Structural studies of CaAl₁₂O₁₉, SrAl₁₂O₁₉, La_2/3+δ Al_12–δO₁₉, and CaAl₁₀NiTiO₁₉ with the hibonite structure; indications of an unusual type of ferroelectricity

Abstract

Various oxides with the hibonite structure were synthesized and structurally analyzed using powder neutron diffraction. The structure of CaAl12O19 at 298 and 11 K shows dipoles that are apparently too dilute to order unless subjected to a suitable electric field. Magnetoplumbites, such as BaFe12O19, are isostructural with hibonite. These compounds possess ferromagnetic properties, which combined with the electric dipoles may influence multiferroic behavior. Our SrAl12O19 sample showed two distinct hexagonal phases, a major phase with the normal hibonite structure and a minor phase having a closely related structure. Our sample of the defect hibonite phase La2/3+δAl12–δO19 shows a distinctly higher δ value (0.25) vs. that reported (~0.15) for samples made from the melt. Finally, we used to advantage the negative scattering length of Ti to determine the site occupancies of Ni and Ti in CaAl10NiTiO19.

Colored oxides with hibonite structure: A potential route to non-cobalt blue pigments

“…hibonite lattice. We have previously reported on one example, CaAl 10 NiTiO 19 , where we revealed from neutron diffraction data that Ni 2+ on the tetrahedral sites in hibonite provides a bright blue color [18]. Here we report on many more Ni 2+ containing hibonite phases.…”

“…In this article we focus on characterization of structure and various properties: color, magnetic, dielectric, and stability. Crystal structures of hibonite type of oxides have been characterized extensively by lab X-ray [8,1017,23,30,35,39], synchrotron [41] and neutron [18,36] diffractions. Other techniques have also been employed to characterize hibonite phases.…”

“…Research on nature hibonites has been focused on their texture, compositions, crystal structures and some physical properties, the major objective of which is to ascertain the origin of hibonite-bearing meteoritic assemblages in hope of understanding the evolution of the primitive solar nebula [39]. [12][13][14][15][16][17][18]. The hexagonal structure of hibonite AM 12 O 19 can be described as, along the c axis, alternating "spinel slabs" or S-blocks (similar to those in -alumina) and intermediate layers or R-blocks (containing A 2+ ions, 5-coordinated MO 5 and isolated face-sharing MO 6 pairs) [5,8].…”

“…None of the three M sites with coordination number (CN) 6 have ideal octahedral symmetry. The M1 atom at the origin has ideal trigonal antiprismatic symmetry, two MO 6 (M4) octahedra form a face-sharing dimer, and the MO 6 octahedra based on the M5 site form a sheet perpendicular to the c axis [18]. The tetrahedral M3 cation inside the "spinel slabs" was found underbonded for M = Al [10,18].…”

“…The M1 atom at the origin has ideal trigonal antiprismatic symmetry, two MO 6 (M4) octahedra form a face-sharing dimer, and the MO 6 octahedra based on the M5 site form a sheet perpendicular to the c axis [18]. The tetrahedral M3 cation inside the "spinel slabs" was found underbonded for M = Al [10,18]. The M2 (M = Al, Ga) cation with trigonal bipyramidal (TBP) symmetry was reported to displace toward one of the apical oxygen atoms and hence be treated with a split-atom model [8,11,1518].…”