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.