Space group and crystal structure of the R-phase in binary NiTi shape memory alloys

The crystal structure parameters of the R-phase in a binary Ni50.8Ti49.2 shape memory alloy were redetermined using a device minimizing texture effects on neutron diffraction intensities. A sphere of 9 mm diameter cut from a Ni50.8Ti49.2 alloy which shows the R-phase state at room temperature was suspended on an air cushion and kept in chaotic rotation motion while neutron diffractograms were obtained. Several models with different space group symmetries were tested in Rietveld refinements using the GSAS software. Our results clearly favour P3¯ symmetry, confirming the results of recent electron diffraction and X-ray diffraction studies.