist. χ2/q subsequently decreases when most of the positrons are trapped at the lattice defects because only one kind of positron annihilation sites exists. For the measurement of CDB, two Ge detectors with an electric cooling system were used. The energies of two γ-rays emitted by the annihilation of positrons with electrons were simultaneously measured using the two Ge detectors, which were located at an angle of 180° relative to each other, to obtain the CBD spectrum. The distance between the sample and the detectors was 20 cm. The CDB spectrum consisted of more than 1 × 107 counts. 2.2. Data Processing of CDB Measurements. The energies of the γ-rays emitted by positron annihilation were simultaneously recorded. These γ-ray energies are expressed by the following equations. E1 = m0c 2 + cpL /2 − E B /2 E2 = m0c 2 − cpL /2 − E B /2

(4)

where pL is the longitudinal component of the positronelectron momentum along the direction of the γ-ray emission, c is the speed of light, m0 is the electron rest mass, and EB is the electron binding energy. Therefore, the total energy and the difference between the energies of the two γ-rays are expressed by the following equations.

(1)

where τ1 is the lifetime of a positron annihilating at an interstitial site of a perfect lattice. When lattice defects such as vacancies and dislocations exist in the lattice, some positrons are trapped and annihilate at the lattice defect sites and the others annihilate at the interstitial sites. In this case, the positron lifetime spectrum consists of discrete multiple components, as described by eq 2.

Et = E1 + E2 = 2m0c 2 − E B ΔE = E1 − E2 = cpL 22239

(5)



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The CDB spectrum was derived from the 2D data under the conditions expressed below. 2m0c 2 − 3.0 (keV) < Et < 2m0c 2 + 3.0 (keV)

The parameters S and W were defined as the ratios of the counts of the detected γ-ray in selected regions to the total counts. The selected regions for S and W were set as 0 ≤ |pL| ≤ 4 × 10−3 m0c and 15 × 10−3m0c ≤ |pL| ≤ 30 × 10−3m0c, respectively. W shows the contribution of the core electrons, while S shows the contributions of the core and the valence electrons. The observed parameters were expressed by the following equations. S = (1 − α)S b + αSD

(6)

W = (1 − α)Wb + αWD

(7)

where Sb (Wb) and SD (WD) are S (W) parameters for samples without and with lattice defects, respectively, and α is the fraction of positrons annihilated at the lattice defects. Both parameters are sensitive to both the concentration and type of the lattice defects. However, the parameter R, defined as R = (S − Sb)/(W − Wb), depends only on the type of the lattice defects and is independent of their concentration.22,23 R was evaluated from the slope of the straight line of the S−W plot. When the slope remains unchanged, the type of lattice defects also remains unchanged. To highlight the difference between the two Doppler broadening spectra and to emphasize the contribution of the core electrons, ratio curves were obtained as defined by the following equation. fsample (pL ) =

Figure 2. Doppler broadening spectra of annealed LaNi5 acquired using one detector and two detectors.

more than 2 orders of magnitudes, although the samples were loaded into a high-pressure sample-holder during the experiments. This is similar to the background levels reported previously using ex situ setup.19 Sufficient peak-to-background ratios were achieved using the in situ CDB equipment used in this study. This implies that the instrumentation we have developed enabled the successful in situ measurement of both positron lifetime and CDB. Figures 3 shows the ratio curves obtained for the LaNi5based alloys and the constituent elements normalized by the CDB spectrum of LaNi5. As seen in eq 8, if the annihilation behavior of a sample is the same as that of LaNi5, the ratio curves should take a constant value of unity. In LaNi5Cu, the intensity of the ratio curve decreased from 5 × 10−3m0c and then increased from 10 × 10−3m0c. This tendency is similar to that observed in pure Cu, indicating that some of the positrons annihilated with the electrons belonging to Cu in LaNi5Cu. In LaNi4.5Al0.5, the intensity of the ratio curves decreased from 3 × 10−3m0c and then gradually increased from 15 × 10−3m0c. When the Al content in LaNi5‑xAlx increased to x = 1, a similar shape of the ratio curve was observed and the magnitude of the change was larger. The shape of these ratio curves was similar to that observed for pure Al. These results indicate that some positrons annihilated with the electrons belonging to Al in LaNi5‑xAlx and the positron annihilation probability with the electrons belonging to Al increased with increase in the Al content. The results obtained from LaNi5Cu and LaNi5‑xAlx demonstrated that the effect of substitution elements on the positron annihilation behavior could be successfully detected using the in situ CDB equipment we developed. The ratio curve of the CDB spectrum of LaNi5 is shown in Figure 4 along with the ratio curve of pure La. Both curves were normalized by the CDB spectrum of pure Ni. If all the positrons were annihilated by the electrons only belonging to Ni in LaNi5, the ratio curve can be expected to show a constant value of unity. If all positrons were annihilated by electrons only belonging to La in LaNi5, the ratio curve can be expected to overlap with that of pure La. However, the observed ratio curve of LaNi5 was located between that of Ni and La. The ratio curve of LaNi5 decreased with increasing pL and became constant above pL = 12 × 10−3m0c. This tendency was similar to that observed for pure La, and the magnitude of the change was much smaller than that shown by pure La. This result suggests

Nsample(pL ) Nreference(pL )

(8)

where fsample(pL) is the ratio curve of the target sample and Nreference(pL) and Nsample(pL) are the Doppler broadening spectra of the reference and target samples, respectively.

3. RESULTS AND DISCUSSION 3.1. Development of the in Situ Positron Annihilation Equipment with Simultaneously Measuring P−C Isotherms at Room Temperature. The positron lifetimes obtained for the completely annealed LaNi5, LaNi4.5Al0.5, LaNi4Al, and LaNi5Cu were around 119, 127, 130, and 120 ps, respectively. These values were close to the positron lifetimes of the completely annealed LaNi5-based alloys without any lattice defects which positron annihilation technique can detect.15,16 Therefore, it was confirmed that lattice defects were absent in the completely annealed samples. In addition, the time resolution (fwhm) obtained with the in situ measurement system used in this study was 172−180 ps. This value was considerably better than that obtained by the analog-type system (209 ps)17 we used previously. In the present system, signals from the detectors are directly recorded using the digital oscilloscope, while in the previous system the signals were recorded with several analog-type devices. It was confirmed that our developed in situ positron lifetime equipment using digital oscilloscope worked better. Figure 2 shows the Doppler broadening spectra of LaNi5 measured with the in situ CDB equipment in the coincidence (double-detector) mode and noncoincidence (single-detector) mode. Measurement in the coincidence mode reduced the background level in the relatively high momentum region by 22240



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Figure 3. Ratio curves of the CDB spectra of LaNi5-based alloys and the substitution elements normalized by the CDB spectrum for LaNi5 represented in (a) normal scale and (b) expanded scale.

In this assumption, the crystal structure, the positron affinity, and the atomic radius were disregarded. The parameter x can be interpreted as the average fraction of positrons annihilating with electrons belonging to La. Therefore, the ratio curve can be re-expressed using eq 10. fLaNi (pL ) ∝ 5

xNLa(pL ) + (1 − x)NNi(pL ) NNi(pL )

∝ (1 − x) + xfLa/Ni (pL )

where f La/Ni(pL) is the ratio curve of pure La normalized by the CDB spectrum obtained from pure Ni. From this equation and the ratio curve, the parameter x for LaNi5 was evaluated as approximately 22%. This was larger than the atomic ratio of La in LaNi5 at 16.7 atom %. The discrepancy is probably because the positron affinities with the constituent elements25 and the atomic radii may have contributed to the observed value of x. However, the results obtained in the present study indicate that information on elements relating to the positron annihilation can be obtained at least qualitatively. For a more precise quantitative analysis, the effects of the positron affinity and the atomic sizes need to be considered using ab initio calculations. 3.2. In Situ Measurements of Positron Lifetimes and CDB in LaNi5Cu during Hydrogenation and Dehydrogenation. Figure 5a shows the P−C isotherms of LaNi5Cu. LaNi5Cu absorbed hydrogen up to 0.77 H/M and exhibited a

Figure 4. Ratio curve of the CDB spectra of annealed LaNi5 (along with that of pure La) normalized by the CDB spectrum for Ni.

that some of the positrons annihilated with the electrons belonging to La and the others annihilated with the electrons belonging to Ni. In addition, the fraction of positron annihilation was roughly estimated from the experimental results. When positrons annihilate with electrons belonging to the constituent elements of binary compounds, the Doppler broadening spectrum can be roughly assumed to be expressed by the following equation.24 NLaNi5(pL ) ∝ xNLa(pL ) + (1 − x)NNi(pL )

(10)

(9)

Figure 5. (a) P−C isotherms and (b) variation in positron lifetime and variance of fit (χ2/q) during hydrogenation and dehydrogenation of LaNi5Cu. 22241



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sloping plateau. Previous reports on in situ X-ray and neutron diffraction indicate that LaNi5Cu did not transform to its hydride phase.18,26 Therefore, the sloping plateau observed here also suggests that the samples did not form hydrides. Moreover, hysteresis between the absorption and desorption isotherms was not observed. These results agree well with previous reports.18,27 Figure 5b shows the variation in the positron lifetime and χ2/q (which indicates the accuracy of the analysis) as a function of the hydrogen content. As mentioned before, the positron lifetime of the completely annealed LaNi5Cu was 120 ps before hydrogenation. The positron lifetime remained unchanged until the hydrogen content reached 0.35 H/M, even though LaNi5Cu absorbed hydrogen. χ2/q also remained constant in this region. The positron lifetime monotonically increased to around 168 ps during hydrogenation when the hydrogen content increased from 0.35 H/M to 0.77 H/M. In this region, χ2/q initially increased and then decreased. As described above, these results clearly indicate that lattice defects were introduced above 0.35 H/M, almost all the positrons were trapped at the lattice defects, and the saturation trap occurred at 0.77 H/M. The positron lifetime at dislocations in LaNi5 was around 150 ps.15,16 The positron lifetime observed was significantly higher than that observed at dislocations. These results indicate that vacancies were created above 0.35 H/M during hydrogenation and almost all the positrons were trapped at the introduced vacancies. The results of multicomponent analysis also showed that the positron lifetime for the lattice defect was around 160−170 ps which is close to the positron lifetime of Ni vacancy15,16,28 and its relative intensity increased with hydrogen content above 0.35 H/M. This is also clear evidence that vacancy start to be created above 0.35 H/M. During dehydrogenation, the positron lifetime slightly decreased above 0.33 H/M, although χ 2 /q remained unchanged. Below 0.33 H/M, the positron lifetime drastically decreased to 124 ps, accompanied by the rapid increase followed by decrease in χ2/q. The positron lifetime after hydrogen desorption was close to that observed before hydrogenation. These results show that a majority of the introduced vacancies were recovered below 0.33 H/M during desorption. From the Doppler broadening spectra, S and W were evaluated. In general, an increase in S indicates the formation of lattice defects, because the probability of positron annihilation with valence electrons increases when positrons are trapped at the lattice defects. Conversely, a decrease in S indicates the recovery of the lattice defects. During hydrogenation, S remained unchanged below 0.35 H/M and increased with increasing hydrogen content above 0.35 H/M. During dehydrogenation, S gradually decreased above 0.33 H/M and subsequently decreased significantly and returned to the initial value. The variation in S was similar to the behavior of the positron lifetime shown in Figure 5b and the relationship between S and the positron lifetime was linear. Therefore, the variation in S observed here also indicates that vacancies were introduced during hydrogenation and that the introduced vacancies were recovered during dehydrogenation. The variation in W was opposite to the behavior of S during the hydrogenation and dehydrogenation cycles, because the positron annihilation with core electrons contributes to the changes in W. In addition, the relationship between W and the positron lifetime was also linear. Figure 6 shows the relationship between S and W (S−W plot) during hydrogenation and dehydrogenation. This S−W

Figure 6. Variation in W with S (S−W plot) during hydrogenation and dehydrogenation of LaNi5Cu.

plot showed a linear relationship. Therefore, R was constant during hydrogenation and dehydrogenation, which indicates that the type of the lattice defects remained unaltered and only their concentration changed during hydrogenation and dehydrogenation. Figure 7 shows the changes in the ratio curves of the CDB spectra of LaNi5Cu during hydrogenation and dehydrogenation. Below 0.35 H/M, the ratio curves were unchanged, and subsequently, with the increase in the hydrogen content to values greater than 0.35 H/M, the ratio curves shifted toward that shown by pure La. This behavior was similar to the variation in positron lifetime and S. The changes in the ratio curves indicate that the probability of positron annihilation with electrons belonging to La increased with the increase in hydrogen content. Using eq 10, the average fraction of positrons annihilating with electrons in La did not change below 0.35 H/M but it was increased with hydrogen content above 0.35 H/M. Finally, it was evaluated to be around 60% at 0.77 H/M. This was similar to the variation in positron lifetime and S. As shown in Figure 5, vacancies were introduced with increasing hydrogen content during hydrogenation. If vacancies are created at the La sites, the probability of annihilation of positrons with electrons in La decreases, because in LaNi5, La is surrounded by only Ni, as shown in Figure 8a. When Ni vacancies are introduced, the signal from the La atom can increase because Ni atoms at both 2c and 3g sites are surrounded by Ni and La atoms (Figure 8b and c). Therefore, the variations in the ratio curves during hydrogenation suggest that the vacancies were formed at Ni sites and not at La sites. To identify the sites (i.e., Ni 2c site or 3g site) at which vacancies were formed from the ratio curves, further quantitative analysis of the Doppler broadening spectrum combined with theoretical calculations are required. From our previous calculations, the positron lifetime in LaNi5 was found to be 241 ps at La vacancies and took the values of 166 ps (at 2c site) and 177 ps (at 3g site) at Ni vacancies, respectively.28 The experimentally observed positron lifetime in the completely hydrogenated LaNi5Cu was around 168 ps, which is significantly lower than the calculated positron lifetime at La vacancy and is closer to that calculated at Ni vacancies (in particular, the value tended toward the lifetime at the 2c sites). In addition, Mizuno et al. reported that in LaNi5 the vacancy formation energies for Ni at 2c sites was about 0.8 eV lower than that for Ni at 3g sites.29 Considering these results, Ni 2c sites can be expected to be the most probable vacancy sites introduced in LaNi5Cu by hydrogenation. 22242



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Figure 7. Variation in the ratio curves of the CDB spectra of LaNi5Cu during (a) hydrogenation and (b) dehydrogenation, normalized by the CDB spectrum of Ni.

Figure 8. Coordination of (a) La 1a site, (b) Ni 2c site, and (c) Ni 3g site in LaNi5.

■

During dehydrogenation, with the decrease in the hydrogen content, the ratio curves reverted toward that shown by LaNi5Cu before hydrogenation. This suggests that the introduced Ni vacancies were recovered during dehydrogenation, which is consistent with the results obtained from the positron lifetime measurements.

4. CONCLUSIONS We successfully developed an equipment to carry out in situ positron lifetime and CDB measurements along with the simultaneous determination of P−C isotherms at room temperature. This equipment clearly detected the differences in the CDB curves of LaNi5 in which a part of Ni was substituted by Al or Cu. The probabilities of positron annihilation with electrons belonging to each constituent element were evaluated qualitatively. The technique we developed can be used to identify the local structure of the lattice defects introduced into materials during hydrogenation. The in situ positron lifetime and CDB measurements in LaNi5Cu indicated that vacancies were introduced at Ni sites (probably at Ni 2c sites) during hydrogenation and the introduced Ni vacancies were completely recovered during dehydrogenation.

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REFERENCES

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Advanced Fundamental Research Project on Hydrogen Storage Materials. 22243



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Identification of Vacancy Formation Sites in LaNi5Cu During

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