The crystal structure of KCaF₃ at 4.2 and 300 K: A re-evaluation using high-resolution powder neutron diffraction

Abstract

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° and 13.2° 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 B21∕m.

Structural Distortion in Perovskite Type KCaH3–xFx (0.54 ≤ x ≤ 3)

“…The refined atomic position parameters agree with those from previous investigations . Interatomic angles and polyhedral distortions are similar to related crystal structures of this type, for example KCaF 3 . Interatomic distances resulting from the structure refinements are in the range of typical ionic hydrides and fluorides , …”

“…[19] Interatomic angles and polyhedral distortions are similar to related crystal structures of this type, for example KCaF 3 . [25] Interatomic distances resulting from the structure refinements are in the range of typical ionic hydrides and fluorides. [16,26] Phases with higher hydrogen content crystallize in the tetragonal SrTiO 3 structure type (space group I4/mcm, no.…”

Parameterization of the Crystal Structures of Centrosymmetric Zone-Boundary-Tilted Perovskites: An Analysis in Terms of Symmetry-Adapted Basis-Vectors of the Cubic Aristotype Phase

“…For CaTiO 3 at ambient temperature (see the reSulTS section for more details), u1/w2 is equal to -1.933 and u2/v2 is equal to -1.013, indicating that there is only a modest distortion of the TiO 6 octahedron from ideality. This is in contrast to the crystal structure of KCaF 3 at 4.2 K, where u1/w2 is equal to -1.874 and u2/v2 is equal to -1.033, both values indicating a more significant deformation of the CaF 6 octahedron in this case (Knight et al 2005b).…”

“…This methodology was not immediately followed up by the crystallographic community at the time, and over thirty years later, Darlington (2002aDarlington ( , 2002b carried out a similar detailed analysis for nine hettotype phases of perovskite and elpasolite. Knight et al (2005aKnight et al ( , 2005b analyzed high-pressure results on SrCeO 3 and low-temperature results on KCaF 3 using the lattice-dynamical analysis of Cochran & Zia (1968), but unfortunately they did not relate the changes in the mode magnitudes to the changes in the crystal structure. Mitchell et al (2007) have presented mode-displacement amplitudes for NaMgF 3 at 3.6 and 300 K. An extremely detailed study of the octahedron tilt modes in the perovskite-related compounds ABX 4 and A 2 BX 4 has been made by Swainson (2005); in that case, displacements have been analyzed in terms of the primitive tetragonal Brillouin zone.…”

High-temperature structural phase transitions in neighborite: a high-resolution neutron powder diffraction investigation

“…In PePI I, laboratory source data were only collected to a Q resolution of 5.23 Å −1 which is insufficient to derive precise structural parameters from a pseudosymmetric crystal structure [see Knight et al (2004) for discussion of an example of an incorrect space group and implausible crystal structure, at ambient temperature and pressure, for KCaF 3 perovskite derived from data collected to a similar resolution]. The derived structural parameters (bond lengths and angles) shown in Table 3 of PePI I are inconsistent with estimated standard deviations (esds) of the structural parameters listed in Table 2, and bond length corrections for thermal motion are based on a model that assumes uncorrelated atomic displacements, which is inappropriate for a 3-dimensional, fully connected, polyhedral network structure.…”