Pressure-Induced Charge-Order Melting and Reentrant Charge Carrier Localization in the Mixed-Valent Pb_3Rh_7O_15

The mixed-valent Pb_3Rh_7O_15 undergoes a Verwey-type transition at T_\rm v \approx 180 K, below which the development of Rh^3+/Rh^4+ charge order induces an abrupt conductor-to-insulator transition in resistivity. Here we investigate the effect of pressure on the Verwey-type transition of Pb_3Rh_7O_15 by measuring its electrical resistivity under hydrostatic pressures up to 8 GPa with a cubic anvil cell apparatus. We find that the application of high pressure can suppress the Verwey-type transition around 3 GPa, above which a metallic state is realized at temperatures below \sim 70 K, suggesting the melting of charge order by pressure. Interestingly, the low-temperature metallic region shrinks gradually upon further increasing pressure and disappears completely at P >7 GPa, which indicates that the charge carriers in Pb_3Rh_7O_15 undergo a reentrant localization under higher pressures. We have constructed a temperature-pressure phase diagram for Pb_3Rh_7O_15 and compared to that of Fe_3O_4, showing an archetype Verwey transition.