High-pressure insulating phase of Mo 4 O 11 with collapsed volume

We investigated the effect of pressure on the crystal structure and transport properties of the Magn\'eli phase \ensuremath{\eta}-${\mathrm{Mo}}_{4}{\mathrm{O}}_{11}$, which at ambient pressure undergoes two successive charge-density-wave (CDW) transitions at ${T}_{\mathrm{CDW}\text{\ensuremath{-}}1}\ensuremath{\approx}105\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{CDW}\text{\ensuremath{-}}2}\ensuremath{\approx}30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, respectively. We find that \ensuremath{\eta}-${\mathrm{Mo}}_{4}{\mathrm{O}}_{11}$ exhibits a structural phase transition from the low-pressure monoclinic $P{2}_{1}/a$ phase to a high-pressure $P{2}_{1}$ phase at ${P}_{c}\ensuremath{\approx}3.5\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Around ${P}_{\mathrm{c}}$, the lattice parameters experience a sudden change with a large volume collapse of $\mathrm{\ensuremath{\Delta}}V/V=\ensuremath{-}8.1%$, while the room-temperature resistivity exhibits a sudden jump by two orders of magnitude, signaling a pressure-induced metal-to-insulator transition. For $Pl{P}_{c}$, the high-pressure resistivity measurements revealed opposite pressure dependences of these two CDW transitions, i.e., ${T}_{\mathrm{CDW}\text{\ensuremath{-}}1}$ is enhanced gradually to \ensuremath{\sim}130 K while ${T}_{\mathrm{CDW}\text{\ensuremath{-}}2}$ is almost suppressed completely by the application of 2.6 GPa pressure. For $P\ensuremath{\ge}{P}_{c}$, the temperature dependence of resistivity changes to an insulating-like behavior, but the activation energy is reduced gradually upon further increasing pressure. We have rationalized the insulating ground state of the high-pressure phase in terms of the structural modifications and charge redistribution based on the refinement of single-crystal x-ray diffraction data at 8.9 GPa.