Pressure-induced instability of the fergusonite phase of EuNbO4 studied by in situ Raman spectroscopy, x-ray diffraction, and photoluminescence spectroscopy

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In this article, we present high-pressure experimental investigations on EuNbO4, an interesting technologically important material, using synchrotron based x-ray powder diffraction, Raman spectroscopy, and europium photoluminescence measurements up to 39.2, 31.6, and 32.4 GPa, respectively. All three techniques show the stability of the ambient monoclinic phase until 20 GPa. Beyond that, a pressure-induced structural phase transition takes place with the coexistence of two phases over a wide pressure range. The structure of the high-pressure phase has been determined as orthorhombic (space group: Imma) with a volume discontinuity of nearly 9% at the transition indicating the nature of transition to be first order. The high-pressure phase remains stable up to ∼40 GPa. The equation of state data for both phases have been determined, and the modulus of incompressibility indicates that the high-pressure phase with bulk modulus as 246(29) GPa is less compressible compared to the low-pressure phase having bulk modulus as 138(4) GPa. The transition involves a change in the coordination polyhedron around europium, increasing the coordination number from 8 in the fergusonite to 12 in the orthorhombic phase, while the coordination around niobium remains 4 in both the phases. All the 18 expected Raman modes in the fergusonite structure have been observed, and their variation as a function of pressure has been reported. The mode Grüneisen parameters for the observed Raman frequencies for monoclinic and orthorhombic phases have been determined. Europium photoluminescence spectra show drastic changes near the transition pressure, which point toward changes in the local environment around the europium ion.