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 phaseV. A reversible broadening of the guest structure is observed below 16.5(2)GPa