Phase formation and crystal structure of Sm2Fe17−yGay compounds during hydrogen disproportionation and desorption recombination (HDDR-process)

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Hydrogen disproportionation was applied to Sm2Fe17−yGay alloys with y=0, 1, 2 using a hydrogen pressure of up to 3 MPa. SmH2+x and b.c.c. (Fe,Ga) solid solution were formed. Ga is completely dissolved in the b.c.c. Fe. The phase formation was studied as a function of the recombination temperature in the range 600°C≤TR≤850°C. At first, metastable phases with the disordered hexagonal R1−sT5+2s structure (R=rare earth, T=transition metal) were formed. The atomic positions of the metastable structures were refined using the Rietveld method. In the disordered hexagonal Sm1−s(Fe,Ga)5+2s structure the Ga atom occupies randomly the atomic position (3g). The phase transformation of Sm1−sFe5+2s and Sm1−s(Fe,Ga)5+2s into the modified rhombohedral Th2Zn17 structure with partial order (denoted m-Sm2Fe17−yGay) starts at TR≥700°C and TR≥750°C, respectively. In the case of Sm2Fe15Ga2, the recombination leads to a phase mixture. A lower Ga concentration in the Th2Zn17 structure is observed after recombination compared to that in the initial state. A completely ordered Th2Zn17 structure is not observed after recombination. During recombination of SmH2+x+b.c.c. (Fe,Ga) solid solution at TR=600°C, Ga3Sm5 and GaSm were formed predominantly.