Martensitic Transformation of Ni<SUB>2.18</SUB>Mn<SUB>0.82</SUB>Ga Single Crystal Observed by Synchrotron Radiation White X-Ray Diffraction

Abstract

A synchrotron radiation white X-ray diffraction of a Heusler-type Ni 2 . 1 8 Mn 0 . 8 2 Ga single crystal was observed by changing the temperature from 400 to 103 K. To investigate precisely the process of the martensitic transformation, changes in Laue spots during the transformation process were observed. The following phenomena were discovered: The single crystal at 300 Kb efore increasing temperature shows a tetragonal structure, the a- or b-axis of which is parallel to the direction of crystal growth. It shows a cubic Heusler structure at 400 K after heating. One of the cubic axes is parallel to the direction of crystal growth. With decreasing temperature, the crystal transforms into many small variants with tetragonal structures. One of the variants evolves so that the a- or b-axis is parallel to the direction of the crystal growth, and the orientation of the other variants can be represented by its rotation about the cubic axis. The rotation angle is at most a few degrees, which changes gradually with decreasing temperature. At 103 K the transformation is almost complete and many tetragonal variants are rearranged into one tetragonal structure, the a- or b-axis of which is parallel to the direction of crystal growth.