Observation of two collapsed phases in CaRbFe4As4

We report the observation of the pressure-induced, fully collapsed tetragonal phase of $\mathrm{CaRbF}{\mathrm{e}}₄\mathrm{A}{\mathrm{s}}₄$ for $P\ensuremath{\sim}22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ via high-pressure x-ray diffraction and magnetotransport measurements. The x-ray measurements, along with resistivity measurements, show that there is an initial half-collapsed tetragonal phase for $6<P<22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, in which superconductivity is continuously suppressed from ${T}_{\mathrm{c}}=35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at $P=3.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ to ${T}_{\mathrm{c}}<2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $P\ensuremath{\ge}17.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as well as signs of the fully collapsed tetragonal phase near $P=22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Density functional calculations suggest that both of these transitions are driven by increased As-As bonding, first across the Ca layer, and then at the second transition, across the Rb layer. Although electrical resistivity measurements in the fully collapsed tetragonal phase do not show superconductivity, there is a change in the slope of both the Hall coefficient and the longitudinal resistance near 22 GPa, suggesting a strong correlation between the electronic and lattice degrees of freedom in this new iron-based superconductor.