Incommensurate structures and radiation damage in Rb₂V₃O₈ and K₂V₃O₈ mixed-valence vanadate fresnoites

The structures and phase transitions to incommensurate structures in Rb2V3O8 and K2V3O8 mixed-valence vanadate fresnoites are studied with synchrotron single-crystal diffraction at low temperatures and ambient pressure. Although mixed satellite reflections are absent, the modulated structure of K2V3O8 below 115 K is better described in (3 + 2)- than in (3 + 1)-dimensional space. The geometries of the VO4 and VO5 building units are rigid and it is mainly slight rotations of these polyhedra and small variation of the intermediate K—O distances that are modulated. Prolonged exposure to the high-brilliance synchrotron beam suppresses the incommensurate phase. The previously postulated phase transition to the incommensurate phase in Rb2V3O8 at 270 K was not observed. One of the reasons could be that the intense radiation also affects the modulation in this material. Strategies to collect and analyse single-crystal diffraction data measured with very intense synchrotron radiation using modern low-noise pixel area detectors are discussed.