New structure and insight on the phase transition within the Cu-Pd-Sn system with 25 at. % Sn

The Cu-Pd-Sn material system has a (Cu, Pd) 3 Sn phase region that shows an interesting and unexplored marten- sitic phase transformation. Literature on this system is limited and the description of the phases present is incom- plete and partly contradictory. With focus on a specific Cu-rich composition in this phase region (Cu 33 Pd 42 Sn 25 ) and having employed a combination of differential scanning calorimetry, X-ray diffraction, optical microscopy and density functional theory (DFT) methods we analyze the phases involved and the phase transformation tem- peratures. The martensitic transformation was found to occur at approximately 378 K upon heating and at 328 K under cooling conditions. The crystal structure of the high temperature (HT) austenite phase was identified as face centered cubic with lattice constant 𝑎 = 6 . 2261(1) Åat 473 K. DFT simulations of plausible atomic arrange- ments indicated that the HT phase was an L2 1 ( 𝐹 𝑚 ̄3 𝑚 ) type structure. The martensitic LT phase was found to be orthorhombic with a D0 11 ( 𝑃 𝑛𝑚𝑎 ) type structure with lattice parameters a = 8.9860(3) Å, 𝑏 = 5 . 9347(2) Åand 𝑐 = 4 . 5328(2) Åat 298 K.