Role of local structural distortions in the variation of martensitic transformation temperature with <i>e</i>/<i>a</i> ratio in Ni₂Mn_1+<i>x</i>Z_1−<i>x</i> (Z = In, Sn or Sb) alloys

Ni₂Mn_1+<i>x</i>Z_1-<i>x</i> (Z = In, Sn or Sb) undergo martensitic transformation with transformation temperature (<i>T</i>_M) scaling with the average valence electron per atom (<i>e</i>/<i>a</i>) ratio. However, the rate of increase of <i>T</i>_M depends on the type of Z atom, with the slope of <i>T</i>_M<i>vs. e</i>/<i>a</i> curve increasing from Z = In to Z = Sb. Local structural distortions are believed to be the leading cause of martensitic transformation in these alloys. A careful study of the Ni and Mn local structures in several Ni₂Mn_1+<i>x</i>Z_1-<i>x</i> alloys with varying <i>e</i>/<i>a</i> ratio and the same Z atom, with the same <i>e</i>/<i>a</i> ratio but different Z atoms and with the same <i>T</i>_M but with different Z atoms and different <i>e</i>/<i>a</i> ratio, revealed that the difference between Ni-Mn and Ni-Z nearest neighbor distances decreases as the Z atom changes from In to Sb. This decrease in the local structural distortion accommodates a higher content of Mn until the <i>L</i>2₁ structure becomes unstable and the alloy undergoes a martensitic transformation.