The crystal structure of KCaF₃ at 4.2 and 300 K: a re-evaluation using the high-resolution powder neutron diffractometer

The crystal structure of the perovskite phase KCaF3 has been redetermined at 4.2 and 300 K using powder neutron diffraction collected at the highest resolution. At both temperatures the phase was found to be orthorhombic in space group Pnma, with lattice parameters a = 0.622 879(5) nm, b = 0.870 031(7) nm, c = 0.611 210(5) nm at 4.2 K, and a = 0.621 488(6) nm, b = 0.876 360(8) nm, c = 0.616 481(6) nm at 300 K. The CaF6 octahedron is regular at both temperatures with octahedral rotations of 9.6 degrees and 13.2 degrees for the in-phase and anti-phase tilts, respectively, at 4.2 K. No evidence was found to support the recent revision of the space group from Pnma to the monoclinic space group B2(1)/m. (c) 2005 International Centre for Diffraction Data.