Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

For the first time, the 7M modulated structure, frequently observed in ferromagnetic shape memory Ni–Mn–Ga martensitic phases, is solved by powder diffraction analysis. Two polycrystalline samples with composition Ni 2Mn 1.2Ga 0.8 and Ni 2.15Mn 0.85Ga, respectively, showing a 7M martensitic state st...

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Bibliographic Details
Published in:Acta materialia 2008-09, Vol.56 (16), p.4529-4535
Main Authors: Righi, L., Albertini, F., Villa, E., Paoluzi, A., Calestani, G., Chernenko, V., Besseghini, S., Ritter, C., Passaretti, F.
Format: Article
Language:English
Subjects:
Applied sciences
Crystal structure
Diffraction
Exact sciences and technology
Martensite
Martensitic phase
Metals. Metallurgy
Modulation
Ni 2MnGa
Nickel
Rietveld method
Shape memory
Stacking
Structural modulation
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Summary:For the first time, the 7M modulated structure, frequently observed in ferromagnetic shape memory Ni–Mn–Ga martensitic phases, is solved by powder diffraction analysis. Two polycrystalline samples with composition Ni 2Mn 1.2Ga 0.8 and Ni 2.15Mn 0.85Ga, respectively, showing a 7M martensitic state stable at room temperature, were studied. The determination of the modulated crystal structure of Ni 2Mn 1.2Ga 0.8 martensite was achieved by refining the X-ray powder diffraction pattern by the Rietveld method. The basic structure belongs to monoclinic symmetry. The crystal structure, solved within the superspace approach, is found to show an incommensurate 7M modulation with q = 0.308 c ∗. The Rietveld refinement for Ni 2.15Mn 0.85Ga martensite on the basis of neutron powder data surprisingly provides a very similar incommensurate 7M structure with the same periodicity and analogous modulation function. The incommensurate structure presents typical displacive modulation with several analogies with the Zhdanov (5, 2 ¯ ) 2 stacking sequence.
ISSN:1359-6454
1873-2453
DOI:10.1016/j.actamat.2008.05.010