Synthesis and structure of the new oxide fluoride Ba2ZrO3F2·xH2O (x ≈ 0.5)

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.