Powder X-ray diffraction study of the thermoelastic martensitic transition in Ni₂Mn_1.05Ga_0.95

Abstract

Results of temperature-dependent magnetic susceptibility and powder x-ray diffraction (XRD) measurements on Ni₂Mn_1.05Ga_0.95 and Ni_2.13Mn_0.87Ga magnetic shape memory alloys are compared. The transformation behavior of these two alloys is found to be entirely different. Detailed LeBail and Rietveld analyses of powder XRD data of Ni₂Mn_1.05Ga_0.95 alloy show that the martensite phase belongs to the Pnnm space group with 7M modulation. The limits of the supercooled austenite and the superheated martensite phases have been determined by Rietveld analysis of powder XRD data recorded at close temperature intervals. It is shown that the martensite and the austenite phases coexist over ~30 K temperature range around the martensitic transition temperature. The transformation strains during cooling in [001], [010], and [100] directions are found to be -4%, +1.6%, and 2.1%, respectively, while the volume change is only 0.06%.