The Crystal Structure of α-K₃AlF₆: Elpasolites and Double Perovskites with Broken Corner-Sharing Connectivity of the Octahedral Framework

Abstract

The crystal structure of alpha-K(3)AlF(6) was solved and refined from a combination of powder X-ray and neutron diffraction data (a = 18.8385(3)A, c = 33.9644(6)A, S.G. I4(1)/a, Z = 80, R(P)(X-ray) = 0.037, R(P)(neutron) = 0.053). The crystal structure is of the A(2)BB'X(6) elpasolite type with the a = b approximately a(e) square root(5), c = 4a(e) superstructure (a(e), parameter of the elpasolite subcell) and rock-salt-type ordering of the K and Al cations over the B and B' positions, respectively. The remarkable feature of alpha-K(3)AlF(6) is a rotation of 2/5 of the AlF(6) octahedra by approximately pi/4 around one of the crystal axes of the elpasolite subcell, coinciding with the 4-fold symmetry axes of the AlF(6) octahedra. The rotation of the AlF(6) octahedra replaces the corner-sharing between the K and Al polyhedra by edge-sharing, resulting in an increase of coordination numbers of the K cations at the B positions up to 7 and 8. Due to significant deformations of the K polyhedra, the corner-sharing connectivity of the octahedral elpasolite framework is broken and the rotations of the AlF(6) octahedra do not have a cooperative character. Elpasolites and double perovskites with similar structural organization are discussed. The difference in ionic radii of the B and B' cations as well as the tolerance factor are proposed to be the parameters governing the formation of elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework.

Co-Precipitation Synthesis and Optical Properties of Mn4+-Doped Hexafluoroaluminate w-LED Phosphors

“…The space groups of Na 3 AlF 6 , K 2 NaAlF 6 and K 3 AlF 6 are listed in Table 2. K 2 NaAlF 6 has a highly symmetric cubic crystal structure (space group is Fm$\overline{3}$m ), while Na 3 AlF 6 and K 3 AlF 6 have a crystal structure with lower symmetry (space groups are P 2 1 / n and I 4 1 / a , respectively) [29,30,31]. In the M 3 AlF 6 crystal structure, the Al 3+ ions are octahedrally coordinated by six F − ions.…”

Multi-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23

“…In materials such as Sr 3 WO 6 large rotations of the small BX 6 octahedra occur to give the larger A cation, which resides on a B-site, a coordination number larger than 6. [11][12][13][14][15] There are also oxygen deficient cryolites and related double perovskite compositions, such as Sr 3 SbO 5.5 , which have simple cubic crystal structures but locally have a large rearrangement of the atoms around the oxygen vacancy such that the actual coordination environments of the cations are similar to those seem in stoichiometric cryolites. 5 There exists a small class of materials which have cryolite-like formulas but with small concentrations of vacancies on the A-sites and anion sites.…”

The strongly defective double perovskite Sr11Mo4O23: crystal structure in relation to ionic conductivity

“…It is generally assumed that the possible modes of octahedral tilting are limited by the fact that the corner-sharing connectivity of the (B 0 ,B 00 )O 6 network must be maintained. While almost all known perovskites obey this restriction, in the past few years there have been reports of compounds where the corner-sharing connectivity is not maintained (Abakumov et al, 2009). This only occurs when there is a very large difference in ionic radius (Ár i ) between the B 0 and B 00 cations and a small tolerance factor.…”