High-pressure synthesis of a new compound of the Mg–Co system

https://doi.org/10.1016/j.ijhydene.2020.04.020Get rights and content

Highlights

  • New compound of Mg–Co system as Mg44Co7 was explored via high pressure synthesis.

  • The crystal structural parameters of the new compound were refined by Rietveld method.

  • The hydrogenation temperature of the compound affected H-content and Tdes.

  • The maximum hydrogen content of hydrogenated Mg44Co7 was 5.0 mass%.

Abstract

High-pressure synthesis is an effective technique to obtain novel compounds. Under high pressure of the order of GPa, the atomic radius of Mg undergoes large shrinkage compared with that of transition metals (TM) and novel compounds with unusual atomic sizes of Mg and TMs can be synthesized. This study describes the high-pressure synthesis of a novel Mg–Co alloy of formula Mg44Co7 at above 5 GPa and 973 K for 8 h. Investigation of the crystal structure of Mg44Co7 using X-ray powder diffraction reveals its Mg44Rh7-type structure (space group F-43 m) with lattice parameter a = 20.127 (1) Å. Mg44Co7 decomposes into Mg and MgCo phases at 663 K in an Ar atmosphere via an exothermic reaction. Hydrogenation of Mg44Co7 at 573 K–623 K under 8 MPa of H2 results in its decomposition into MgH2, Mg2CoH5 and Mg6Co2H11 with a total hydrogen content of 3.5–5.0 mass%.

Introduction

The investigation of the effect of a high pressure, of the order of GPa, on the properties of metals has given access to novel compounds that are unstable at ambient pressure. Generally, GPa pressures reduce the atomic volumes of the elements and change their physical and chemical properties, which has prompted the study of the high-pressure synthesis of many compounds in various fields.

In particular, high-pressure techniques are expected to afford novel alloys and compounds of Mg, because the other element in the alloy would change its crystal structure in order to match the higher atomic compressibility of Mg. Mg-based alloys and compounds have attracted attention because they are lightweight and have a high gravimetric hydrogen capacity, which has resulted in many reports dealing with the high-pressure synthesis of Mg-based compounds for use as new hydrogen-storage alloys, such as MgCu [1], Mg51Cu20 [1], the high-pressure phase of MgNi2 [2], MgNi [3], and Mg6Ni [4]. MgNi2 which does not react with H2 at ambient pressure was synthesized as MgNi2H3.2 under high-pressure conditions in the presence of an additional H source [2]. Notably, a MgNi compound synthesized at 673 K and 5 GPa, was later reported to exhibit reversible hydrogen reactivity with a hydrogen content of 1.89 mass% H [3].

Hydrides of Mg and transition metal (TM), characterized by the general formula Mg2TMHy (TM = Fe, Co, and Ni), are popular because of their high theoretical hydrogen-storage capacities [[5], [6], [7], [8], [9]]. Regarding to Mg–Co hydrides, the hydrogen contents of Mg2CoH5 and Mg6Co2H11 are 4.5 and 4 mass%, respectively [10,11]; moreover, Mg2Co and Mg3Co can be considered as the respective starting materials employed for their syntheses. To date, many studies have focused on the synthesis of Mg1+yCo (y = 0 or 1) compounds [[12], [13], [14], [15], [16], [17], [18], [19]] using mechanical alloying processes to elucidate the hydrogenation characteristics of Mg2CoH5 and Mg6Co2H11. Moreover, the BCC structure of Mg–Co alloy prepared via mechanical alloying [[20], [21], [22], [23], [24]] has been reported, and an evaluation of its hydrogenation properties indicates that the BCC Mg–Co alloy can absorb hydrogen at −15 °C with 2.67 mass% for Mg50Co50 [21,22] and 3.24 mass% for Mg55Co45 [23,24].

Theoretical studies by Trivedi and Bandyopadhyay on the hydrogen-storage behavior of the MgnCo (n = 1–10) series revealed Mg4Co and Mg6Co to be stable and that Mg5Co to be an effective agent of hydrogen-storage [25]. Therefore, it seems reasonable to assume that these Mg–Co compounds can be synthesized under high-pressure conditions and used as hydrogen-storage materials.

The purpose of the present study is to synthesize novel Mg-rich Mg–Co compounds by using a high-pressure (GPa-order pressure) cubic anvil-type apparatus. The crystal structure and thermal stability of the new compounds were investigated, along with the hydrogen content of their Mg–Co hydrides.

Section snippets

Experimental procedure

MgH2 powder (98 mass%) and Co powder (99 mass%, 5 μm) were used as raw materials. MgH2 powder was dehydrogenated at 723 K for 2 h in an autoclave with argon gas to obtain fine Mg powder. The fine Mg and Co powders were mixed and pressed into pellets by using a hydraulic press machine at 20 MPa for 2 min, and then assembled into a cubic high-pressure sample cell in an Ar gas-filled glove box. The cubic high-pressure sample cell consisted of a BN container acting as the sample insulator, a carbon

Results and discussion

To synthesize a novel Mg-rich Mg–Co compound, various compositions of Mg–Co mixture were investigated. Fig. 1 shows the XRD pattern for Mg–x at% Co (x = 10, 15, 16.67, 20, 25, 33, and 50) samples prepared at 973 K and 6 GPa for 8 h. The Mg–50 at% Co sample showed a single phase, which was assumed to consist of Mg1+yCo (y = 0 or 1) because the same XRD pattern was previously observed for MgCo and Mg2Co [12,14,[17], [18], [19],27]. In this study, Mg1+yCo was assigned to be MgCo based on the

Conclusion

A novel compound of the Mg–Co system was synthesized under a high pressure. At 973 K and above 5 GPa of pressure for 8 h, Mg44Co7 was obtained from the dehydrogenation of MgH2 and Co as raw materials using a cubic anvil-type apparatus. The new compound Mg44Co7 exhibited Mg44Rh7-type structure (space group F-43 m, No.216) with a lattice parameter of a = 20.127 (1) Å. It decomposed into Mg and MgCo phases at 663 K through an exothermic reaction. Upon hydrogenation at 573 K–623 K under 8 MPa of H2

Acknowledgments

This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP19K04485.

References (35)

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