Abstract

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 (≈2%) along with a large expansion along c (≈23%): Sr₂TiO₃F₂, a = b = 3.8017(1), c = 15.536(1) Å. 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.