Rare-earth orthovanadates (RVO₄, with R = Sc, Y, La-Lu) crystallize, typically, in I4₁/amd space group (zircon type structure). They transform to the scheelite-type structure (I41/a space group) under high-pressure conditions of several GPa. As the transition is irreversible, it is possible to study the properties of scheelite polymorph at ambient pressure. The first detailed information on transition of this kind at hydrostatic conditions was reported for YVO₄ [1, 2]. Physical properties of zircon-type rare-earth orthovanadates are better recognized, and various applications have been proposed for them, in particular they can be applied as catalysts. The orthovanadates with the scheelite structure attract attention because some representatives such as EuVO₄ show strong luminescence [3] creating an opportunity for application in optoelectronic devices. The above-mentioned features resulted in an increasing interest in structural phase transitions, their mechanisms and in equations of state for both RVO₄ polymorphs.

The aim of the present investigation was the in-situ observation of the zircon-scheelite phase transition under pressure and determination of elastic properties for two members of the RVO₄ family, NdVO₄ and EuVO₄, for both zircon and scheelite type polymorphs. The samples of zircon structures were obtained by solid state reaction between R₂O₃ (R = Nd, Eu) and V₂O₅ at temperatures of 1100°C for NdVO₄ and 950°C for EuVO₄. The samples transformed to scheelite-type structure after compression of zircon-type phase at elevated temperature.

In-situ high-pressure X-ray diffraction experiments were performed in the range up to 15 GPa at I711 beamline MAX II (Lund, Sweden) using a diamond-anvil cell. The samples studied were placed in a hole drilled in a pre-indented stainless-steel gasket which was subsequently clamped between two diamond anvils having 0.3 mm diameter culets. A water-ethanol-methanol mixture was used as pressure-transmitting medium providing hydrostatic compression conditions up to about 10 GPa. The applied pressure was measured by laser-excited ruby luminescence. Data acquisition was carried out using MAR165 area detector. Using the aforementioned experimental approach, the zircon-type NdVO₄ and scheelite-type EuVO₄ were studied. The acquired experimental data were analyzed using Rietveld method providing the information on structure of the material. For NdVO₄, the zircon-scheelite phase transition was observed in the pressure range from 4.35 to 5.75 GPa. Bulk modulus values were calculated for both materials from the volume-pressure relationship.

References

[1] A. Jayaraman, G.A. Kourouklis, G.P. Espinosa, A.S. Cooper, L.G. Van Uitert, "A high-pressure Raman study of yttrium vanadate (YVO₄) and the pressure-induced transition from the zircon-type to the scheelite-type structure," J. Phys. Chem. Solids  48, 8 (1987) 755-759.

[2] X. Wang, I. Loa, K. Syassen, M. Hanfland, B. Ferrand, "Structural properties of the zircon- and scheelite-type phases of YVO₄ at high pressure," Phys. Rev. B  70 (2004) 064109.

[3] G. Chen, R.G. Haire, J.R. Peterson, M.M. Abraham, "A luminescence study of EuVO₄ at high pressure," J. Phys. Chem. Solids  55, 4 (1994) 313-316.

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