Abstract

In this paper we report the synthesis of the new oxide fluoride Ba₂ZrO₃F₂·xH₂O (x ≈ 0.5) from the reaction of Ba₂ZrO₄ with NH₄F or transition metal difluorides (CuF₂, ZnF₂) at low temperature (250 °C). The fluorination reaction represents a substitution of 1 oxygen by 2 fluorines, thus increasing the anion content and resulting in a large expansion of the unit cell in the c direction (tetragonal, I4/mmm, a = b = 4.1721(3), c = 16.376(2) Å). Powder neutron diffraction studies have shown that the material has a K₂NiF₄-type structure similar to the precursor Ba₂ZrO₄, with the extra anions occupying interstitial sites within the rock salt layers, which accounts for the large expansion in the unit cell along the c direction. The anion content determined from the neutron diffraction data refinement is higher than expected for the simple oxide fluoride Ba₂ZrO₃F₂. This is attributed to the additional incorporation of water as OH groups, which is supported by TGA and high temperature XRD studies. On heating to 500 °C a mass loss consistent with the loss of 0.5 moles of H₂O was observed along with a reduction in the cell parameters to a = b = 4.180(6), c = 15.45(3) Å for the dehydrated phase. On heating to higher temperatures (>500 °C) decomposition to BaF₂ and the perovskite phase BaZrO₃ was observed.

These results and preliminary work showing the successful fluorination of related phases demonstrate both the versatility of these low temperature fluorination routes, and the ready ability of the K₂NiF₄ structure to incorporate extra anions.