Putting the Squeeze on Lead Chromate Nanorods.

We have studied by means of X-ray diffraction and Raman spectroscopy the high-pressure behavior of PbCrO4 nanorods. We have found that these nanorods follow a distinctive structural sequence that differs from that of bulk PbCrO4. In particular, a phase transition from a monoclinic monazite-type PbCrO4 to a novel monoclinic AgMnO4-type polymorph has been discovered at 8.5 GPa. The crystal structure, Raman-active phonons, and compressibility of this novel high-pressure phase are reported for the first time. The experimental findings are supported by ab initio calculations that provide information not only on structural and vibrational properties of AgMnO4-type PbCrO4 but also on the electronic properties. The discovered phase transition triggers a band gap collapse and a subsequent metallization at 44.2 GPa, which has not been observed in bulk PbCrO4. This suggests that nanoengineering can be a useful strategy to drive metallization under compression.