Direct Zircon-to-Scheelite Structural Transformation in YPO4 and YPO4:Eu3+ Nanoparticles Under High Pressure

The high-pressure behavior of zircon-structured YPO4 (with/without Eu3+ doping) nanoparticles was examined at room temperature using in situ synchrotron X-ray diffraction (XRD) and photoluminescence (PL) measurements. In contrast with the reported XRD results of bulk YPO4 upon compression, the nanoparticles showed a distinct transition sequence: zircon phase → scheelite phase (∼18 GPa) without the metastable monazite phase. By the return to ambient pressure, both XRD and PL results revealed that the scheelite phase could be reserved. Further Raman experiments helped us to identify the valuable mode ν1(Ag) of the scheelite structure in the quenched samples. The dopants effect, quasi-hydrostatic stress, and nanoscale-induced surface energy difference are considered to explain the high-pressure behavior of the nanoparticles. It is proposed that the nanoscale-induced higher surface energy contribution plays a crucial role in the distinctive high-pressure behavior of the nanoparticles.