Pressure-induced structural transitions triggering dimensional crossover in the lithium purple bronze Li 0.9 Mo 6 O 17

At ambient pressure, lithium molybdenum purple bronze (${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$) is a quasi-one-dimensional solid in which the anisotropic crystal structure and the linear dispersion of the underlying bands produced by electronic correlations possibly bring about a rare experimental realization of Tomomaga-Luttinger liquid physics. It is also the sole member of the broader purple molybdenum bronzes family where a Peierls instability has not been identified at low temperatures. The present paper reports a pressure-induced series of phase transitions between 0 and 12 GPa. These transitions are strongly reflected in infrared spectroscopy, Raman spectroscopy, and x-ray diffraction. The most dramatic effect seen in optical conductivity is the metalization of the $c$ axis, concomitant to the decrease in conductivity along the $b$ axis. This indicates that high pressure drives the material away from its quasi-one-dimensional behavior at ambient pressure. Although the first pressure-induced structure of the series is resolved, the identification of the underlying mechanisms driving the dimensional change in the physics remains a challenge.