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

✍ Scribed by L. Righi; F. Albertini; E. Villa; A. Paoluzi; G. Calestani; V. Chernenko; S. Besseghini; C. Ritter; F. Passaretti

• Publisher: Elsevier Science
• Year: 2008
• Tongue: English
• Weight: 540 KB
• Volume: 56
• Category: Article
• ISSN: 1359-6454
• DOI: 10.1016/j.actamat.2008.05.010

✦ Synopsis

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 2 Mn 1.2 Ga 0.8 and Ni 2.15 Mn 0.85 Ga, respectively, showing a 7M martensitic state stable at room temperature, were studied. The determination of the modulated crystal structure of Ni 2 Mn 1.2 Ga 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 = 0.308c * . The Rietveld refinement for Ni 2.15 Mn 0.85 Ga 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.