Polymorphism of rare-earth orthovanadates under high pressure

Resumen

Rare-earth orthovanadates constitute a paradigmatic family of ternary oxides, due to their striking properties and their manifold polymorphs. Many of their different phases can be achieved by submitting these crystal structures under extreme pressure and temperature conditions. In this dissertation we study the behaviour of NdVO4, GdVO4, PrVO4 and TbVO4 under high pressures and different temperature conditions. Different samples of these compounds were characterized by means of several experimental techniques such as X-ray diffraction (XRD), Raman scattering and optical absorption. In addition to this, many of our experimental results are also supported by ab initio calculations, which allowed a better understanding of the pressure behaviour of these compounds. With regard to NdVO4, our experiments under quasi-hydrostatic conditions showed that there is a zircon-to-scheelite phase transition at 6.5 GPa and a scheelite-to-fergusonite phase transition at 20 GPa. Similar findings were found in GdVO4 at 7.0 and 20 GPa respectively by means of XRD, Raman spectroscopy and optical absorption measurements. Moreover, the Raman scattering measurements found a third phase transition to a post-fergusonite phase near 30 GPa. With respect to PrVO4, XRD experiments at high pressures found a zircon-to-monazite and a monazite-to-PbWO4-III phase transition at 5.5 and 12.7 GPa, respectively. Motivated by theoretical predictions, we also synthesized the scheelite phase of PrVO4 at high pressures and high temperatures in a Paris-Edinburgh large volume press cell. The recovered scheelite phase was later studied upon compression by using Raman spectroscopy. In general, the equations of state of the different phases observed for NdVO4, GdVO4 and PrVO4 are also reported, as well as the pressure evolution of the Raman active modes. The experiments and calculations on these orthovanadates show that kinetic barriers play a crucial role in the zircon-scheelite/monazite systematics of RVO4 compounds. By gathering the data available in the most recent literature as well as our ab initio calculations we could unveil the mechanisms that govern in these structural sequences. In particular, the zircon-to-scheelite and zircon-to-monazite phase transitions are triggered by dynamical and mechanical instabilities respectively. Finally, we explored the Jahn-Teller effect of TbVO4 at low temperatures (and high pressures). Optical absorption experiments at low temperatures and room pressure show a progressive widening of the electronic band gap below 35 K as a consequence of the Jahn-Teller distortion. XRD measurements at low temperatures and low pressures show that the transition temperature considerably increases with pressure. A tentative pressure-temperature phase diagram is proposed for TbVO4.