Phase equilibria in the Tb-Mg-Co system at 500 °C, crystal structure and hydrogenation properties of selected compounds

Journal of Solid State Chemistry 232 (2015) 228–235

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Phase equilibria in the Tb-Mg-Co system at 500 °C, crystal structure and hydrogenation properties of selected compounds V.V. Shtender a, R.V. Denys b, I.Yu. Zavaliy a,n, O.Ya. Zelinska c, V. Paul-Boncour d, V.V. Pavlyuk c a

Karpenko Physico-Mechanical Institute, NAS of Ukraine, 5 Naukova Street, 79601 Lviv, Ukraine Hystorsys AS, P.O. Box 45, Kjeller NO-2027, Norway c Department of Inorganic Chemistry, Ivan Franko National University of Lviv, 6 Kyryla i Mefodiya Street, 79005 Lviv, Ukraine d Institut de Chimie et des Matériaux de Paris Est, CMTR, CNRS and U-PEC, 2-8 rue H. Dunant, 94320 Thiais, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 28 July 2015 Received in revised form 23 September 2015 Accepted 26 September 2015 Available online 30 September 2015

The isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been built on the basis of XRD analysis of forty samples prepared by powder metallurgy. The existence of two ternary compounds Tb4Mg3Co2 and Tb4MgCo was confirmed. The formation of two solid solutions, Tb1
xMgxCo3 (0 r x r0.4) and Tb1-
xMgxCo2 (0 rx r 0.6), was found for the first time. It is shown that Tb5Mg24 also dissolves a small amount of Co. Other binary compounds do not dissolve the third component. The Tb4MgCo and TbMgCo4 compounds form hydrides (12.7 and 5.3 at.H/f.u. capacity, respectively) that retain the original structure of metallic matrices. Upon thermal desorption the Tb4MgCoH12.7 hydride was stable up to 300 °C and disproportionated at higher temperature. Two other hydrides, Tb4Mg3Co2H∼4 and Tb2MgCo9H12, are unstable in air and decompose into the initial compounds. & 2015 Elsevier Inc. All rights reserved.

Keywords: Rare earth compounds Magnesium compounds Isothermal section Crystal structure Hydrogen storage Metal hydrides

1. Introduction In recent years Mg-based ternary intermetallic compounds were extensively studied in terms of optimization of the synthesis conditions and determination of their crystal structures and properties. Despite the great interest to R–Mg–T compounds and alloys (R ¼rare-earth; T ¼transition metal) the phase diagrams for most of the systems have not been studied yet. The phase equilibria were investigated and the isothermal sections were constructed only for three systems so far: Ce–Mg–Ni at 200 °C [1], La– Mg–Ni at 500 °C [2,3] and Tb–Mg–Ni at 400 °C [4]. The existence of 4, 7 and 11 ternary compounds was revealed in these systems, respectively. Solid solutions on the basis of the binary compounds RT5, R2T7, RT3 and RT2 with R atoms substituted by Mg were found for the systems with La and Tb [3,4]. There are several works devoted to the study of R–Mg–Ni systems, which confirm the formation of RMgNi4 and RMg2Ni9 (R¼ Y, Ce, Nd, Pr) ternary compounds but contain the contradictive phase equilibriums in Ni-rich region [5–8]. The R–Mg–Ni compounds and alloys attract attention as hydrogen absorbing materials and particularly as effective MH electrodes for Ni–MH chemical power sources [9]. It was established that the n

Corresponding author. E-mail address: [email protected] (I.Yu. Zavaliy).

http://dx.doi.org/10.1016/j.jssc.2015.09.031 0022-4596/& 2015 Elsevier Inc. All rights reserved.

partial replacement of lanthanum by magnesium leads to significant changes in the characteristics of hydrides of intermetallic compounds. In particular, the compounds with R replaced by Mg do not undergo hydrogen induced amorphization if compare with the most relevant binary compounds and as a result the crystalline hydrides can be obtained. Hydrides of the La–Ni intermetallic compounds (e.g. La2Ni7) are characterized by anisotropic cell expansion during the hydrogenation, whereas the compounds with partial substitution of lanthanum by magnesium (e.g. La1.5Mg0.5Ni7) demonstrate isotropic expansion [10]. Many new ternary compounds were recently found in the R–Mg–Ni systems [9,11–32]. Most of these intermetallics should be studied for hydrogen storage purposes. Researchers paid also much attention to the systems with other transition metals. For example, De Negri et al. [33] studied the interaction of the components in the La–Mg–Co system at 400 °C (Cor50 at%) on the basis of X-ray and metallographic analysis. These authors determined the existence of the homogeneity range for already known compound La4
xMg1 þ xCo (0 rx r0.15) (str. ̄ type Gd4RhIn, sp. group F43 m), and found the limits of existence as well as the crystal structure for the new compound La23
x Mg4 þ xCo7 (
0.50 rxr 0.60) (str. type Pr23Ir7Mg4, sp. group P63mc). The crystal structure of the compound ∼La38Co55Mg7 remains undetermined. The existence of the La2Co17
xMgx phase was reported and indicates the stabilizing role of magnesium on the formation of the binary phase La2Co17.

V.V. Shtender et al. / Journal of Solid State Chemistry 232 (2015) 228–235

Fig. 1. Isothermal section of the Tb–Mg–Co phase diagram at 500 °C (binary equilibrium regions marked by gray color).

Our previous works were devoted to the study of R–Mg–Co systems, namely, the synthesis of new compounds, study of their crystal structures and hydrogenation properties. In particular, it was demonstrated that the hydrogen storage capacity increased substantially for the compounds RMgT4 (R¼Y, Ce, Nd) with Co for Ni substitution [20–22]. This paper reports the isothermal section of Tb–Mg–Co phase diagram at the 500 °C, the crystal structure of ternary compounds and solid solutions Tb1
xMgxCo2 and Tb1-xMgxCo3 as well as their hydrogenation properties, crystal structure and thermodesorption of hydrogen for the corresponding hydrides. The results about the hydrogenation of R4Mg3Co2 and R4MgCo compounds have been described more detailed in [34,35].

2. Experimental part The R and Co ingots (with purities Z99.9%), and Mg powder (Alfa Aesar, 325 mesh, 99.8%) were used as starting materials for synthesis of R–Mg–Co alloys. In the first step, R–Co precursors were prepared by arc melting of corresponding metals in purified argon atmosphere. Then they were ground and mixed with Mg powder in certain proportions. In order to avoid the deviation from nominal composition due to Mg evaporation3 wt% excess of Mg was added. The powder mixtures obtained by the grinding were pressed into pellets (d ¼8 mm), loaded into tantalum containers, and sealed in stainless steel tubes under argon. After that the samples were heated step by step from 500 to 800 °C for 8– 15 h. Finally, the alloys were slowly cooled down to 500 °C, annealed at this temperature for ∼250 hours and quenched in cold water. Phase analysis of the samples was carried out with the use of powder X-ray diffraction data (DRON-3.0M and Brucker D8 diffractometers with Fe and Cu Kα-radiation respectively). The obtained powder diffraction profiles were analyzed by Rietveld method using Fullprof software [36]. XRD profiles for the known binary and ternary compounds were generated from Pearson's Crystal Data [37] and used for the phase analysis also. Microstructures and elemental compositions of the alloys were examined by scanning electron microscopy (SEM) using an EVO 40XVP microscope equipped with Inca Energy 350 spectrometer for energy dispersive X-ray analysis (EDXS). The quantitative

229

analysis of Tb–Mg–Co alloys was limited by energy spectrometer resolution, which leads to serious peaks overlap between the spectra of Mg and Tb. As a result, the ratio of the Mg/Tb concentrations was inflated. According to our observations the concentration of Mg was higher for 5–10 at%. Another problem was the brittleness of the samples. EDXS quantitative analysis was performed on selected alloys. The hydrogen absorption properties were studied in a conventional Sieverts type apparatus, in order to determine hydrogen storage capacity by the volumetric method. The samples were activated by heating up to 200 °C under dynamic vacuum (10
5 mbar) and, afterward, cooled to the room temperature. Hydrogenation was carried out by two methods. First, the samples were hydrogenated at room temperature and 10 bar H2 final pressure (Tb4Mg3Co2, TbMgCo4) and 25 bar H2 final pressure for Tb2MgCo9. The hydrogen was added with 1 bar step for full saturation. In the second method, the hydrogenation of Tb4MgCo was performed at higher temperature and lower hydrogen pressure. The sample was maintained at 120 °C, hydrogen gas was added with 0.5–1 bar step, final pressure was 10 bar. The samples easily absorbed hydrogen and kept at final saturation pressure within 20 h. In situ XRD of the Tb4MgCoH12.7 sample was studied during the thermal desorption (2 °C/min heating in argon atmosphere). Powder XRD patterns were measured using Bruker D2 Discover powder diffractometer equipped Cu sealed tube, Göbbel mirror, Vantec 500 area detector and Anton Paar DHS 1100 Domed Heating Stage.

3. Results and discussions 3.1. Phase equilibria in the Tb-Mg-Co system Forty samples of different compositions (indicated as points on the isothermal section) were synthesized for the study of the phase equilibria in the Tb–Mg–Co system. The isothermal section of the Tb–Mg–Co phase diagram at 500 °C constructed on the basis of the XRD and SEM data is presented in Fig. 1. The results of the XRD phase analysis of 21 selected samples are listed in Table 1. XRD patterns (experimental, calculated and differential) of several ternary alloys are shown in Fig. 2 as an example. EDXS quantitative analysis was performed for Nos. 4, 15, 16 alloys (and few others, which are not marked on the diagram). The result of their studies were confirmed by XRD data, which are presented in Table 1. All presented binary compounds have been confirmed during investigation of the ternary Tb–Mg–Co system at 500 °C. The existence of two ternary compounds Tb4Mg3Co2 (str. type Nd4Mg3Co2, sp. gr. P2/m) [38] and Tb3.77Mg1.23Co (str. type ̄ Gd4RhIn, sp. gr. F43m) [39] in the Tb-rich region is confirmed in this study also. The extended solid solutions based on the TbCo2 and TbCo3 binary compounds were found for the first time. The crystal structure peculiarities of the Tb1
xMgxCo2 and Tb1
xMgxCo3 alloys are detailed in the next section. We also observed a small Co solubility in Tb5Mg24 compound (up to 5 at%) accompanied by slight decrease of unit cell volume. Other binary compounds did not dissolve the third component. Usually this depends on the crystal and electronic structure of binary compounds. For R–Mg–T systems the Laves phases and phases, which contain this fragment (RT3, R2T7 and/or R5T19) are capable to dissolve the magnesium as third component. In this work this dissolution is confirmed experimentally for the Tb1
xMgxCo2 and Tb1
xMgxCo3 phases. For more detailed analysis and explanations quantum-chemical calculations can be used [40]. For Tb–Mg–Co system (unlikely to R–Mg–Ni systems) the intermetallic compounds for Tb: Co ¼1: 1 ratio as well as

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Table 1 Phase compositions and structural characteristics of some Tb–Mg–Co alloys. No.

Composition

1

Tb5Mg10Co85

2

Tb19Mg3Co78

4 5

Tb17Mg8Co75 Tb15Mg10Co75

6

Tb8Mg17Co75

8 9 10

Tb13Mg20Co67 Tb17Mg16Co67 Tb22Mg11Co67

11 12 13

Tb25Mg8Co67 Tb28Mg5Co67 Tb33Co67

14

Tb60Mg10Co30

15

Tb4Mg60Co36

16

Tb50Mg25Co25

17

Tb70Mg10Co20

18

Tb80Mg10Co10

19

Tb70Mg20Co10

20

Tb58Mg32Co10

21

Tb48Mg42Co10

22

Tb37Mg53Co10

23

Tb17Mg67Co16

Phase

MgCo2 Tb2Co17 Co Tb2Co7 Tb1
xMgxCo3 TbCo5 Tb2MgCo9 Tb1.8Mg1.2Co9 TbCo5 MgCo2 TbCo5 Tb1
xMgxCo3 Tb0.8Mg1.2Co4 TbMgCo4 Tb1
xMgxCo2 TbCo3 Tb1
xMgxCo2 Tb1
xMgxCo2 TbCo2 TbCo3 Tb4
хMg1 þ хCo TbCo2 Tb4Co3 Mg MgCo2 Tb5Mg24
xCox Tb4
хMg1 þ хCo Tb4Mg3Co2 TbCo2 Tb4
хMg1 þ хCo Tb3Co Tb4
хMg1 þ хCo Tb Tb4-хMg1 þ хCo htTb TbMg Tb4
хMg1 þ хCo TbMg Tb4Mg3Co2 TbMg Tb1
xMgxCo2 TbMg2 Tb1-xMgxCo2 TbMg Tb5Mg24
xCox Tb1
xMgxCo2 TbMg3

Space group

P63/mmc P63/mmc P63/mmc ̄ R3m ̄ R3m P6/mmm ̄ R3m ̄ R3m P6/mmm P63/mmc P6/mmm ̄ R3m ̄ F43 m ̄ F43 m ̄ F43 m ̄ R3m ̄ Fd3m ̄ Fd3m ̄ Fd3m ̄ R3m ̄ F43 m ̄ Fd3m P63/m P63/mmc P63/mmc ̄ I43 m ̄ F43 m P2/m ̄ Fd3m ̄ F43 m Pnma ̄ F43 m P63/mmc ̄ F43 m ̄ Im3m ̄ Pm3m ̄ F43 m ̄ Pm3m P2/m ̄ Pm3m ̄ Fd3m P63/mmc ̄ Fd3m ̄ Pm3m ̄ I43 m ̄ Fd3m ̄ Fm3m

Lattice parameters, Å

Phase content, wt%

a

b

c

4.8880(5) 8.3530(12) 2.5042(7) 5.0056(7) 5.0031(15) 4.9561(8) 4.9870(3) 4.9807(1) 4.9196(15) 4.9249(6) 4.8339(5) 4.9259(7) 7.0517(2) 7.0762(2) 7.1089(13) 5.0179(11) 7.1544(3) 7.1657(9) 7.2080(2) 5.029(2) 13.5579(17) 7.2094(8) 11.551(4) 3.2058(8) 4.9295(13) 11.042(5) 13.5062(11) 7.497(4) 7.2082(10) 13.5632(12) 7.012(2) 13.5999(14) 3.6059(11) 13.529(9) 4.028(3) 3.799(3) 13.521(3) 3.772(2) 7.396(8) 3.739(4) 7.178(6) 6.040(2) 7,199(2) 3.794(3) 11.2558(8) 7.1902(4) 7.4376(7)

– – – – – – – – – – – – – – – – – – – – – – – – – – – 3.723(2), β ¼109.56(4)°

7.994(1) 8.148(2) 4.088(3) 36.258(8) 24.092(9) 3.975(1) 24.053(2) 24.027(1) 4.001(2) 8.080(2) 4.067(1) 24.184(4) – – – 24.386(7) – – – 24.421(13) – – 3.992(2) 5.206(2) 8.029(4) – – 8.189(4) – – 6.257(1) – 5.667(3) – – – – – 8.040(2) – – 9.827(3) – – – – –

compounds with incommensurated structures in Mg-rich region were not observed. Our results and literature data demonstrated significant changes in the formation of compounds in R–Mg–T (T ¼Fe, Co, Ni) systems, which correlate with the changes in the radii of transition metal atoms and the electron occupation of 3dsublevels. For R–Mg–T systems no compounds are observed for T ¼Fe [4], 3 and 4 compounds for T ¼Co, 9 and 11 compounds for T ¼Ni (the data presented for R¼La and Tb respectively) [33,2–4]. 3.2. Crystal structure of the ternary and pseudobinary compounds of the Tb-Mg-Co system The refined lattice parameters for both Tb4Mg3Co2 and Tb4MgCo compounds (see Table 2) are in good agreement with the published data [38,39]. We also confirmed the deviation from a stoichiometric composition 4:1:1 of the compound Tb4MgCo. The homogeneity range of the Tb4
xMg1 þ xCo compound (∼4 at%) was found as a result of the analysis of the lattice parameters and

– 9.387(2) – – – – – – – 3.795(4), β ¼ 108.96(5)° – – – – – – –

79.6 13.3 7.1 37.7 36.3 26.0 100 95.0 5.0 44.8 36.6 18.6 100 100 92.5 7.5 100 100 90.8 9.2 59.7 30.0 10.3 67.5 21.3 11.2 58.1 30.8 11.1 76.0 24.0 93.3 6.7 74.8 22.0 3.2 63.0 37.0 73.5 19.5 7.0 80.0 17.1 2.9 47.5 37.5 15.0

chemical compositions of 2 samples with different Mg content [35]. The calculated and observed XRD diffraction profiles for the TbMgCo4 and Tb2MgCo9 compositions of the Tb1
xMgxCo2 and Tb1
xMgxCo3 solid solutions are shown in Figs. 3 and 4, correspondingly. The refined values of lattice parameters, positions and thermal displacement parameters of atoms for these pseudobinary compounds are presented in Table 3. The maximum solubility of Mg in TbCo3 and TbCo2 is 10 and 20 at% respectively (see Fig.1). Tb1
xMgxCo3 (x¼ 0–0.4) can be clearly identified as a solid solution of substitution (Tb by Mg) in TbCo3 with a corresponding decrease in cell parameters (see Table 2). Tb1
xMgxCo3 preserves the structure of the binary compound (str. type PuNi3) with the substitution of Tb by Mg in 6c site only. Tb2MgCo9 is isostructural to R2MgNi9 (R¼La, Nd, Pr) [15–17]. In case of the Tb1
xMgxCo2 solid solution the change of the crystal structure from MgCu2-type for TbCo2 to MgCu4Sn-type for TbMgCo4 has been observed. The superstructure peaks hkl indexes [2 0 0], [4 2 0], [6 0 0] appeared for the latter compound (see

V.V. Shtender et al. / Journal of Solid State Chemistry 232 (2015) 228–235

231

Fig. 2. Refined XRD patterns of the selected Tb–Mg–Co alloys (the first number is the number of sample in Table 1, the two or three other numbers correspond to the refined phases in the figure). (a) No. 2-(1-Tb2Co7 þ 2-TbCo5 þ 3-Tb2MgCo9); (b) No. 14– (1-Tb4MgCo þ2-Tb4Co3 þ 3-TbCo2); (c) No. 16 – (1-Tb4MgCoþ 2-Tb4Mg3Co2 þ 3-TbCo2); (d) No. 17– (1-Tb4MgCoþ 2-Tb3Co); (e) No. 18 – (1-Tb4MgCoþ 2-Tb); (f) No. 20 – (1-Tb4MgCoþ 2-TbMg). ̄

̄

Fig. 5). The structural transition Fd3m-F43m (see Fig.6), which occurs after the addition of x 40.3 Mg to TbCo2, is related to ordering of Tb and Mg atoms. Full ordering Mg in 4c site was observed for the TbMgCo4 alloys. The same phenomena was observed for RMgNi4 isostructural alloys (R¼La, Tb) [2,4]. It should be noted also that Tb1
xMgxCo2 is Laves phase whereas Tb1
xMgxCo3 is a hybrid, constituted by layers of Laves and AB5 Haucke phases. In both cases Mg substitutes Tb in 4 c and 6 c sites respectively, which belong to Laves phase layers [24,40].

3.3. Hydrogenation properties of the ternary and pseudobinary Tb-Mg-Co compounds Tb4Mg3Co2 and Tb4MgCo compounds were described a long time ago, but their hydrogenation properties have not been studied yet. The Tb4Mg3Co2 alloy absorbed only ∼0.5 wt% of hydrogen at 25 °C and 10 bar hydrogen pressure. The obtained hydride was unstable in air at atmospheric pressure and prior to XRD analysis decomposed back to Tb4Mg3Co2 initial phase [34]. It is possible,

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Table 2 Crystallographic parameters of the ternary and. pseudobinary compounds in the Tb–Mg–Co system. #

τ1 τ2

Compound

Tb4
xMg1 þ xCo x ¼
0.2C0.2 Tb4Mg3Co2

Structure type

Space group

̄

Gd4RhIn

F43 m

Nd4Mg3Co2

P12/m1

Lattice parameters (Å) a

b

c

13.621(3)* 13.5747(4)
13.5999(14) 7.504(2)

– – 8.195(2)

[39] This work [38]

8.1913(5)

This work

7.2080(2) – 7.0517(2)

– – 3.7286(6) β ¼109.48(3) 3.7253(2) β ¼109.587(2) –

–

This work

5.0242(2) – 4.9807(1)

–

24.422(2)–24.0265(14)

This work

7.5034(4) τ3 τ4

Tb1
xMgxCo2 x ¼0C0.6 Tb1
xMgxCo3 x ¼0C0.4

MgCu2 – MgCu4Sn PuNi3

̄

̄

Fd3m – F43 m ̄

R3m

Reference

*at x ¼0.23. Table 3 Crystallographic parameters of the Tb1
xMgxCo2 and Tb2MgCo9 alloys. obtained from XRD data. Atoms

Site

x

y

Biso (Å2)

z

̄

TbMgCo4 (Sp. gr. F43m; a¼ 7.0755(2) Å; V ¼354.22(2) Å3) Tb 4a 0 0 0 Mg 4c 1/4 1/4 1/4 Co 16e 0.6244(2) x x

1.5(1) 0.2(2) 1.7(1)

̄

Tb0.75Mg0.25Co2 (Sp. gr. Fd3m; a ¼7.1544(3) Å; V ¼366.20(6) Å3) 0.23(2)Mg/ 0.77(2)Tb 8b 0.375 x x 1.3(1) Co 16c 0 0 0 1.4(3) ̄ Tb2MgCo9 (Sp. gr. R3m; a ¼4.9855(3) Å; c ¼24.044(2) Å; V ¼517.55(7) Å3) Tb1 3a 0 0 0 1.5(3) 0.5 Mg/0.5Tb 6c 0 0 0.1434(3) 1.6(2) Co1 3b 0 0 1/2 0.7(4) Co2 6c 0 0 0.3345(5) 1.5(3) Co3 18h 0.5040(9)
x 0.0830(4) 0.9(2)

Fig. 3. XRD pattern of the TbMgCo4 sample. Phase composition: 1–98% TbMgCo4 þ 2–1% Mg þ3–1% Ta.

̄

Fig. 5. Comparison of the XRD patterns of the TbCo2 (sp.gr. Fd3m) and TbMgCo4 ̄ (sp.gr. F43m) compounds. Fig. 4. XRD pattern of the Tb2MgCo9 sample. Phase composition: 90% Tb2MgCo9 þ 8%Tb2Co7 þ 2% Ta.

that higher hydrogen content can be reached at higher hydrogen pressure. Tb4MgCo compound slowly absorbed hydrogen at 25 °C and 10 bar hydrogen pressure. Increasing the temperature up to 120 °C

led to a faster absorption. Tb4MgCoH12.7 hydride easily formed at low pressure and moderate temperatures, keeping the structure type of Tb4MgCo metal matrix with volume expansion by ∼24%. Hydrogenation capacity of Tb4MgCo compound was ∼2 wt% (Table 4). Formation of the Tb4MgCoH12.7 was irreversible.

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233

TbMgCo4H5.3 is accompanied by ∼20% volume expansion (see Table 4). As can be seen from the XRD pattern (Fig. 8) the hydrogenation of TbMgCo4 does not change the original cubic structure. Crystallographic parameters of the TbMgCo4H5.3 hydride are listed in Table 5. Hydrogenation behavior of the TbMgCo4 compound and crystal structure of its hydride are comparable to the isostructural CeMgCo4 and YMgCo4 compounds [20,21]. Difference in hydrogen behavior are caused by the difference of R:T (or R:Mg:T) ratio and the crystal structure of the intermetallic compounds. Both these fundamental characteristics determine the number of interstitial sites available for the insertion of hydrogen atoms which depends on three criteria: i) the size of the interstitial site (r Z0.4 Å), ii) the distance between two H atoms (dH–H Z2.1 Å) and iii) the chemical affinity to hydrogen of the H neighbors atoms. Therefore, due to the difference in R:Mg:T ratio and crystal structure of the studied Tb–Mg–Co compounds different hydrogenation properties are observed. Fig. 6. Dependence of the unit cell parameter versus the Mg content for Tb1
xMgxCo2.

In this work the hydrogen thermal desorption from Tb4MgCoH12.7 has been studied by in situ XRD analysis (heating in argon from 25 to 650 °C). The previous XRD phase analysis showed the presence of Tb4MgCoH12.7 as a main phase (95 wt%) and TbH3 as minor phase (5 wt%). In situ XRD analysis demonstrates that the Tb4MgCoH12.7 hydride is stable up to 300 °C (see Fig. 7). The traces of this hydride phase are present up to 400 °C. It means that additionally the TbH3 hydride and some amorphous phases can be formed as a result of Tb4MgCoH12.7 decomposition. We observed also the formation of TbH2 at 4 600 °C. Obviously this is the result of TbH3 decomposition [41]. These data are in good agreement with TDS spectra in vacuum [35], where the several peaks at 197, 352 and 570 °C indicated the multistep process of hydrogen desorption. The Tb2MgCo9 compound absorbed hydrogen up to 12 at.H/f.u. (1.4 wt%) at 25 °C and 25 bar H2 pressure. The crystal structure of the Tb2MgCo9 hydride could not be determined because of its decomposition (in air or even at pressure less than 10 bar H2) to the initial compound. Hydrogen storage capacity of Tb2MgCo9 (12 at.H/f.u.) is the same as that for Ce-based compound [20]. The TbMgCo4 compound absorbed hydrogen up to ∼5.3 at. H/f.u. (1.3 wt%) at 25 °C and 10 bar H2 pressure. Formation of

4. Conclusions In this work, the isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been fully investigated with the use of XRD analysis of 40 alloys, which were prepared by a powder metallurgy process. This study confirms the existence of two ternary compounds (Tb4MgCo and Tb4Mg3Co2) and installs the formation of two novel solid solutions (Tb1
xMgxCo3 (0 rx r0.4) and Tb1
xMgxCo2 (0 rx r0.6)). The study of the Tb1
xMgxCo2 solid ̄ solution indicates a structural change from cubic Fd3m space ̄ group for xr0.3 to F43m space group for x 40.3. A small amount of Co (up to 5 at%) can be dissolved in Tb5Mg24, other binary compounds do not dissolve the third component. The Tb4MgCo and TbMgCo4 ternary alloys absorb hydrogen and form stable hydrides with hydrogen content 2 and 1.3 wt%, respectively. These hydrides retain the original structure of their parent compounds with a cell volume expansion reaching 24 and 20% respectively. The TbMgCo4H5.3 hydride released hydrogen in vacuum, whereas Tb4MgCoH12.7 was stable in vacuum and disproportionated upon the heating. It was shown by in situ XRD analysis that upon thermal desorption the Tb4MgCoH12.7 hydride was stable up to 300 °C and disproportionated at higher temperature. Two other compounds, Tb4Mg3Co2 and Tb2MgCo9, absorb hydrogen at higher pressure, but their hydrides are unstable in air and decompose to the initial compounds.

Fig. 7. In situ XRD patterns (Cu-Kα) for thermal desorption of the Tb4MgCoH12.7 hydride.

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Table 4 Hydrogenation and lattice parameters of the hydrides of Tb–Mg–Co alloys. Hydride

Tsynthesis (°C)

Tb4Mg3Co2H4 Tb4MgCoH12.5 Tb4MgCoH12.7 Tb2MgCo9H12 TbMgCo4H5.3

25 25 120 25 25

Psynthesis (bar)

Lattice parameters

10 15 10 25 10

a (Å)

V (Å3)

– 14.5322(3) 14.6004(5) – 7.5212(9)

– 3068.99(9) 3112.42(18) – 425.26(9)

Fig. 8. XRD patterns of the TbMgCo4Н5.3 hydride. Phase composition: 1–92% TbMgCo4Н5.3 þ2–5% Mg þ3–3% MgO. Table 5 ̄ Crystal structure of the TbMgCo4H5.3 hydride (str. type MgCu4Sn, sp. gr. F43m). Atoms

Site

х

y

z

Biso (Å2)

Tb Mg Co

4a 4c 16e

0 1/4 0.6207(4)

0 1/4 x

0 1/4 x

2.8(2) 2.5(8) 0.4(1)

Occupancy factors are equal 1.0.

Acknowledgments Dr. Peter Y.Zavalij and Mr. Daniel D. Taylor are highly appreciated for high temperature XRD measurements at X-ray Crystallographic Center, University of Maryland, College Park, MD 20742, USA.

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CH (wt%)

ΔV/V (%)

0.53 1.7 1.8 1.4 1.3

Unstable 22.6 23.9 Unstable 20.1

ΔV/nH  Z (Å3/H atom)

2.84 2.96 3.3

Ref.

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