Contrasting Structural Stabilities and New Pressure-Induced Polymorphic Transitions of Scheelite- and Zircon-Type ZrGeO4

As promising optical materials, the structures of scheelite- and zircon-type ZrGeO4 have been investigated by Raman spectroscopy at pressures up to 51.6 GPa using diamond anvil cells. For scheelite-type ZrGeO4, two reversible phase transitions at 16.2 and 41.2 GPa were identified from the Raman spectral profiles and pressure dependence of the Raman shift upon compression. The first transition can be interpreted as the change of the tetragonal scheelite structure to the monoclinic fergusonite structure. By comparison with a previously reported isomorphic crystal, we infer that the second high pressure phase likely has a monoclinic structure. The zircon-type ZrGeO4 initially transforms to a scheelite structure at ∼10.1 GPa, while such phase transformation is completed at around 24.6 GPa, and remains stable up to 37.6 GPa. Moreover, the zircon-to-scheelite phase transition is irreversible, and the high pressure scheelite structure cannot be recovered upon decompression. These phase transitions for different starting ZrGeO4 materials were unambiguously identified for the first time, and their contrasting structural stabilities as well as the transition mechanisms can be understood from the intrinsic characteristics of the crystal lattices. These results contribute to the understanding of the pressure behavior of very sparsely reported germinate compounds in the ABO4 family.