Pressure dependent incommensuration in Rb-IV.

Rb-IV is found to have an incommensurate composite structure, comprising a tetragonal host framework and a simple body-centered tetragonal guest. This does not have the unexpectedly short Rb-Rb distances of the previously reported structure [U. Schwarz et al., Phys. Rev. Lett. 83, 4085 (1999)]. The ratio of the c-axis lattice parameters is strongly pressure dependent and approaches the commensurate value of 5/3 at the transition to phase V. A reversible broadening of the guest structure is observed below 16.5(2) GPa.