Article

The high-temperature polymorphs of K3AlF6.

Lujan Neutron Scattering Center, Los Alamos National Laboratory, MS H805, Los Alamos, New Mexico 87545, USA.
Inorganic Chemistry (Impact Factor: 4.79). 08/2011; 50(16):7792-801. DOI: 10.1021/ic200956a
Source: PubMed

ABSTRACT The crystal structures of the three high-temperature polymorphs of K(3)AlF(6) have been solved from neutron powder diffraction, synchrotron X-ray powder diffraction, and electron diffraction data. The β-phase (stable between 132 and 153 °C) and γ-phase (stable between 153 to 306 °C) can be described as unusually complex superstructures of the double-perovskite structure (K(2)KAlF(6)) which result from noncooperative tilting of the AlF(6) octahedra. The β-phase is tetragonal, space group I4/m, with lattice parameters of a = 13.3862(5) Å and c = 8.5617(3) Å (at 143 °C) and Z = 10. In this phase, one-fifth of the AlF(6) octahedra are rotated about the c-axis by ∼45° while the other four-fifths remain untilted. The large ∼45° rotations result in edge sharing between these AlF(6) octahedra and the neighboring K-centered polyhedra, resulting in pentagonal bipyramidal coordination for four-fifths of the K(+) ions that reside on the B-sites of the perovskite structure. The remaining one-fifth of the K(+) ions on the B-sites retain octahedral coordination. The γ-phase is orthorhombic, space group Fddd, with lattice parameters of a = 36.1276(4) Å, b = 17.1133(2) Å, and c = 12.0562(1) Å (at 225 °C) and Z = 48. In the γ-phase, one-sixth of the AlF(6) octahedra are randomly rotated about one of two directions by ∼45° while the other five-sixths remain essentially untilted. These rotations result in two-thirds of the K(+) ions on the B-site obtaining 7-fold coordination while the other one-third remain in octahedral coordination. The δ-phase adopts the ideal cubic double-perovskite structure, space group Fm ̅3m, with a = 8.5943(1) Å at 400 °C. However, pair distribution function analysis shows that locally the δ-phase is quite different from its long-range average crystal structure. The AlF(6) octahedra undergo large-amplitude rotations which are accompanied by off-center displacements of the K(+) ions that occupy the 12-coordinate A-sites.

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    ABSTRACT: Phase transitions in the elpasolite-type K{sub 3}AlF{sub 6} complex fluoride were investigated using differential scanning calorimetry, electron diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures T{sub 1}=132 deg. C, T{sub 2}=153 deg. C and T{sub 3}=306 deg. C. The {alpha}-K{sub 3}AlF{sub 6} phase is stable below T{sub 1} and crystallizes in a monoclinic unit cell with a=18.8588(2)A, b=34.0278(2)A, c=18.9231(1)A, {beta}=90.453(1){sup o} (a=2a{sub c}-c{sub c}, b=4b{sub c}, c=a{sub c}+2c{sub c}; a{sub c}, b{sub c}, c{sub c}-the basic lattice vectors of the face-centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate {beta} phase exists only in very narrow temperature interval between T{sub 1} and T{sub 2}. The {gamma} polymorph is stable in the T{sub 2}<T<T{sub 3} temperature range and has an orthorhombic unit cell with a=36.1229(6)A, b=17.1114(3)A, c=12.0502(3)A (a=3a{sub c}-3c{sub c}, b=2b{sub c}, c=a{sub c}+c{sub c}) at 250 deg. C and space group Fddd. Above T{sub 3} the cubic {delta} polymorph forms with a{sub c}=8.5786(4)A at 400 deg. C and space group Fm3-bar m. The similarity between the K{sub 3}AlF{sub 6} and K{sub 3}MoO{sub 3}F{sub 3} compounds is discussed.
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    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.
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