Martensitic phase transformation and electronic structure of NiTi and PdTi

Total-energy calculations of B2-, B19'- and L1(0)-NiTi and B2-, B19- and L1(0)-PdTi were performed by means of the full potential linear augmented plane wave (FLAPW) method. In agreement with experiment, the martensitic B19 and B19' phases, respectively, were found to be most stable at 0 K, whereas the L1(0) phases are unstable for both systems. B2-PdTi was shown to be unstable against both, tetragonal and orthorhombic distortions, leading to either L1(0)- or B19-PdTi. According to our calculation, B2-NiTi is stable against both kinds of distortion. Temperature effects were included by means of a Sommerfeld model for the electronic and a modified Debye model for the lattice contribution to the entropy and specific heat. Elastic constants, specific heat capacities, formation and transformation energies are calculated for the B2 phases and found to agree well with experiment, if available. The calculated shifts of the peak positions of the electronic susceptibility of B2-NiTi and B2-PdTi in the [110] direction with the valence electron concentration explain the strong dependence of the actual mechanism of the martensitic phase transformation on the composition and stoichiometry of the samples.