Synthesis and structure of the new oxide fluoride Ba2ZrO3F2 center dot xH(2)O (x approximate to 0.5)

In this paper we report the synthesis of the new oxide fluoride Ba2ZrO3F2. xH(2)O (x approximate to 0.5) from the reaction of Ba2ZrO4 with NH4F or tra nsition metal difluorides (CuF2, ZnF2) at low temperature (250 degreesC). T he fluorination reaction represents a substitution of 1 oxygen by 2 fluorin es, 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) Angstrom). 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 lay ers, which accounts for the large expansion in the unit cell along the c di rection. The anion content determined from the neutron diffraction data ref inement is higher than expected for the simple oxide fluoride Ba2ZrO3F2. Th is is attributed to the additional incorporation of water as OH groups, whi ch is supported by TGA and high temperature XRD studies. On heating to 500 degreesC a mass loss consistent with the loss of 0.5 moles of H2O was obser ved along with a reduction in the cell parameters to a = b = 4.180(6), c = 15.45(3) Angstrom for the dehydrated phase. On heating to higher temperatur es (> 500 degreesC) decomposition to BaF2 and the perovskite phase BaZrO3 w as observed. These results and preliminary work showing the successful fluorination of r elated phases demonstrate both the versatility of these low temperature flu orination routes, and the ready ability of the K2NiF4 structure to incorpor ate extra anions.