High Pressure Synthesis of Novel Compounds in Mg-TM Systems (TM = Ti Zn)* 1

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1 Materials Transactions, Vol. 45, No. 4 (2004) pp to 1354 #2004 The Japan Institute of Metals High Pressure Synthesis of Novel Compounds in Mg-TM Systems (TM = Ti Zn)* 1 Hiroaki Watanabe* 2, Yasuyuki Goto* 2, Hirofumi Kakuta, Atsunori Kamegawa, Hitoshi Takamura and Masuo Okada Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai , Japan High pressure synthesis under 6 GPa using a cubic-anvil-type apparatus were applied to investigation of new compounds in Mg-TM systems. The crystal structure, thermal stability and reactivity with hydrogen for the newly synthesized compounds were studied. The Mg 4 Ni compound belonging to space group F43m with a lattice parameter of a ¼ 1:9987ð1Þ nm was synthesized as a new compound at 1173 K for 8 h under 6 GPa. It decomposed to Mg and Mg 2 Ni phase at 545 K as exothermic reaction. In Mg-Cu systems, two unreported phases were synthesized at 1073 K for 8 h under 6 GPa. The one is the MgCu compound which has CsCl-type structure with a lattice parameter of a ¼ 0:31616ð7Þ nm and the other is Mg 51 Cu 20 belonging to space group Immm with a lattice parameter of a ¼ 1:3929ð7Þ nm, b ¼ 1:428ð1Þ nm and c ¼ 1:392ð1Þ nm. The MgCu compound decomposed to Mg 2 Cu and MgCu 2 phases at 500 K and the Mg 51 Cu 20 compound did to Mg and Mg 2 Cu phase at 430 K. (Received December 1, 2003; Accepted February 16, 2004) Keywords: hydrogen storage alloy, magnesium, high pressure 1. Introduction Hydrogen storage materials have been paid much attentions for the use in hydrogen energy systems. Various studies based on the metallurgical process such as melting, mechanical alloying process, sintering method and so on have been done for the development of Mg-based alloys as hydrogen storage medium so far. The high-pressure synthesis method is one of the promising techniques to explore new compounds. The high-pressure up to GPa orders has been employed using a cubic-anvil-type apparatus. A large number of new hydrides, i.e. Mg 3 MnH 7 1) and Mg 2 Ni 3 H 3:4 2) and high pressure phase such as MgNi 2 3) with new type structure which is different from normal pressure phase have been synthesized by using this method, which indicates that it is effective to examine this method as a means of searching for a new compound. In addition, this method has various advantages, such as possible proceeding of solid phase reaction above the melting point of Mg since a melting point of Mg will be raised under high pressure and no compositional change of the sample is applicable because the sample is synthesized in completely closed environment. Thus, the purpose of this work is to explore the new compounds in magnesium-transitional metals systems using a cubic-anvil-type apparatus, where Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn were selected as the transition-metals because these elements were relatively light weight in transitionmetals. The crystal structure, the reactivity with hydrogen and thermal stability for the newly-found compounds were also studied. 2. Experimental Procedures Raw materials such as Mg (purity 99.9 mass%), Ti (purity 99 mass%), V (purity 99.5 mass%), Cr (purity 99.9 mass%), Mn (purity 99.9 mass%), Fe (purity 99 mass%), Co (purity 99 mass%), Ni (purity 99.8 mass%), Cu (purity 99.9 mass%), Zn (purity 99.9 mass%) powders are mixed with a molar ratio of Mg:TM = 1:1. They were pressed into pellets in a glove box filled with pure Ar. Especially, in the Mg-Ni and Mg-Cu systems, the Mg-20 at%ni and Mg-20 at%cu samples were also prepared since the Mg 2 Ni and Mg 2 Cu phases exist in Mg-rich composition in these systems, respectively. The pellets of samples were put into BN containers. Then, the container was set into a graphite tube heater, and placed into a pressure media made from pyrophyllite shown in Fig. 1. The high-pressure synthesis was carried out in a cubicanvil-type apparatus. Samples were prepared at K SUS ring Graphite heater BN Mo disk sample * 1 This Paper was Presented at the Autumn Meeting of the Japan Institute of Metals Held in Hokkaido, on October 12, 2003 * 2 Graduate Student, Tohoku University Fig mm Pyrophyllite Schematic illustration of details of the cell used in this study.

2 High Pressure Synthesis of Novel Compounds in Mg-TM Systems (TM = Ti Zn) 1351 for 8 h under 6 GPa. The pressure was increased up to 6 GPa about 1 h. By making pass electric current through a graphite tube heater, a temperature of samples was controlled. After soaking samples for 8 h at a target temperature under 6 GPa, the electric current was turned off. Since it took a few seconds for the sample to return to room temperature from the setting temperature, samples can be regarded as quenched. After that, samples were taken out in a glove box. Phase identification was carried out by X-ray diffraction (XRD) using a Cu-K radiation. Thermal stability of samples was investigated using a differential scanning calorimeter (DSC) under an Ar flow. As for the reactivity of the sample with hydrogen, hydrogenation was performed at K for 3 days under hydrogen pressure of 9 MPa. 3. Results and Discussion Figure 2 shows X-ray diffraction patterns of Mg-50%TM (TM = Ti Zn) samples prepared at K for 8 h under 6 GPa. Synthetic temperatures are also shown in the Fig. 2. Under the above conditions, only starting elements or reported phases were observed for the composition ratio 1:1 in Mg-Ti, V, Cr, Mn, Fe, Co, Ni, Zn systems. On the other hand, a phase which didn t exist in binary phase diagrams 4) was found at the composition of Mg-50 at%cu. 6GPa, T K, 8h Mg-Ti<1:1>(T=1223) Mg Ti Intensity (a.u.) 6GPa, TK, 8h Mg-Ni<4:1>(T=1173) Figure 3 shows X-ray diffraction patterns of Mg-20 at%ni, Cu samples prepared at K for 8 h under 6 GPa. Synthetic temperature is shown in the Fig. 3, together with each composition. In this case, phases which didn t exist in binary phase diagrams 4,5) were found at the composition of Mg-20 at%ni and Mg-20 at%cu. There is a possibility that the phases belonged to a newlysynthesized compound. In the following, the newly appeared phases were further studied. 3.1 Mg-Ni systems Figure 4 shows X-ray diffraction patterns of Mg-xat%Ni (x ¼ 10, 15, 20, 25 and 33) samples prepared at 1173 K for 8 h under 6 GPa. The sample of Mg-20 at%ni only showed an Mg-Cu<4:1>(T=1073) Mg Fig. 3 X-ray diffraction patterns of Mg-20 at%ni, Cu samples prepared at K for 8 h under 6 GPa. Mg-V<1:1>(T=1223) Mg V Mg-Cr<1:1>(T=1223) Mg Cr Mg - Xat% Ni 6 GPa, 1173 K, 8 h Mg Mg 2 Ni X=33 Intensity (a.u.) Mg-Mn<1:1>(T=1223) Mg Mn Mg-Fe<1:1>(T=1223) Mg Fe Mg-Co<1:1>(T=1223) Mg 2 Co MgCo 2 X=25 X=20 Mg-Ni<1:1>(T=1173) Mg 2 Ni MgNi 2 3) Mg-Cu<1:1>(T=1073) X=15 Mg-Zn<1:1>(T=673) Mg MgZn 2 Mg 51 Zn 20 X=10 Fig. 2 X-ray diffraction patterns of Mg-50 at% (TM = Ti Zn) samples prepared at 1223 K for 8 h under 6 GPa. Fig. 4 X-ray diffraction patterns of Mg-xat%Ni (x ¼ 10, 15, 20, 25 and 33) samples prepared at 1173 K for 8 h under 6 GPa.

3 1352 H. Watanabe et al Mg 4 Ni Simulation Fig. 5 X-ray diffraction pattern of Mg 4 Ni phase and simulational result of X-ray diffraction. phase which did not exist in binary phase diagrams, but the sample with other composition consists of the other known phases, Mg or Mg 2 Ni. It seems that the novel compound might be expressed as a composition formula of Mg 4 Ni. In addition, this sample was stable in air. The sample of Mg 4 Ni was further studied by the X-ray diffraction. We found that the appeared X-ray diffracted peaks were so similar to that of Mg 6 Pd 6) compound. Each peak was indexed as the equivalent ones to the space group of F 43m of Mg 6 Pd compound with the lattice parameter of a ¼ 1:9987ð1Þ nm. The simulation of X-ray diffraction pattern was performed using RIETAN ) as shown in Fig. 5, assuming that the compound has the same structure of Mg 6 Pd with the lattice parameter of a ¼ 1:9987ð1Þ nm. Figure 5 shows comparison of X-ray diffraction pattern of the compound with Mg-20 at%ni with the simulated pattern assuming atomic coordination of Mg 6 Pd compound. It is found that the X-ray diffraction pattern of this compound almost corresponds to the result of this simulation. Therefore, it will be reasonable to judge that the crystal structure of this compound is Mg 6 Pdy-type structure, where the Mg sites might be randomly occupied by Mg and Ni atoms. Figure 6 shows the DSC curve of Mg 4 Ni sample. The exothermic reaction was observed at 545 K. The X-ray diffraction analysis showed that Mg 4 Ni compound decomposed to Mg and Mg 2 Ni phase with increasing temperature. No hydrogenation of Mg 4 Ni compound was observed under the condition of keeping at 423 K for 3 days under a hydrogen pressure of 9 MPa. Heat Flow, Q / W. g -1 Mg 4 Ni 350 Fig Temperature, T / K K (5 K / min) 0.05 MPa Ar flow 545 K DSC curve of Mg 4 Ni sample Mg-Cu systems Figure 7 shows X-ray diffraction patterns of Mg-xat%Cu (x ¼ 45, 50 and 55) samples prepared at 1073 K for 8 h under 6 GPa. The Mg 2 Cu and MgCu 2 phases were found in Mg-rich and in Cu-rich composition, respectively, in addition to the new phase of 1:1 composition. The single phase of newlyfound compound was expected to be expressed as compositional formula of MgCu. In addition, this sample was stable in air. Judging from the X-ray diffraction pattern, the crystal structure of MgCu compound is a CsCl-type structure and its diffracted patterns were well indexed as shown in Fig. 8 with a lattice parameter of a ¼ 0:31616ð7Þ nm. Figure 8 shows X-ray diffraction patterns of Mg-xat%Cu (x ¼ 20, 25, 28, 30 and 33) samples prepared at 1073 K for 8 h under 6 GPa. The sample of Mg-28 at%cu showed an phase like a single phase. The sample with richer or poorer Cu content corresponded to Mg 2 Cu or Mg, respectively. The sample of Mg-28 at%cu was further studied by the X- ray diffraction. The appeared X-ray diffracted peaks are so similar to those of Mg 51 Zn 20 8) compound. Each peak was indexed as the equivalent one to the space group of Immm of Mg 51 Zn 20 phase with the lattice parameter of a ¼ 1:3929ð7Þ nm, b ¼ 1:428ð1Þ nm and c ¼ 1:392ð1Þ nm. The simulation of X-ray diffraction pattern was performed using RIETAN- 2000, assuming that the phase has the same structure of

4 High Pressure Synthesis of Novel Compounds in Mg-TM Systems (TM = Ti Zn) 1353 Mg - Xat% Cu 6 GPa, 1073 K, 8 h MgCu 2 Mg 2 Cu Mg - Xat% Cu 6 GPa, 1073 K, 8 h Mg Mg 2 Cu X=33 X= X= X=30 X=28 X=25 X=20 Fig. 7 X-ray diffraction patterns of Mg-xat%Cu (x ¼ 45, 50 and 55) samples prepared at 1073 K for 8 h under 6 GPa. Fig. 8 X-ray diffraction patterns of Mg-xat%Cu (x ¼ 20, 25, 28, 30 and 33) samples prepared at 1073 K for 8 h under 6 GPa Mg 51 Cu Simulation Fig. 9 X-ray diffraction pattern of the phase which was nearly single phase in composition ratio of Mg-28 at%cu and simulational result of X-ray diffraction. Mg 51 Zn 20 with the lattice parameter of a ¼ 1:3929ð7Þ nm, b ¼ 1:428ð1Þ nm and c ¼ 1:392ð1Þ nm. Figure 9 shows comparison of X-ray diffraction pattern of the phase with Mg-28 at%cu with the simulated pattern assuming atomic coordination of Mg 51 Zn 20 compound as well. The X-ray diffraction pattern of this phase almost corresponds to the result of this simulation. Therefore, it is reasonable to say that this phase will be expressed as composition formula Mg 51 Cu 20. Figure 10 shows the DSC curves of newly synthesized MgCu and Mg 51 Cu 20 samples. For the thermal stability of these samples, MgCu and Mg 51 Cu 20 phase decomposed to Mg 2 Cu and MgCu 2 phase at 500 K and to Mg and Mg 2 Cu phase at 430 K with increasing temperature, respectively, both of which are exothermic. No hydrogenation of MgCu and Mg 51 Cu 20 phases was observed under the condition of keeping at 423 K and 403 K for 3 days under 9 MPa of hydrogen pressure, respectively. As mentioned above, three new compounds were synthesized in this study, but none of them was hydrogenated. The decomposition temperatures of the presently-synthesized

5 1354 H. Watanabe et al. Heat Flow, Q / W. g -1 MgCu Mg 51 Cu Fig K Temperature, T / K K (5 K / min) 0.05 MPa Ar flow 500 K 550 DSC curves of MgCu and Mg 51 Cu 20 samples :9987ð1Þ nm was synthesized as a new compound at 1173 K for 8 h under 6 GPa, which decomposed to the Mg and Mg 2 Ni phases at 545 K as exothermic reaction. In Mg-Cu system, two new compounds such as the MgCu compound which has CsCl-type structure with a lattice parameter of a ¼ 0:31616ð7Þ nm and Mg 51 Cu 20 compound belonging to space group Immm with a lattice parameter of a ¼ 1:3929ð7Þ nm, b ¼ 1:428ð1Þ nm and c ¼ 1:392ð1Þ nm were synthesized at 1073 K for 8 h under 6 GPa. The MgCu compound decomposed to the Mg 2 Cu and MgCu 2 phases at 500 K and Mg 51 Cu 20 compound did to the Mg and Mg 2 Cu phases at 430 K, respectively. For the reactivity with hydrogen of these new compounds, none of them was hydrogenated in this study. Mg 4 Ni, MgCu and Mg 51 Cu 20 compounds are 545 K, 500 K and 430 K, respectively. Therefore, it seems likely that optimum conditions for hydrogenation will be in temperatures below their decomposition temperatures. The various hydrogenation conditions were already tried, but it was observed that the some decomposition reaction of the compound started below the exothermic temperatures measured by DSC. Then the present conditions of hydrogenation were adopted, but none of them were succeeded. But further detailed studies will be required to find the hydrogenation conditions since Mg 51 Zn 20 9) compound which is the same structure of Mg 51 Cu 20 compound is reported to react with hydrogen at 523 K. 4. Conclusion The novel compounds were synthesized in Mg-Ni and Mg- Cu systems by high-pressure technique using a cubic-anviltype apparatus. In Mg-Ni system, Mg 4 Ni compound belonging to space group F 43m with a lattice parameter of a ¼ Acknowledgement This work has been supported in part by New Energy and Industrial Technology Development Organization (NEDO). REFERENCES 1) M. Bortz, B. Bertheville, K. Yvon, E. A. Movlaev, V. N. Verbetsky and F. Fauth: J. Alloys Compd. 279 (1998) L8-L10. 2) H. Takamura, H. Kakuta, A. Kamegawa and M. Okada: J. Alloys Compd (2002) ) Kakuta, H., Ph. D. thesis, Tohoku University, 2000 (in Japanese) 4) T. B. Massalski: Binary Alloy Phase Diagrams. vol. 2 (1990) ) T. B. Massalski: Binary Alloy Phase Diagrams. vol. 3 (1990) ) Samson, S: Acta Cryst. B28 (1972) ) F. Izumi and T. Ikeda: Mater. Sci. Forum, (2000) ) I. Higashi, N. Shiotani, M. Uda, T. Mizoguchi and H. Katoh: J. Solid State Chem. 36 (1981) ) G. Bruzzone, G. Costa, M. Ferretti and G. L. Olcese: Int. J. Hydrogen Energy 8 (1983) 459.