Phase relations and hydrogenation behavior of Sr(Al1−xMgx)2

doi:10.1016/S0925-8388(03)00320-7

Journal of Alloys and Compounds

Volume 360, Issues 1–2, 6 October 2003, Pages 143-150

https://doi.org/10.1016/S0925-8388(03)00320-7 Get rights and content

Abstract

The phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂ alloys were studied. The pseudobinary C36-type Laves phase Sr(Al,Mg)₂ was found as a structural intermediate between the Zintl phase and the C14 Laves phase. The single-phase regions for the Zintl phase, C36 phase and C14 phase, were determined to be x=0–0.10, 0.45–0.68 and 0.80–1, respectively. The Mg-substituted Zintl phase Sr(Al_0.95Mg_0.05)₂ can be hydrogenated to Sr(Al,Mg)₂H₂ at about 473 K. However, the Sr(Al,Mg)₂H₂ directly decomposes into SrH₂ and Sr(Al,Mg)₄ starting at 513 K. When the temperature is 573 K, the C36 Laves phase Sr(Al_0.5Mg_0.5)₂ can be hydrogenated into SrMgH₄ and Al, while the C14 Laves phase Sr(Al_0.1Mg_0.9)₂ is hydrogenated into SrMgH₄, Mg₁₇Al₁₂ and Mg.

Introduction

The hydrogenation properties of the Zintl compound SrAl₂ have been studied for the purpose of hydrogen storage applications [1], [2], [3], [4], [5]. It was ascertained that the hydrogenation process proceeds in three steps [1], [2]. SrAl₂ can be first hydrogenated to SrAl₂H₂ at about 463 K. Increasing the hydrogenation temperature to 513 K, SrAl₂H₂ further absorbs hydrogen to form Sr₂AlH₇ and Al. Sr₂AlH₇ decomposes to SrH₂, Al and H₂ when the temperature is raised to about 563 K.

Although Ca and Ba belong to the alkaline earth elements like Sr, the maximum solubility of Ca or Ba in the Zintl compound SrAl₂ is less than 0.11. In the Sr_1−xCa_xAl₂ alloys, the single Zintl phase only exists in the composition range from x=0 to 0.108 [3]. Similarly, the single Zintl phase appears in the Sr_1−xBa_xAl₂ alloys when x is smaller than 0.082 [4]. The Ca- or Ba-substituted SrAl₂ can also be hydrogenated to (Sr,M)₂AlH₇ (M=Ca or Ba) and Al [3], [4].

Mg is a lightweight alkaline earth metal. Mg-based alloys are considered as promising candidates for hydrogen storage materials [6]. Thus our investigations were started with the purpose to study the effect of partially substituting Sr by Mg on crystal structure and hydrogenation properties of (Sr,Mg)Al₂. It was surprising that Mg atoms always occupied the Al site in the magnesium-substituted Zintl phase. The binary SrMg₂ is a C14 Laves phase with an atomic radius ratio of 1.34 [7]. Therefore, the phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂ were finally studied.

Section snippets

Experimental details

The (Sr_1−xMg_x)Al₂ (x=0.05, 0.1, 0.2 and 0.3) and Sr(Al_1−xMg_x)₂ (x=0.05, 0.3, 0.5, 0.6, 0.7, 0.85 and 0.9) alloys were prepared by induction melting of Sr (Furuuchi Chemical, 99.9%), Al (Furuuchi Chemical, 99.99%) and Mg (Furuuchi Chemical, 99.99%) metals. Before the preparation, the loss of Mg during induction melting was determined to be about 10 wt.%. On the basis of stoichiometric amounts of starting materials, thus, an extra 10 wt.% of Mg was added to compensate the loss of Mg during

Phase relations of (Sr_1−xMg_x)Al₂

Fig. 1 shows the XRD patterns for the (Sr_1−xMg_x)Al₂ (x=0.05, 0.1, 0.2 and 0.3) alloys. It can be seen that the patterns are indexed to SrAl₂ and SrAl₄, and the relative intensities of the peaks from SrAl₄ increase with x. This means that the substitution of Mg for Sr leads to the formation of SrAl₄. However, the atomic ratios of (Sr+Mg)/Al in the alloys were kept at 1:2. Thus, this result aroused our suspicion that the Mg atoms do not occupy Sr sites but Al sites in the SrAl₂ and SrAl₄ phases.

Conclusions

The phase relations and hydrogenation characteristics of Sr(Al_1−xMg_x)₂ alloys were studied. The major conclusions are summarized in the following paragraphs.

With the increase of x, the Sr(Al_1−xMg_x)₂ alloys gradually transform from the Zintl phase to the C36 Laves phase, finally to the C14 Laves phase. The single-phase regions for the Zintl phase, C36 phase and C14 phase are x=0–0.10, 0.45–0.68 and 0.80–1, respectively.

The C36 phase Sr(Al,Mg)₂ is a first example of the structural intermediate

Acknowledgements

This work was carried out as a part of the ‘WE-NET’ project. Q.A. Zhang would like to thank the New Energy and Industrial Technology Development Organization (NEDO) of Japan for providing the NEDO fellowship. The authors thank Dr H. Enoki of the National Institute of Advanced Industrial Science and Technology of Japan for technical assistance.

References (20)

F. Gingl et al.
J. Alloys Comp.
(2000)
Q.A. Zhang et al.
J. Alloys Comp.
(2001)
Q.A. Zhang et al.
J. Alloys Comp.
(2002)
D.J. Thoma et al.
J. Alloys Comp.
(1995)
F. Gingl et al.
J. Alloys Comp.
(1994)
F. Gingl et al.
J. Alloys Comp.
(1992)
N.E. Brese et al.
J. Solid State Chem.
(1990)
Q.A. Zhang, H. Enoki, Y. Nakamura, E. Akiba, in: Proceedings of the 131th JIM Fall Meeting, Osaka, Japan, 2002, p....
Q.A. Zhang et al.
Inorg. Chem.
(2002)
S. Orimo et al.
Appl. Phys. A
(2001)

There are more references available in the full text version of this article.

Cited by (15)

Study of the physical aspects of Pt<inf>3</inf>T (T = Mn, Ni) intermetallic compounds using first principles method
2024, Physica B: Condensed Matter
Platinum based transitional intermetallic compounds creates significant interest in the research area because of having their promising physical, optical, and electronic properties with the large scale of thermodynamic applications. The main purpose of this study is to examine the several physical properties of intermetallic compounds Pt₃T (T = Mn, Ni) explored through density functional theory (DFT). The reliability of this work was made by comparing the investigated mechanical features with previously studied compounds of similar types. The calculated structural parameters have a similarity with experimental values. For the first time, we have calculated as well as presented the elastic constants, Cauchy pressure, bulk, shear, and Young's moduli. The mechanical stability of these compounds is proven by the calculation of elastic constants which meets necessary stability requirements. Metallic and ductile nature of Pt₃Mn and Pt₃Ni is represented by the Cauchy pressure and Pugh's ratio respectively. High machinability index of Pt₃Ni ensured its low friction and huge application in industry. Band structure and DOS calculations also represented the metallic behavior of these compounds. High reflectivity in UV region refers the high conductance characteristics of Pt₃T (T = Mn, Ni) in low energy regions. Very high absorption in the UV region is observed here. In the infrared region high refractive index is noticed. The dielectric constant's ε₁(ω) large negative value in the real part demonstrates the Drude-like behaviors which are the metallic system's common characteristics. Very high melting temperature of Pt₃T (T = Mn, Ni) ensured their probable applications in the high temperature technology. TBC nature of these phases has been ensured from their very low thermal conductivity values.
First-principles study on the phase transition, elastic properties and electronic structure of Pt<inf>3</inf>Al alloys under high pressure
2014, Journal of Alloys and Compounds
Citation Excerpt :
The Al and Pt atoms are located at the site (0, 0, 0) and (0.5, 0.5, 0), respectively. Each Al atom is surrounded by twelve Pt atoms [26]. The other phase is the tetragonal structure (space group: P4/mmm, No: 123) with experimental lattice parameters: a = b = 3.832 Å and c = 3.894 Å.
The phase transition, formation enthalpies, elastic properties and electronic structure of Pt₃Al alloys are studied using first-principle approach. The calculated results show that the pressure leads to phase transition from tetragonal structure to cubic structure at 60 GPa. With increasing pressure, the elastic constants, bulk modulus and shear modulus of these Pt₃Al alloys increase linearly and the bond lengths of Pt–Al metallic bonds and the peak at E_F decrease. The cubic Pt₃Al alloy has excellent resistance to volume deformation under high pressure. We suggest that the phase transition is derived from the hybridization between Pt and Al atoms for cubic structure is stronger than that of tetragonal structure and forms the strong Pt–Al metallic bonds under high pressure.
Hydrogen storage properties of the Zintl phase alloy SrAl<inf>2</inf> doped with TiF<inf>3</inf>
2010, Journal of Alloys and Compounds
In this paper, the structural and hydrogenation characteristics of TiF₃-doped Zintl phase alloy SrAl₂ were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and hydrogenation measurements. The results show that the hydrogenation kinetics of the Zintl phase alloy SrAl₂ is improved greatly after doping with TiF₃. By adjusting the doping amount and ball milling time, the optimal doping conditions were obtained. The catalytic mechanism of TiF₃ for the hydrogenation of SrAl₂ was also investigated. SrAl₂ does not react with TiF₃ during the ball milling process. However, it reacts with TiF₃ to form SrAl₂H₂, SrF₂, SrAl₄ and Ti during the hydrogenation process, among which Ti plays an important role in the hydrogenation kinetics of SrAl₂.
Effect of Si substitution for Al on the structural and hydrogenation properties of the Zintl phase alloy SrAl<inf>2</inf>
2009, Journal of Alloys and Compounds
In this paper, Al was partially substituted by Si in the Zintl phase alloy SrAl₂ and the structural and hydrogenation properties of the SrAl_2−xSi_x (x = 0.2, 0.4, 0.6, 0.8 and 1.0) alloys were studied by X-ray diffraction (XRD), optical microscopy (OM) and hydrogenation measurements. With the substitution of Al by Si, SrAlSi phase appears in the alloy and the SrAl_2−xSi_x (x = 0.2, 0.4, and 0.6) alloys are all composed of SrAl₂ and SrAlSi phases. The phase abundance of SrAl₂ and SrAlSi is decreased and increased, respectively, when x increases from 0.2 to 0.6. However, the alloy consists of a single SrAlSi phase when x = 0.8 and 1.0. Hydrogenation measurements were made at 573 K under 3 MPa hydrogen pressure. The hydrogen absorption capacity is decreased with increasing the Si substitution for Al, which can be attributed to the smaller hydrogen absorption capacity and slower hydrogenation rate of SrAlSi phase than SrAl₂ phase.
Effect of Al content on the phase structure and the hydrogenation characteristic of La(Mg<inf>1-x</inf>Al<inf>x</inf>) alloys
2006, Rare Metals
La(Mg_1-xAl_x) (x = 0.2, 0.4, 0.6, 0.8) alloys have been prepared using induction melting followed by annealing. It is found that partial substitution of Mg by Al does not lead to a change in crystal structure, and the alloys have a single LaMg phase when x ≤ 0.4. The lattice parameter of the LaMg phase decreases obviously after the partial substitution of Mg by Al. However, further substitution of Mg by Al leads to the coexistence of multiple phases when x ≥ 0.6. The alloys consist of the LaMg, LaAl, LaAl₂, and La₅Al₄ phases. The LaMg phase decreases, whereas the La₅Al₄ phase increases with the increase in x. The Al-substituted La(Mg_0.6Al_0.4) alloy can be hydrogenated into the tetragonal LaH₃, cubic LaH₃, MgH₂, and LaAl under 5 MPa at 473 K for 5 d.
Structure and stability of Laves phases part II - Structure type variations in binary and ternary systems
2005, Intermetallics
In Part I, various models for the prediction of the occurrence and stability of Laves phases have been discussed. In the present Part II paper, an overview is given on the various types of manifestation of Laves phases in intermetallic alloy systems with emphasis on transition metal systems. Temperature- and composition-dependent as well as stress-induced phase transformations between the cubic C15 and the hexagonal C14 and C36 structural polytypes of Laves phases were observed in various binary and ternary systems. The phase fields of the different polytypes are generally separated by small two-phase fields. Some general rules for the occurrence of the different Laves phase polytypes are derived from a study of the results of experimental phase diagram investigations of various binary and ternary systems. The solubilities and site occupancies of ternary additions in the different Laves phase polytypes and the mutual miscibility of different binary Laves phases are discussed. The need for careful experimental investigations of phase equilibria is demonstrated. Existing models discussed in the preceding Part I paper are shown to fail to predict the structure and stability of the presented Laves phases in many cases.

View all citing articles on Scopus

View full text

Phase relations and hydrogenation behavior of Sr(Al1−xMgx)2

Abstract

Introduction

Section snippets

Experimental details

Phase relations of (Sr1−xMgx)Al2

Conclusions

Acknowledgements

J. Alloys Comp.

J. Alloys Comp.

J. Alloys Comp.

J. Alloys Comp.

J. Alloys Comp.

J. Alloys Comp.

J. Solid State Chem.

Inorg. Chem.

Appl. Phys. A

Phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂

Phase relations of (Sr_1−xMg_x)Al₂