Dynamic Disorder of Fe 3+ Ions in the Crystal Structure of Natural Barioferrite

摘要

A natural barioferrite, BaFe 3+ 12 O 19 , from a larnite–schorlomite–gehlenite vein of paralava within gehlenite hornfels of the Hatrurim Complex at Har Parsa, Negev Desert, Israel, was investigated by Raman spectroscopy, electron probe microanalysis, and single-crystal X-ray analyses acquired over the temperature range of 100–400 K. The crystals are up to 0.3 mm × 0.1 mm in size and form intergrowths with hematite, magnesioferrite, khesinite, and harmunite. The empirical formula of the barioferrite investigated is as follows: (Ba 0.85 Ca 0.12 Sr 0.03 ) ∑1 (Fe 3+ 10.72 Al 0.46 Ti 4+ 0.41 Mg 0.15 Cu 2+ 0.09 Ca 0.08 Zn 0.04 Mn 2+ 0.03 Si 0.01 ) ∑11.99 O 19 . The strongest bands in the Raman spectrum are as follows: 712, 682, 617, 515, 406, and 328 cm ?1 . The structure of natural barioferrite ( P 6 3 / mmc , a = 5.8901(2) ?, c = 23.1235(6) ?, V = 694.75(4) ? 3 , Z = 2) is identical with the structure of synthetic barium ferrite and can be described as an interstratification of two fundamental blocks: spinel-like S -modules with a cubic stacking sequence and R -modules that have hexagonal stacking. The displacement ellipsoids of the trigonal bipyramidal site show elongation along the [001] direction during heating. As a function of temperature, the mean apical Fe–O bond lengths increase, whereas the equatorial bond lengths decrease, which indicates dynamic disorder at the Fe2 site.