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Structural peculiarities in the β phase of the La0.75Ce0.25Ni4.8Al0.2 deuterides
Journal of Alloys and Compounds 788 (2019) 533e540
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Structural peculiarities in the b phase of the La0.75Ce0.25Ni4.8Al0.2 deuterides J. Czub a, *, W. Jamka a, J. Przewo znik a, A. Zarzecka a, A. Hoser b, D. Wallacher b, N. Grimm b, Ł. Gondek a a b
w, Poland AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Mickiewicza 30, 30-059, Krako Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109, Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2017 Received in revised form 20 February 2019 Accepted 23 February 2019 Available online 25 February 2019
In this contribution the structural, magnetic and sorption properties of the novel La0.75Ce0.25Ni4.8Al0.2 alloy and its deuterides are discussed. The collected data indicate good stability of the alloy during the cycle-life and thermal treatment tests. Particularly, the desorption pressure is almost constant versus the cycle number. The results of the in-situ neutron diffraction studies show the unusual behaviour of the deuterium rich b phase. Unexpectedly, the b phase belongs to the same P6/mmm space group as the hydrogen poor a phase. Moreover, the b phase behaves like a solid solution on the plateau of the pressure-composition-temperature (PCT) curve. The deuterium sites and their occupations in both a and b phases are determined from the neutron diffraction data, giving the first-hand insight into the mechanism of the PCT hysteresis. As far as the magnetic properties are concerned, cerium is trivalent in the investigated alloy. However, no magnetic ordering down to 3 K is observed. It can be concluded that the sorption properties of the crystal depend on the cerium valence, what distinguishes the alloy from the other compositions. © 2019 Elsevier B.V. All rights reserved.
Keywords: Hydrides Sorption properties Magnetism Magnetic ordering Neutron diffraction
1. Introduction The AB5 alloys have been extensively studied since the seventies because of their excellent hydrogen sorption properties that can be easily tuned in order to meet the requirements of solid state hydrogen storage systems [1]. Those properties include the maximum H2 capacity, thermodynamics, kinetics, reversibility, cycle-life and corrosion resistance. The gravimetric hydrogen storage capacity is an exception. Although it does not exceed 2 wt% for the LaNi5 derivatives, those compositions are still the most promising materials for stationary hydrogen storage and hydrogen purification units. It was also shown that the thermodynamic and cycle-life properties of the AB5 alloys can be improved as a result of partial substitution for the A or/and the B [2e5]. For instance, Marshall et al. found the correlation between the unit cell volume and the plateau pressure for the LaNi5-xAlx [6]. Namely, the higher Al concentration resulted in an increase of the lattice parameters and a
* Corresponding author. E-mail address: [email protected] (J. Czub). https://doi.org/10.1016/j.jallcom.2019.02.259 0925-8388/© 2019 Elsevier B.V. All rights reserved.
decrease of the plateau pressure. Since then, extensive studies on different dopants have been conducted [7e14]. For example, Uchida et al. studied the series of the La1-xRxNi5 (where R ¼ Ce - Lu) compounds. It was shown that an addition of Ce led to a significant increase of the equilibrium pressure and the heat of formation [14]. Moreover, cycling stability of the LaNi5 alloys can be enhanced by Ce substitution, what prevents Ni sedimentation [15]. This is important for long-term stability that is crucial for hydrogenstorage/compression/purifying applications. On the other hand, replacement of Ni with a low amount of Al reduced the plateau slope without a significant decrease of the hydrogen capacity [16]. Moreover, substitution of Al for Ni significantly lowered the equilibrium pressure for hydrogen desorption [13]. Similar results were obtained recently for the LaNi5-xAlx (x ¼ 0, 0.25, 0.5, 0.75, 1) [17]. It was shown that an increase of the Al content led to a decrease of the formation energy and reduced the plateau values. However, it was presented that long-term cycling led to an increase of the PCT slope for the LaNi4Al and a decrease of the hydrogen concentration. Therefore, it can be concluded that the material degraded. Particularly, disproportionation and nanocrystallization occurred [18]. On the other hand, substitution of Al and Bi in the LaNi5 can reduce absorption/desorption pressures and
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prevents decrepitation during cycling [19]. The other recently proposed method of improving the performance of the AB5 alloys was to introduce the surface modification by Pd/Pt in order to decrease chemical activity towards hydrogen impurities. Particularly, the MmB5 (Mm ¼ the mixture of La, Ce, Pr, Nd; B ¼ the mixture of Ni, Co, Al, Mn) alloy was modified by ballmilling followed by autocatalytic deposition of Pd [20]. It was concluded that such a procedure led to a marginal improvement in stability for low concentrations of Pd, while for higher concentrations (>2 wt%) disproportionation of the primary MmB5 material occurred. Substitution of Ce and Al for the A and B elements respectively yielded very interesting results for the AB3 alloys [21]. While Ce doping increased the equilibrium pressure, introduction of Al reduced the hydrogen capacity and increased the plateau slope of the pressure-composition-temperature (PCT) curves. It was concluded that Ce and Al doping can be used for fine-tuning of the AB3 systems [22e24]. Additionally, multi-substitution for several elements is a well-established route to improve the sorption properties of the alloys. Recently, such an approach has been successfully applied for the Mg-based alloys [25,26]. Although the LaNi5 and its derivatives have been thoroughly studied and their extremely interesting behaviour under hydrogen/ deuterium pressure has been reported, the information on the structural aspects of the hydrides formation is limited. It is known that the structure of the a phase belongs to the hexagonal P6/mmm space group. However, the structure of the hydrogen rich b phase is a matter for debate. The issue seems to be resolved for the LaNi5Dx (5.0 x 6.7) by the results of the comprehensive research reported by Lartigue et al. [27]. According to Refs. [27e29], the structure of the b phase belongs to the original P6/mmm or P6mm space group up to x ¼ 5.0. The superstructure appeared for the higher deuterium concentrations due to ordering of deuterium. Consequently, the appearance of additional reflections was related to the P63mc space group (hkl ¼ 201, 203, 205) that emerged when the original P6/mmm c lattice parameter was doubled. It is worth noting that among the superstructure reflections, the 203 one was the most prominent as it did not interfere with the others. Moreover, it was noticed that the 203 reflection was indeed visible in the previously reported neutron diffraction patterns [27]. It should be also highlighted that usually 4 deuterium sites (4 h, 6 m, 12n, 12 ) were considered for the P6/mmm space group, while the number of possible sites increased for the P6mm and P63mc space groups. That observation is important especially for doping with elements with different hydrogen/deuterium affinities because that enables preferential site occupation depending on the substituents. Therefore, it leads to the conclusion that the occurrence of the superstructure would be even more probable for the doped LaNi5 alloys. The superstructure was indeed reported for the non-stoichiometric LaNi5þxDy alloy for the deuterium concentration above 5.45 [30]. It is known that the evolution of the superstructure involves mainly hydrogen/deuterium atoms. For that reason, it cannot be detected with X-ray diffraction. To the best of our knowledge, the superstructure has been reported only as the result of the neutron diffraction experiments conducted for the b phase close to saturation. The nature of the a to b phase transformation is another interesting structural aspect of hydriding of the LaNi5-related alloys. Namely, the occurrence of an additional g phase was reported [31e34]. The g phase coexisted with the a and b phases for the narrow hydrogen concentration range. The g phase was initially observed for the alloys that underwent temperature treatment. However, its occurrence was also reported for the raw material as a result of non-equilibrium measurements conditions [34]. It was confirmed by the recent studies conducted by Gray et al. [35].
In this contribution, we report the results of the comprehensive studies on the La0.75Ce0.25Ni4.8Al0.2 alloy and the corresponding deuterides. The report includes the results of the X-ray and in-situ neutron diffraction experiments, the Sieverts volumetric H2 sorption and cycle-life studies and finally the VSM magnetometric measurements. 2. Experimental details The La0.75Ce0.25Ni4.8Al0.2 alloy was synthesised by induction melting of the high purity constituents (3 N for rare earths and 5 N for Ni and Al). The initial content of Al was 5 wt% higher than the stoichiometric quantity. The sample was remelted and then annealed at 550 C for 28 days in order to achieve homogeneity. It is worth noting that annealing at higher temperatures leads to some sedimentation visible on the surface of quartz tubes. The resulting alloy was examined by X-ray diffraction and fluorescence spectroscopy that confirmed high quality of the sample with the proper stoichiometry and without noticeable traces of impurity phases. The X-ray diffraction studies (XRD) were performed by means of the PANalytical Empyrean diffractometer using the Cu Ka radiation. The neutron diffraction (ND) studies were conducted by means of the E6 focusing diffractometer at the Helmholtz-Zentrum Berlin. A high temperature furnace with a gas absorption insert (operating up to 20 bar) was used for the in-situ measurements. The insert was connected to the Sieverts apparatus. The sample of about 6 g was placed in an inconel steel container. Prior to the measurements, the calibration patterns of an empty container at different temperatures and D2 pressures were collected for a precise extraction of the contribution originating in the container and the furnace from the data. For the high temperature neutron diffraction measurements, the sample was placed in a quartz tube. The contribution originating in the quartz tube was extracted using the results of the calibration measurements performed for an empty tube in the entire temperature range. The measurements were performed in the inert gas (Ar) atmosphere at 1 bar overpressure. The diffraction patterns were refined using the Rietveld FullProf software [36]. The sorption properties, including the PCT and cycle-life, were investigated using the Setaram PCT-PRO Sieverts apparatus. The cycle life measurement were performed at 25 C and the pressure of 20 bar was applied and maintained until no further progress of hydrogen absorption was noticed. Then the desorption process was performed and followed by 5 min of vacuum pumping. The magnetometric data were collected by means of the Quantum Design PPMS-9 system in the temperature range 3e350 K and magnetic fields up to 9 T. 3. Results and discussion 3.1. Structural properties and thermal stability The XRD and ND experiments revealed that at temperature of 25 C the La0.75Ce0.25Ni4.8Al0.2 crystallises in the hexagonal CaCu5 structure with the P6/mmm symmetry with the following lattices parameters: a ¼ 4.99284(12) Å and c ¼ 4.00528(12) Å. The best refinements coefficients are obtained for the model where Ce atoms substitute La at the position 1a, while Al atoms are located only in the 3g sites. It is worth noting that while the a lattice parameter is somewhat lower than the value for the pure LaNi5, the c parameter is higher. The similar relation between the refined parameters can be found for the LaNi4.75Al0.25 [29]. For the La for Ce substitution, it is well established that both lattice parameters are lower than for the pure alloy (the lanthanides contraction). Comparing our data with those reported for the La0.8Ce0.2Ni5, one can find that both refined lattice parameters for the
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La0.75Ce0.25Ni4.8Al0.2 are higher [37]. The conclusion is that simultaneous introduction of Ce and Al into the LaNi5 lattice leads to an anisotropic change of the lattice parameters, what is very convenient for tuning of the sorption properties. The issue of thermal stability mentioned in the experimental section was addressed by high temperature neutron diffraction (Fig. 1). As one can see, the crystal structure is maintained up to the highest investigated temperature. However, an additional line at 71 of 2Q appears at temperatures above 550 C. This line is related to the CeAl2 intermetallic alloy. The sedimentation of the Al-rich phase is reflected in a decrease of the c lattice parameter that starts above 625 C as depicted in Fig. 2. Such behaviour corresponds with the crystal properties of the Ce or Al doped LaNi5 alloy discussed above. The alloy is not supposed to operate at such elevated temperatures. Therefore, recycling (e.g. desorbing of surface impurities) has to be conducted at temperatures below 500 C.
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3.2. Sorption and cycle-life properties The maximum hydrogen concentration for the investigated alloy at 0 C corresponds to the formal composition La0.75Ce0.25Ni4.8Al0.2H6.75(2). After the full activation (10 cycles), the desorption mid-plateau pressure is almost equal to 0.6 bar, while the corresponding absorption pressure is 1.1 bar (Fig. 3). For 25 C, the corresponding values are 3.9 and 2.1 bar for absorption and desorption, respectively. The absorption pressure at 25 C is much higher than for the pure LaNi5 or LaNi4.8Al0.2. The values around 1.4 bar are reported for those compositions [38]. On the other hand, the values of absorption plateau pressure are significantly higher than 5 bar for the LaNi5 alloys doped only with Ce [14,37,39]. A closer look at Table 1, in which the thermodynamic properties are gathered, yields an interesting observation. The absorption enthalpy change (DH) is close to the values obtained for the alloys doped only with Ce that contain the similar amount of Ce [37]. On the contrary, the desorption related DH values are much higher. However, those values are almost equal to those for the alloys doped only with Al [40,41]. For the detailed comparison one can refer to Table 2. It may be concluded that Al doping for Ni in the primary LaNi5 alloy leads to a significant increase of the modulus of the formation enthalpy. It yields almost 47 kJ/mol H2 for the LaNi4Al. On the contrary, substitution of Ce for La leads to a decrease of the -DH, however it seems that there is no apparent correlation between the DH and the Ce concentration (Table 2). The -DH value is 24.3 kJ/mol H2 for the La0.85Ce0.15Ni5 alloy, while 26.6 kJ/mol H2 was reported for the La0.8Ce0.2Ni5. Mischmetal (Mm) is used instead of pure La for the other typical compositions. Those
Fig. 1. The high temperature neutron diffraction patterns for the La0.75Ce0.25Ni4.8Al0.2. Only a group of the main reflections is presented for clarity.
Fig. 3. The Pressure-Concentration-Temperature (PCT) absorption and desorption curves for the La0.75Ce0.25Ni4.8Al0.2.
Table 1 The thermodynamic properties of hydrogen absorption/desorption for the just activated (10 cycles) and cycled La0.75Ce0.25Ni4.8Al0.2. Absorption
Fig. 2. Temperature variations of the lattice parameters for the La0.75Ce0.25Ni4.8Al0.2 derived from the neutron diffraction data.
After activation After 500 cycles
Desorption
-DH [kJ/mole H2]
-DS [J/mole H2 K]
DH [kJ/mole H2]
DS [J/mole H2 K]
28.61 32.37
107.45 117.71
30.54 28.18
108.85 100.9
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Table 2 Comparison of the enthalpy and the entropy of formation for the investigated compound and the chosen AB5 alloys. Mm denotes the La-rich mixture of light rare earths (La, Ce, Pr, Nd). Alloy
-DH [kJ/mole H2]
-DS [J/mole H2 K]
Ref.
MmNi5 MmNi4.7Al0.3 La0.85Ce0.15Ni5 MmNi4.5Al0.5 MmNi4.8Al0.2 La0.9Ce0.1Ni5 La0.8Ce0.2Ni5 MmNi4Al La0.4Ce0.4Ca0.2Ni5 La0.75Ce0.25Ni4.8Al0.2 La0.7Ca0.2Ce0.1Ni5 LaNi4.8Al0.2 LaNi5 MmNi4.7Sn0.3 LaNi4.7Al0.3 La0.5Mm0.5Ni4.7Sn0.3 LaNi4.7Sn0.3 LaNi4.5Al0.5 LaNi4Al
20.3 22.6 24.30 24.4 25.1 25.30 26.6 26.2 28.20 28.61 29.4 30.40 31.80 31.83 32.7 33.80 36.51 36.90 45.85
101.4 96.7 91.28 97.0 80.0 106 107.3 79.9 115.3 107.45 107 101.6 110.0 106.8 106 111.2 122.6 108.55 114.10
[42] [42] [38] [42] [42] [43] [37] [42] [44] This work [45] [46] [47] [48] [45] [48] [48] [40] [40]
alloys exhibit lower values of -DH in comparison with the LaNi5 due to the presence of Ce, Pr and Nd apart from La. The flatness of the desorption plateau is the other important factor. It was evidenced for the MnNi5-xAlx that the introduction of Al increases the slope of PCT plateau [42]. However, the flatness of the plateau is retained when only Ce is used as the La substituent [37]. Therefore, the investigated alloy exhibits the good flatness of the plateau comparable to those for the La1-xCexNi5 alloys [37]. At the same time, the range of desorption pressures desired for fuel cells applications can be achieved. Additionally, the cycle-life tests were performed to track the thermodynamic changes (Fig. 4.) It was found that the hysteresis for the La0.75Ce0.25Ni4.8Al0.2 is significantly reduced after 500 cycles.
Fig. 4. Results of the cycle-life studies for the La0.75Ce0.25Ni4.8Al0.2. The main chart presents the changes of the PCT curves collected at 25 C after cycling. A change of the maximum hydrogen concentration during cycling is presented in the inset. The red line is a fit of a simple exponential decay. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Namely, the absorption mid-plateau pressure is reduced to 3.0 bar, while the desorption pressure remains unaffected yielding value of 2.1 bar, what is the desired behaviour for possible applications. It was found that cycling does not affect the kinetics of the absorption/desorption reaction. Moreover, the maximum hydrogen capacity seems to obey a single exponential decay with the value of 5.83H/f.u. after 500 cycles. This value is higher than the value for the pure LaNi5. As a matter of facts, it resembles the behaviour reported for the LaNi5-xSnx [49,50]. The very recent studies on the LaNi5-xAlx showed an importance of Al doping for stabilising the crystal structure during long-term cycling [51]. The results presented by Liu at al apparently indicate better repeatability of the uptake mid-plateau pressures for x ¼ 0.25 and x ¼ 0.5 after 1000 cycles. However, the analogous mid-plateau pressures for desorption were more affected by cycling.
3.3. Structural changes during the cycle-life treatment The stability of the alloys is the key feature considering their potential use for hydrogen storage or compression. The structural aspects of ageing cover the issues of decrepitation, disproportionation, amorphisation and strain development that affect the parameters of the PCT curve (equilibrium pressures, slope of the plateau, hysteresis). Therefore, the XRD measurements were performed for the investigated alloy after 175 and 500 cycles (Fig. 5). According to our results, the sample does not exhibit traces of spurious phases after 500 cycles, what can be seen in Fig. 5. Moreover, the XRD patterns collected after 175 and 500 cycles do not differ significantly. That is in agreement with the cycle-life data presented in the previous paragraph. The Rietveld analysis of the collected patterns reveals significant strain development after 175 cycles, however the strain was slightly increased after another 325 cycles (Table 3). Similarly, the grains size is reduced significantly after 175 cycles, while further cycling does not affect this parameter within the uncertainty level. The lattice parameters do not change significantly and only a small expansion of the unit cell can be noticed. It was suggested in a number of papers that cycling of the LaNi5-related compounds leads to Ni precipitation. As a result, lowering of the unit cell volume during cycling was reported [50]
Fig. 5. The X-ray diffraction patterns for the cycled La0.75Ce0.25Ni4.8Al0.2. Details of the Rietveld refinement are presented for the raw sample.
J. Czub et al. / Journal of Alloys and Compounds 788 (2019) 533e540 Table 3 Results of the Rietveld refinement of the XRD data collected for the cycled La0.75Ce0.25Ni4.8Al0.2. a [Å]
c [Å]
V [Å3]
Strain [%] Size [nm]
Raw sample 4.99284(12) 4.00528(12) 86.479(3) 0.457(5) After 175 cycles 4.99324(11) 4.00536(12) 86.484(3) 0.906(3) After 500 cycles 4.99321(32) 4.00580(34) 86.506(6) 0.988(3)
>500 301(73) 282(67)
what is contrary to our case. For that reason, additional magnetometric measurements were performed for the La0.75Ce0.25Ni4.8Al0.2 to address this issue. Magnetometry is extremely sensitive even for traces of ferromagnetic precipitations and therefore can be used to track the traces of Ni sedimentation. According to Ref. [51], cycling of the LaNi5-xAlx alloys brought interesting findings. Namely, the XRD and EXAFS experiments revealed that substitution of Al or Ni reduced growth of structural disorder reflected in strain development. Moreover, the EXAFS experiment proved diminishing of the Ni-Ni coordination number with a simultaneous (smaller) increase of that quantity for the Ni-La. Apart from mixing of the La and Ni sublattices, that effect might be indicative of Ni sedimentation [51]. Unfortunately, neither magnetic studies nor extended EXAFS analyses were performed to address that issue. 3.4. Magnetometric measurements The magnetic properties of the La0.75Ce0.25Ni4.8Al0.2 are displayed in Fig. 6. The alloy is weakly magnetic at room temperature, likely due to the traces of ferromagnetic Ni nanoclusters. It can be deduced from the ferromagnetic part of the curve collected at 300 K that the share of Ni nanoclusters is less than 0.3% (atomic). The nickel contribution is an important parameter concerning the hydrogen absorption/desorption cycle-life studies, as mentioned in the introduction. Magnetometric measurements are well suited to track even very small quantities of nickel so this technique can be used for evaluation of nickel sedimentation after cycling of the alloy. The data presented in Fig. 6 (right bottom insert) show that the magnetisation curves are nearly the same after 500 cycles. That corresponds with the XRD results that showed that no visible impurities due to pure Ni were detected after the cycle-life measurements. The susceptibility data exhibit an increase at low temperatures,
Fig. 6. Magnetic properties of the La0.75Ce0.25Ni4.8Al0.2 compound. Magnetisation versus magnetic field at chosen temperatures is presented as the main diagram. The inserts present magnetic susceptibility and hysteresis loops for the raw and cycled sample.
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what might be associated with magnetic ordering of the Ce sublattice. Unfortunately, the inverse magnetic susceptibility does not obey the Curie-Weiss law (pure and modified), hence the paramagnetic Curie temperature and the effective magnetic moment could not been evaluated. Cerium with the electron configuration 4f15d16s2 can form alloys and compounds with two different valence states: Ce3þ with the 4f15d06s0 electron configuration (usually referred as 4f1) and Ce4þ with the 4f05d06s0 electron configuration (referred as 4f0). Magnetic studies are crucial for determining the cerium configuration. The partially filled 4f states carry the magnetic moment originating from the spin and orbital component of the single 4f electron. On the other hand, the Ce4þ configuration, which is equivalent to the La3þ one, is intrinsically non-magnetic. The determined magnetic properties strongly suggest that cerium exhibit the 4f1 electronic configuration (Ce3þ) in the La0.75Ce0.25Ni4.8Al0.2, while the 4f0 configuration was reported for the CeNi5 [52]. Therefore, the investigation of magnetic properties sheds more light on an influence of Ce doping. Namely, the Ce3þ ion exhibits a larger radius than Ce4þ, so there is no lattice contraction of the investigated alloy observed in comparison with the purely Ce-doped LaNi5 derivatives, as discussed in section 3.1. Apart from an increased size of Ce3þ, also a change of the density of states (DOS) at the Fermi level could be expected. The latter is crucial for screening the repulsive interaction between hydrogen/deuterium atoms located at different sites in the crystal unit cell. Moreover, the electronic DOS is the key factor that establishes the enthalpy of hydride formation. Both effects are important for hydrogen uptake as the origin of the Westlake and Switendick criterions respectively. 3.5. In-situ neutron diffraction In order to get the information about the crystal properties of the La0.75Ce0.25Ni4.8Al0.2 during deuterium uptake/release, the insitu diffraction studies following the simultaneous PCT run were done. Several examples of the collected neutron diffraction patterns are presented in Fig. 7. One can notice that typical coexistence of the deuterium poor (a phase) and deuterium rich (b phase) occurs. Usually, the a phase is considered a metal-hydrogen/ deuterium solid solution. Indeed, the values of both the a and c lattice parameters of the a phase increase significantly with the deuterium equilibrium pressure as can be deduced from Fig. 8. Coexistence of both phases can be seen in the diffraction pattern collected at 4 bar (Figs. 7 and 8). It is interesting the c lattice parameter of the a phase significantly decreases when the b phase appears (at 4 bar). This could indicate that some deuterium transfers between both phases, what seems quite surprising. It is worth mentioning that coexistence of both phases is noticed only for that pressure. Moreover, an apparent increase of the lattice parameters in the b phase is evidenced when deuterium pressure is applied, what has not been reported so far. This increase is not as prominent as for the a phase, however it is still above the uncertainty level. It means that the b phase can be treated to some extent as a solid solution, similarly to the hydrogen poor phase. This behaviour is confirmed for the desorption process for which the lattice parameters for the b phase decrease (Fig. 8). Concerning deuterium desorption, the lattice parameters of the b phase significantly decrease as the process continues. Due to hysteresis evidenced by the PCT data, the deuterium rich phase is stable down to 2 bar of the D2 equilibrium pressures and only the a phase is present at 1.25 bar. A non-linear crystal volume expansion as a function of deuterium concentration was explained for AB2 alloys by Hirata model [53], at least for low D concentration. That approach uses the bulk compressibility theory, in which the pressure originates from the deuterium/hydrogen concentration. An
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Fig. 7. The neutron diffraction patterns collected at different pressures of D2 as marked in the PCT diagram (at 25 C). The contribution from the sample holder was extracted.
Fig. 8. The lattice parameters versus D2 pressure determined by in-situ neutron diffraction. It should be noted that the pressure scale covers both absorption (0e20 bar) and desorption (20 - 0 bar).
additional assumption that an expansion of the crystal (after hydriding) may open some positions primary excluded from D/H occupation by the Westlake/Switendick criterion was proposed in Ref. [54]. The latter model predicts the complex behaviour of the lattice volume versus the hydrogen concentration, what is apparently in agreement with the data presented in Fig. 8. Last, but certainly not least, the crystal symmetries and occupied deuterium positions will be discussed. According to the literature
reviewed in the introduction, we expected that the crystal structure of the b phase would exhibit lower symmetry like the P6mm or P63mc, at least for high deuterium concentrations [27e30]. Surprisingly, even for high deuterium concentration (about 6 D atoms/ f.u.) the diffraction patterns can be completely indexed by the primary P6/mmm space group with no additional reflections visible (see Fig. 7). It seems that such behaviour has not been reported yet. All positions of deuterium with derived occupancies are presented in Fig. 9. It can be seen in the picture that deuterium occupies two positions in the a phase: 6 m and 12n. At the D2 equilibrium pressure of 4 bars, the refined stoichiometry of this phase is La0.75Ce0.25Ni4.8Al0.2D1.33(4). The stoichiometry of the b phase is La0.75Ce0.25Ni4.8Al0.2D3.82(3) at the same pressure with deuterium occupying mainly the 12n and the 6 m positions. Additionally, the 4 h and 12 positions are occupied in that phase. Taking the refined shares of the a and b phases into account, the total amount of deuterium in the sample is equal to 3.45(5) D/f.u., what is in a good agreement with the corresponding PCT data. Further uptake of deuterium is related to filling of the 4 h and the 12 positions in the b phase, what can be clearly seen in Fig. 9. The deuterium content is close to 6.3(1) D/f.u. at the equilibrium pressure of 20 bar. Interestingly, the release of deuterium is related to the 6 m and the 12n positions, from which more deuterium is released than from the 4 h and 12 sites. That is an origin of observed hysteresis between absorption and desorption cycles. 4. Concluding remarks In this paper, we report the structural and sorption properties of the novel La0.75Ce0.25Ni4.8Al0.2 alloy. We propose complex control
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Fig. 9. The occupancies of the deuterium positions in the a phase and b phase derived from in-situ neutron diffraction.
strategy for cycle-life including the X-ray diffraction and magnetometry measurements. The obtained PCT curves along with good stability towards cycling makes the investigated compound a good candidate as a supply for fuel cells, H2 compressors, heat storage or soft actuating systems. According to the results of the magnetometric measurements, the Ce exhibit 4f1 configuration due to Al doping, what is an origin of good sorption properties. The magnetometry data gathered after cycle-life experiment confirms extremely low Ni sedimentation, what is the key factor for the stability towards extended absorption/desorption operation. Investigation of the La0.75Ce0.25Ni4.8Al0.2 structural properties at different stages of deuterium uptake presented in section 3.4 brings very intriguing results. At first, both the a and b phases have the same hexagonal crystal structure P6/mmm. This is distinctive in the context of the data presented for the other LaNi5 derivatives, for which lowering of the symmetry has been evidenced. Moreover, our data suggest clearly the pressure dependent deuterium distribution over different available sites in the b phase, which is usually considered as saturated. As a matter of fact, the b phase might be treated as saturated only away from the PCT plateau region. Finally, we show that hysteresis exhibited by the PCT curves is associated with the non-homogeneous release of deuterium from the occupied sites in the b phase. Acknowledgements This work was supported by the Polish-Norwegian Research Programme through the project ‘Nanomaterials for hydrogen storage’ No. 210733, the European Commission under the 7th Framework Programme through the ‘Research Infrastructure’ action of the 'Capacities' Programme, the NMI3-II Grant number 283883 and by the AGH UST statutory tasks No. 11.11.220.01/6 subsidized by the Ministry of Science and Higher Education. We thank the HZB for the allocation of the neutron beamtime. JC, WJ and ŁG thankfully acknowledge the financial support from the HZB. References [1] J.H.N. van Vucht, F.A. Kuijpers, H.C.A.M. Bruning, Reversible roomtemperature absorption of large quantities of hydrogen by intermetallic compounds, Philips Res. Rep. 25 (1970) 133e140. [2] X. Yuan, H.-S. Liu, Z.-F. Ma, N. Xu, Characteristics of LaNi5-based hydrogen storage alloys modified by partial substituting La for Ce, J. Alloys Compd. 359
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Structural peculiarities in the b phase of the La0.75Ce0.25Ni4.8Al0.2 deuterides J. Czub a, *, W. Jamka a, J. Przewo znik a, A. Zarzecka a, A. Hoser b, D. Wallacher b, N. Grimm b, Ł. Gondek a a b
w, Poland AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Mickiewicza 30, 30-059, Krako Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109, Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2017 Received in revised form 20 February 2019 Accepted 23 February 2019 Available online 25 February 2019
In this contribution the structural, magnetic and sorption properties of the novel La0.75Ce0.25Ni4.8Al0.2 alloy and its deuterides are discussed. The collected data indicate good stability of the alloy during the cycle-life and thermal treatment tests. Particularly, the desorption pressure is almost constant versus the cycle number. The results of the in-situ neutron diffraction studies show the unusual behaviour of the deuterium rich b phase. Unexpectedly, the b phase belongs to the same P6/mmm space group as the hydrogen poor a phase. Moreover, the b phase behaves like a solid solution on the plateau of the pressure-composition-temperature (PCT) curve. The deuterium sites and their occupations in both a and b phases are determined from the neutron diffraction data, giving the first-hand insight into the mechanism of the PCT hysteresis. As far as the magnetic properties are concerned, cerium is trivalent in the investigated alloy. However, no magnetic ordering down to 3 K is observed. It can be concluded that the sorption properties of the crystal depend on the cerium valence, what distinguishes the alloy from the other compositions. © 2019 Elsevier B.V. All rights reserved.
Keywords: Hydrides Sorption properties Magnetism Magnetic ordering Neutron diffraction
1. Introduction The AB5 alloys have been extensively studied since the seventies because of their excellent hydrogen sorption properties that can be easily tuned in order to meet the requirements of solid state hydrogen storage systems [1]. Those properties include the maximum H2 capacity, thermodynamics, kinetics, reversibility, cycle-life and corrosion resistance. The gravimetric hydrogen storage capacity is an exception. Although it does not exceed 2 wt% for the LaNi5 derivatives, those compositions are still the most promising materials for stationary hydrogen storage and hydrogen purification units. It was also shown that the thermodynamic and cycle-life properties of the AB5 alloys can be improved as a result of partial substitution for the A or/and the B [2e5]. For instance, Marshall et al. found the correlation between the unit cell volume and the plateau pressure for the LaNi5-xAlx [6]. Namely, the higher Al concentration resulted in an increase of the lattice parameters and a
* Corresponding author. E-mail address: [email protected] (J. Czub). https://doi.org/10.1016/j.jallcom.2019.02.259 0925-8388/© 2019 Elsevier B.V. All rights reserved.
decrease of the plateau pressure. Since then, extensive studies on different dopants have been conducted [7e14]. For example, Uchida et al. studied the series of the La1-xRxNi5 (where R ¼ Ce - Lu) compounds. It was shown that an addition of Ce led to a significant increase of the equilibrium pressure and the heat of formation [14]. Moreover, cycling stability of the LaNi5 alloys can be enhanced by Ce substitution, what prevents Ni sedimentation [15]. This is important for long-term stability that is crucial for hydrogenstorage/compression/purifying applications. On the other hand, replacement of Ni with a low amount of Al reduced the plateau slope without a significant decrease of the hydrogen capacity [16]. Moreover, substitution of Al for Ni significantly lowered the equilibrium pressure for hydrogen desorption [13]. Similar results were obtained recently for the LaNi5-xAlx (x ¼ 0, 0.25, 0.5, 0.75, 1) [17]. It was shown that an increase of the Al content led to a decrease of the formation energy and reduced the plateau values. However, it was presented that long-term cycling led to an increase of the PCT slope for the LaNi4Al and a decrease of the hydrogen concentration. Therefore, it can be concluded that the material degraded. Particularly, disproportionation and nanocrystallization occurred [18]. On the other hand, substitution of Al and Bi in the LaNi5 can reduce absorption/desorption pressures and
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prevents decrepitation during cycling [19]. The other recently proposed method of improving the performance of the AB5 alloys was to introduce the surface modification by Pd/Pt in order to decrease chemical activity towards hydrogen impurities. Particularly, the MmB5 (Mm ¼ the mixture of La, Ce, Pr, Nd; B ¼ the mixture of Ni, Co, Al, Mn) alloy was modified by ballmilling followed by autocatalytic deposition of Pd [20]. It was concluded that such a procedure led to a marginal improvement in stability for low concentrations of Pd, while for higher concentrations (>2 wt%) disproportionation of the primary MmB5 material occurred. Substitution of Ce and Al for the A and B elements respectively yielded very interesting results for the AB3 alloys [21]. While Ce doping increased the equilibrium pressure, introduction of Al reduced the hydrogen capacity and increased the plateau slope of the pressure-composition-temperature (PCT) curves. It was concluded that Ce and Al doping can be used for fine-tuning of the AB3 systems [22e24]. Additionally, multi-substitution for several elements is a well-established route to improve the sorption properties of the alloys. Recently, such an approach has been successfully applied for the Mg-based alloys [25,26]. Although the LaNi5 and its derivatives have been thoroughly studied and their extremely interesting behaviour under hydrogen/ deuterium pressure has been reported, the information on the structural aspects of the hydrides formation is limited. It is known that the structure of the a phase belongs to the hexagonal P6/mmm space group. However, the structure of the hydrogen rich b phase is a matter for debate. The issue seems to be resolved for the LaNi5Dx (5.0 x 6.7) by the results of the comprehensive research reported by Lartigue et al. [27]. According to Refs. [27e29], the structure of the b phase belongs to the original P6/mmm or P6mm space group up to x ¼ 5.0. The superstructure appeared for the higher deuterium concentrations due to ordering of deuterium. Consequently, the appearance of additional reflections was related to the P63mc space group (hkl ¼ 201, 203, 205) that emerged when the original P6/mmm c lattice parameter was doubled. It is worth noting that among the superstructure reflections, the 203 one was the most prominent as it did not interfere with the others. Moreover, it was noticed that the 203 reflection was indeed visible in the previously reported neutron diffraction patterns [27]. It should be also highlighted that usually 4 deuterium sites (4 h, 6 m, 12n, 12 ) were considered for the P6/mmm space group, while the number of possible sites increased for the P6mm and P63mc space groups. That observation is important especially for doping with elements with different hydrogen/deuterium affinities because that enables preferential site occupation depending on the substituents. Therefore, it leads to the conclusion that the occurrence of the superstructure would be even more probable for the doped LaNi5 alloys. The superstructure was indeed reported for the non-stoichiometric LaNi5þxDy alloy for the deuterium concentration above 5.45 [30]. It is known that the evolution of the superstructure involves mainly hydrogen/deuterium atoms. For that reason, it cannot be detected with X-ray diffraction. To the best of our knowledge, the superstructure has been reported only as the result of the neutron diffraction experiments conducted for the b phase close to saturation. The nature of the a to b phase transformation is another interesting structural aspect of hydriding of the LaNi5-related alloys. Namely, the occurrence of an additional g phase was reported [31e34]. The g phase coexisted with the a and b phases for the narrow hydrogen concentration range. The g phase was initially observed for the alloys that underwent temperature treatment. However, its occurrence was also reported for the raw material as a result of non-equilibrium measurements conditions [34]. It was confirmed by the recent studies conducted by Gray et al. [35].
In this contribution, we report the results of the comprehensive studies on the La0.75Ce0.25Ni4.8Al0.2 alloy and the corresponding deuterides. The report includes the results of the X-ray and in-situ neutron diffraction experiments, the Sieverts volumetric H2 sorption and cycle-life studies and finally the VSM magnetometric measurements. 2. Experimental details The La0.75Ce0.25Ni4.8Al0.2 alloy was synthesised by induction melting of the high purity constituents (3 N for rare earths and 5 N for Ni and Al). The initial content of Al was 5 wt% higher than the stoichiometric quantity. The sample was remelted and then annealed at 550 C for 28 days in order to achieve homogeneity. It is worth noting that annealing at higher temperatures leads to some sedimentation visible on the surface of quartz tubes. The resulting alloy was examined by X-ray diffraction and fluorescence spectroscopy that confirmed high quality of the sample with the proper stoichiometry and without noticeable traces of impurity phases. The X-ray diffraction studies (XRD) were performed by means of the PANalytical Empyrean diffractometer using the Cu Ka radiation. The neutron diffraction (ND) studies were conducted by means of the E6 focusing diffractometer at the Helmholtz-Zentrum Berlin. A high temperature furnace with a gas absorption insert (operating up to 20 bar) was used for the in-situ measurements. The insert was connected to the Sieverts apparatus. The sample of about 6 g was placed in an inconel steel container. Prior to the measurements, the calibration patterns of an empty container at different temperatures and D2 pressures were collected for a precise extraction of the contribution originating in the container and the furnace from the data. For the high temperature neutron diffraction measurements, the sample was placed in a quartz tube. The contribution originating in the quartz tube was extracted using the results of the calibration measurements performed for an empty tube in the entire temperature range. The measurements were performed in the inert gas (Ar) atmosphere at 1 bar overpressure. The diffraction patterns were refined using the Rietveld FullProf software [36]. The sorption properties, including the PCT and cycle-life, were investigated using the Setaram PCT-PRO Sieverts apparatus. The cycle life measurement were performed at 25 C and the pressure of 20 bar was applied and maintained until no further progress of hydrogen absorption was noticed. Then the desorption process was performed and followed by 5 min of vacuum pumping. The magnetometric data were collected by means of the Quantum Design PPMS-9 system in the temperature range 3e350 K and magnetic fields up to 9 T. 3. Results and discussion 3.1. Structural properties and thermal stability The XRD and ND experiments revealed that at temperature of 25 C the La0.75Ce0.25Ni4.8Al0.2 crystallises in the hexagonal CaCu5 structure with the P6/mmm symmetry with the following lattices parameters: a ¼ 4.99284(12) Å and c ¼ 4.00528(12) Å. The best refinements coefficients are obtained for the model where Ce atoms substitute La at the position 1a, while Al atoms are located only in the 3g sites. It is worth noting that while the a lattice parameter is somewhat lower than the value for the pure LaNi5, the c parameter is higher. The similar relation between the refined parameters can be found for the LaNi4.75Al0.25 [29]. For the La for Ce substitution, it is well established that both lattice parameters are lower than for the pure alloy (the lanthanides contraction). Comparing our data with those reported for the La0.8Ce0.2Ni5, one can find that both refined lattice parameters for the
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La0.75Ce0.25Ni4.8Al0.2 are higher [37]. The conclusion is that simultaneous introduction of Ce and Al into the LaNi5 lattice leads to an anisotropic change of the lattice parameters, what is very convenient for tuning of the sorption properties. The issue of thermal stability mentioned in the experimental section was addressed by high temperature neutron diffraction (Fig. 1). As one can see, the crystal structure is maintained up to the highest investigated temperature. However, an additional line at 71 of 2Q appears at temperatures above 550 C. This line is related to the CeAl2 intermetallic alloy. The sedimentation of the Al-rich phase is reflected in a decrease of the c lattice parameter that starts above 625 C as depicted in Fig. 2. Such behaviour corresponds with the crystal properties of the Ce or Al doped LaNi5 alloy discussed above. The alloy is not supposed to operate at such elevated temperatures. Therefore, recycling (e.g. desorbing of surface impurities) has to be conducted at temperatures below 500 C.
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3.2. Sorption and cycle-life properties The maximum hydrogen concentration for the investigated alloy at 0 C corresponds to the formal composition La0.75Ce0.25Ni4.8Al0.2H6.75(2). After the full activation (10 cycles), the desorption mid-plateau pressure is almost equal to 0.6 bar, while the corresponding absorption pressure is 1.1 bar (Fig. 3). For 25 C, the corresponding values are 3.9 and 2.1 bar for absorption and desorption, respectively. The absorption pressure at 25 C is much higher than for the pure LaNi5 or LaNi4.8Al0.2. The values around 1.4 bar are reported for those compositions [38]. On the other hand, the values of absorption plateau pressure are significantly higher than 5 bar for the LaNi5 alloys doped only with Ce [14,37,39]. A closer look at Table 1, in which the thermodynamic properties are gathered, yields an interesting observation. The absorption enthalpy change (DH) is close to the values obtained for the alloys doped only with Ce that contain the similar amount of Ce [37]. On the contrary, the desorption related DH values are much higher. However, those values are almost equal to those for the alloys doped only with Al [40,41]. For the detailed comparison one can refer to Table 2. It may be concluded that Al doping for Ni in the primary LaNi5 alloy leads to a significant increase of the modulus of the formation enthalpy. It yields almost 47 kJ/mol H2 for the LaNi4Al. On the contrary, substitution of Ce for La leads to a decrease of the -DH, however it seems that there is no apparent correlation between the DH and the Ce concentration (Table 2). The -DH value is 24.3 kJ/mol H2 for the La0.85Ce0.15Ni5 alloy, while 26.6 kJ/mol H2 was reported for the La0.8Ce0.2Ni5. Mischmetal (Mm) is used instead of pure La for the other typical compositions. Those
Fig. 1. The high temperature neutron diffraction patterns for the La0.75Ce0.25Ni4.8Al0.2. Only a group of the main reflections is presented for clarity.
Fig. 3. The Pressure-Concentration-Temperature (PCT) absorption and desorption curves for the La0.75Ce0.25Ni4.8Al0.2.
Table 1 The thermodynamic properties of hydrogen absorption/desorption for the just activated (10 cycles) and cycled La0.75Ce0.25Ni4.8Al0.2. Absorption
Fig. 2. Temperature variations of the lattice parameters for the La0.75Ce0.25Ni4.8Al0.2 derived from the neutron diffraction data.
After activation After 500 cycles
Desorption
-DH [kJ/mole H2]
-DS [J/mole H2 K]
DH [kJ/mole H2]
DS [J/mole H2 K]
28.61 32.37
107.45 117.71
30.54 28.18
108.85 100.9
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Table 2 Comparison of the enthalpy and the entropy of formation for the investigated compound and the chosen AB5 alloys. Mm denotes the La-rich mixture of light rare earths (La, Ce, Pr, Nd). Alloy
-DH [kJ/mole H2]
-DS [J/mole H2 K]
Ref.
MmNi5 MmNi4.7Al0.3 La0.85Ce0.15Ni5 MmNi4.5Al0.5 MmNi4.8Al0.2 La0.9Ce0.1Ni5 La0.8Ce0.2Ni5 MmNi4Al La0.4Ce0.4Ca0.2Ni5 La0.75Ce0.25Ni4.8Al0.2 La0.7Ca0.2Ce0.1Ni5 LaNi4.8Al0.2 LaNi5 MmNi4.7Sn0.3 LaNi4.7Al0.3 La0.5Mm0.5Ni4.7Sn0.3 LaNi4.7Sn0.3 LaNi4.5Al0.5 LaNi4Al
20.3 22.6 24.30 24.4 25.1 25.30 26.6 26.2 28.20 28.61 29.4 30.40 31.80 31.83 32.7 33.80 36.51 36.90 45.85
101.4 96.7 91.28 97.0 80.0 106 107.3 79.9 115.3 107.45 107 101.6 110.0 106.8 106 111.2 122.6 108.55 114.10
[42] [42] [38] [42] [42] [43] [37] [42] [44] This work [45] [46] [47] [48] [45] [48] [48] [40] [40]
alloys exhibit lower values of -DH in comparison with the LaNi5 due to the presence of Ce, Pr and Nd apart from La. The flatness of the desorption plateau is the other important factor. It was evidenced for the MnNi5-xAlx that the introduction of Al increases the slope of PCT plateau [42]. However, the flatness of the plateau is retained when only Ce is used as the La substituent [37]. Therefore, the investigated alloy exhibits the good flatness of the plateau comparable to those for the La1-xCexNi5 alloys [37]. At the same time, the range of desorption pressures desired for fuel cells applications can be achieved. Additionally, the cycle-life tests were performed to track the thermodynamic changes (Fig. 4.) It was found that the hysteresis for the La0.75Ce0.25Ni4.8Al0.2 is significantly reduced after 500 cycles.
Fig. 4. Results of the cycle-life studies for the La0.75Ce0.25Ni4.8Al0.2. The main chart presents the changes of the PCT curves collected at 25 C after cycling. A change of the maximum hydrogen concentration during cycling is presented in the inset. The red line is a fit of a simple exponential decay. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Namely, the absorption mid-plateau pressure is reduced to 3.0 bar, while the desorption pressure remains unaffected yielding value of 2.1 bar, what is the desired behaviour for possible applications. It was found that cycling does not affect the kinetics of the absorption/desorption reaction. Moreover, the maximum hydrogen capacity seems to obey a single exponential decay with the value of 5.83H/f.u. after 500 cycles. This value is higher than the value for the pure LaNi5. As a matter of facts, it resembles the behaviour reported for the LaNi5-xSnx [49,50]. The very recent studies on the LaNi5-xAlx showed an importance of Al doping for stabilising the crystal structure during long-term cycling [51]. The results presented by Liu at al apparently indicate better repeatability of the uptake mid-plateau pressures for x ¼ 0.25 and x ¼ 0.5 after 1000 cycles. However, the analogous mid-plateau pressures for desorption were more affected by cycling.
3.3. Structural changes during the cycle-life treatment The stability of the alloys is the key feature considering their potential use for hydrogen storage or compression. The structural aspects of ageing cover the issues of decrepitation, disproportionation, amorphisation and strain development that affect the parameters of the PCT curve (equilibrium pressures, slope of the plateau, hysteresis). Therefore, the XRD measurements were performed for the investigated alloy after 175 and 500 cycles (Fig. 5). According to our results, the sample does not exhibit traces of spurious phases after 500 cycles, what can be seen in Fig. 5. Moreover, the XRD patterns collected after 175 and 500 cycles do not differ significantly. That is in agreement with the cycle-life data presented in the previous paragraph. The Rietveld analysis of the collected patterns reveals significant strain development after 175 cycles, however the strain was slightly increased after another 325 cycles (Table 3). Similarly, the grains size is reduced significantly after 175 cycles, while further cycling does not affect this parameter within the uncertainty level. The lattice parameters do not change significantly and only a small expansion of the unit cell can be noticed. It was suggested in a number of papers that cycling of the LaNi5-related compounds leads to Ni precipitation. As a result, lowering of the unit cell volume during cycling was reported [50]
Fig. 5. The X-ray diffraction patterns for the cycled La0.75Ce0.25Ni4.8Al0.2. Details of the Rietveld refinement are presented for the raw sample.
J. Czub et al. / Journal of Alloys and Compounds 788 (2019) 533e540 Table 3 Results of the Rietveld refinement of the XRD data collected for the cycled La0.75Ce0.25Ni4.8Al0.2. a [Å]
c [Å]
V [Å3]
Strain [%] Size [nm]
Raw sample 4.99284(12) 4.00528(12) 86.479(3) 0.457(5) After 175 cycles 4.99324(11) 4.00536(12) 86.484(3) 0.906(3) After 500 cycles 4.99321(32) 4.00580(34) 86.506(6) 0.988(3)
>500 301(73) 282(67)
what is contrary to our case. For that reason, additional magnetometric measurements were performed for the La0.75Ce0.25Ni4.8Al0.2 to address this issue. Magnetometry is extremely sensitive even for traces of ferromagnetic precipitations and therefore can be used to track the traces of Ni sedimentation. According to Ref. [51], cycling of the LaNi5-xAlx alloys brought interesting findings. Namely, the XRD and EXAFS experiments revealed that substitution of Al or Ni reduced growth of structural disorder reflected in strain development. Moreover, the EXAFS experiment proved diminishing of the Ni-Ni coordination number with a simultaneous (smaller) increase of that quantity for the Ni-La. Apart from mixing of the La and Ni sublattices, that effect might be indicative of Ni sedimentation [51]. Unfortunately, neither magnetic studies nor extended EXAFS analyses were performed to address that issue. 3.4. Magnetometric measurements The magnetic properties of the La0.75Ce0.25Ni4.8Al0.2 are displayed in Fig. 6. The alloy is weakly magnetic at room temperature, likely due to the traces of ferromagnetic Ni nanoclusters. It can be deduced from the ferromagnetic part of the curve collected at 300 K that the share of Ni nanoclusters is less than 0.3% (atomic). The nickel contribution is an important parameter concerning the hydrogen absorption/desorption cycle-life studies, as mentioned in the introduction. Magnetometric measurements are well suited to track even very small quantities of nickel so this technique can be used for evaluation of nickel sedimentation after cycling of the alloy. The data presented in Fig. 6 (right bottom insert) show that the magnetisation curves are nearly the same after 500 cycles. That corresponds with the XRD results that showed that no visible impurities due to pure Ni were detected after the cycle-life measurements. The susceptibility data exhibit an increase at low temperatures,
Fig. 6. Magnetic properties of the La0.75Ce0.25Ni4.8Al0.2 compound. Magnetisation versus magnetic field at chosen temperatures is presented as the main diagram. The inserts present magnetic susceptibility and hysteresis loops for the raw and cycled sample.
537
what might be associated with magnetic ordering of the Ce sublattice. Unfortunately, the inverse magnetic susceptibility does not obey the Curie-Weiss law (pure and modified), hence the paramagnetic Curie temperature and the effective magnetic moment could not been evaluated. Cerium with the electron configuration 4f15d16s2 can form alloys and compounds with two different valence states: Ce3þ with the 4f15d06s0 electron configuration (usually referred as 4f1) and Ce4þ with the 4f05d06s0 electron configuration (referred as 4f0). Magnetic studies are crucial for determining the cerium configuration. The partially filled 4f states carry the magnetic moment originating from the spin and orbital component of the single 4f electron. On the other hand, the Ce4þ configuration, which is equivalent to the La3þ one, is intrinsically non-magnetic. The determined magnetic properties strongly suggest that cerium exhibit the 4f1 electronic configuration (Ce3þ) in the La0.75Ce0.25Ni4.8Al0.2, while the 4f0 configuration was reported for the CeNi5 [52]. Therefore, the investigation of magnetic properties sheds more light on an influence of Ce doping. Namely, the Ce3þ ion exhibits a larger radius than Ce4þ, so there is no lattice contraction of the investigated alloy observed in comparison with the purely Ce-doped LaNi5 derivatives, as discussed in section 3.1. Apart from an increased size of Ce3þ, also a change of the density of states (DOS) at the Fermi level could be expected. The latter is crucial for screening the repulsive interaction between hydrogen/deuterium atoms located at different sites in the crystal unit cell. Moreover, the electronic DOS is the key factor that establishes the enthalpy of hydride formation. Both effects are important for hydrogen uptake as the origin of the Westlake and Switendick criterions respectively. 3.5. In-situ neutron diffraction In order to get the information about the crystal properties of the La0.75Ce0.25Ni4.8Al0.2 during deuterium uptake/release, the insitu diffraction studies following the simultaneous PCT run were done. Several examples of the collected neutron diffraction patterns are presented in Fig. 7. One can notice that typical coexistence of the deuterium poor (a phase) and deuterium rich (b phase) occurs. Usually, the a phase is considered a metal-hydrogen/ deuterium solid solution. Indeed, the values of both the a and c lattice parameters of the a phase increase significantly with the deuterium equilibrium pressure as can be deduced from Fig. 8. Coexistence of both phases can be seen in the diffraction pattern collected at 4 bar (Figs. 7 and 8). It is interesting the c lattice parameter of the a phase significantly decreases when the b phase appears (at 4 bar). This could indicate that some deuterium transfers between both phases, what seems quite surprising. It is worth mentioning that coexistence of both phases is noticed only for that pressure. Moreover, an apparent increase of the lattice parameters in the b phase is evidenced when deuterium pressure is applied, what has not been reported so far. This increase is not as prominent as for the a phase, however it is still above the uncertainty level. It means that the b phase can be treated to some extent as a solid solution, similarly to the hydrogen poor phase. This behaviour is confirmed for the desorption process for which the lattice parameters for the b phase decrease (Fig. 8). Concerning deuterium desorption, the lattice parameters of the b phase significantly decrease as the process continues. Due to hysteresis evidenced by the PCT data, the deuterium rich phase is stable down to 2 bar of the D2 equilibrium pressures and only the a phase is present at 1.25 bar. A non-linear crystal volume expansion as a function of deuterium concentration was explained for AB2 alloys by Hirata model [53], at least for low D concentration. That approach uses the bulk compressibility theory, in which the pressure originates from the deuterium/hydrogen concentration. An
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Fig. 7. The neutron diffraction patterns collected at different pressures of D2 as marked in the PCT diagram (at 25 C). The contribution from the sample holder was extracted.
Fig. 8. The lattice parameters versus D2 pressure determined by in-situ neutron diffraction. It should be noted that the pressure scale covers both absorption (0e20 bar) and desorption (20 - 0 bar).
additional assumption that an expansion of the crystal (after hydriding) may open some positions primary excluded from D/H occupation by the Westlake/Switendick criterion was proposed in Ref. [54]. The latter model predicts the complex behaviour of the lattice volume versus the hydrogen concentration, what is apparently in agreement with the data presented in Fig. 8. Last, but certainly not least, the crystal symmetries and occupied deuterium positions will be discussed. According to the literature
reviewed in the introduction, we expected that the crystal structure of the b phase would exhibit lower symmetry like the P6mm or P63mc, at least for high deuterium concentrations [27e30]. Surprisingly, even for high deuterium concentration (about 6 D atoms/ f.u.) the diffraction patterns can be completely indexed by the primary P6/mmm space group with no additional reflections visible (see Fig. 7). It seems that such behaviour has not been reported yet. All positions of deuterium with derived occupancies are presented in Fig. 9. It can be seen in the picture that deuterium occupies two positions in the a phase: 6 m and 12n. At the D2 equilibrium pressure of 4 bars, the refined stoichiometry of this phase is La0.75Ce0.25Ni4.8Al0.2D1.33(4). The stoichiometry of the b phase is La0.75Ce0.25Ni4.8Al0.2D3.82(3) at the same pressure with deuterium occupying mainly the 12n and the 6 m positions. Additionally, the 4 h and 12 positions are occupied in that phase. Taking the refined shares of the a and b phases into account, the total amount of deuterium in the sample is equal to 3.45(5) D/f.u., what is in a good agreement with the corresponding PCT data. Further uptake of deuterium is related to filling of the 4 h and the 12 positions in the b phase, what can be clearly seen in Fig. 9. The deuterium content is close to 6.3(1) D/f.u. at the equilibrium pressure of 20 bar. Interestingly, the release of deuterium is related to the 6 m and the 12n positions, from which more deuterium is released than from the 4 h and 12 sites. That is an origin of observed hysteresis between absorption and desorption cycles. 4. Concluding remarks In this paper, we report the structural and sorption properties of the novel La0.75Ce0.25Ni4.8Al0.2 alloy. We propose complex control
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Fig. 9. The occupancies of the deuterium positions in the a phase and b phase derived from in-situ neutron diffraction.
strategy for cycle-life including the X-ray diffraction and magnetometry measurements. The obtained PCT curves along with good stability towards cycling makes the investigated compound a good candidate as a supply for fuel cells, H2 compressors, heat storage or soft actuating systems. According to the results of the magnetometric measurements, the Ce exhibit 4f1 configuration due to Al doping, what is an origin of good sorption properties. The magnetometry data gathered after cycle-life experiment confirms extremely low Ni sedimentation, what is the key factor for the stability towards extended absorption/desorption operation. Investigation of the La0.75Ce0.25Ni4.8Al0.2 structural properties at different stages of deuterium uptake presented in section 3.4 brings very intriguing results. At first, both the a and b phases have the same hexagonal crystal structure P6/mmm. This is distinctive in the context of the data presented for the other LaNi5 derivatives, for which lowering of the symmetry has been evidenced. Moreover, our data suggest clearly the pressure dependent deuterium distribution over different available sites in the b phase, which is usually considered as saturated. As a matter of fact, the b phase might be treated as saturated only away from the PCT plateau region. Finally, we show that hysteresis exhibited by the PCT curves is associated with the non-homogeneous release of deuterium from the occupied sites in the b phase. Acknowledgements This work was supported by the Polish-Norwegian Research Programme through the project ‘Nanomaterials for hydrogen storage’ No. 210733, the European Commission under the 7th Framework Programme through the ‘Research Infrastructure’ action of the 'Capacities' Programme, the NMI3-II Grant number 283883 and by the AGH UST statutory tasks No. 11.11.220.01/6 subsidized by the Ministry of Science and Higher Education. We thank the HZB for the allocation of the neutron beamtime. JC, WJ and ŁG thankfully acknowledge the financial support from the HZB. References [1] J.H.N. van Vucht, F.A. Kuijpers, H.C.A.M. Bruning, Reversible roomtemperature absorption of large quantities of hydrogen by intermetallic compounds, Philips Res. Rep. 25 (1970) 133e140. [2] X. Yuan, H.-S. Liu, Z.-F. Ma, N. Xu, Characteristics of LaNi5-based hydrogen storage alloys modified by partial substituting La for Ce, J. Alloys Compd. 359
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