Synthesis and structure of the new oxide fluoride Sr2TiO3F2 from the low temperature fluorination of Sr2TiO4: an example of a staged fluorine substitution/insertion reaction

In this paper we report the synthesis of the new oxide fluoride, Sr2TiO3F2, from the low temperature reaction of Sr2TiO4 with fluorinating agents (NH4F, transition metal difluorides). The reaction involves involves substitution/insertion with 2 F replacing 1 O, with the extra anions entering interstitial sites within the structure. The fluorination results in a significant reduction in the unit cell along a, b (approximate to2%) along with a large expansion along c (approximate to23%): Sr2TiO3F2, a = b = 3.8017(1), c = 15.536(1) Angstrom. Structural refinement using powder neutron diffraction data indicates that the reaction is a staged process leading to alternate layers of full and empty interstitial sites (space group P4/nmm). The occupancy of the interstitial sites also results in a substantial distortion in the Ti coordination away from the octahedral coordination of the Sr2TiO4 starting material, with one very long bond such that the coordination approximates to square pyramidal.