High-Pressure X‑ray Diffraction Investigation of Fe0.9Al0.1VO4

Abstract

This study demonstrates that the influence of cationic composition on the phase behavior of vanadates under high pressure must be meticulously considered. In this investigation, we report an in situ high-pressure powder X-ray diffraction investigation on triclinic Fe_0.9Al_0.1VO₄ (space group P1̅) up to 11 GPa. The structural sequence of Fe_0.9Al_0.1VO₄ is different than that of FeVO₄. Our analysis shows that Fe_0.9Al_0.1VO₄ undergoes a first-order structural phase transition at 2.85 GPa to another triclinic structure described by the same space group with a volume collapse of ∼9%. At 6.1 GPa, we observed the onset of a second phase transition to a monoclinic structure (space group P2/c), with coexistence of both phases until 8.55 GPa. The transformation to the second phase is completed at 9.15 GPa, with a volume collapse of ∼13%. On release of pressure to ambient conditions, we have observed the coexistence of the second and first high-pressure phases. The compressibility of the three phases of the compound has been studied too. We have observed variations in structural sequence and compressibility behavior due to Al incorporation. Since electronic properties could be modified by tuning the crystal structure, the present results could have an impact on applications of the studied compound such as photocatalysis and batteries.

1

Crystal structure of phases I, I′, and IV of FeVO₄ (Fe_0.9Al_0.1VO₄). V (Fe/Al) coordination polyhedra are shown in blue (brown). Oxygen atoms are shown in red.

2

Rietveld refinement of the XRD pattern of triclinic Fe_0.9Al_0.1VO₄ at ambient conditions (λ = 0.4956 Å). Data are shown as black crosses (×), while the red solid line represents the result of the refinement. The blue (green) line represents the residuals of the fitting (the background). Vertical pink (black) bars identify the Bragg reflections of Fe_0.9Al_0.1VO₄ (copper).

3

X-ray powder diffraction patterns of Fe_0.9Al_0.1VO₄ at selected pressures (λ = 0.4956 Å). The patterns measured under pressure release are identified by (rel). Cu indicates the diffraction peaks of copper at 0.1 GPa, 6.1 GPa, and 0.1 MPa. At 6.1 GPa, (+) symbols identify diffraction peaks assigned to the triclinic HP phase, while (*) symbols identify emerging diffraction peaks assigned to the monoclinic HP phase.

4

Rietveld refinement of the XRD pattern of the HP triclinic Fe_0.9Al_0.1VO₄ (phase I′) at 2.85 GPa (λ = 0.4956 Å). Data are shown as black crosses (×), while the red solid line represents the result of the refinement. The blue (green) line represents the residuals of the fitting (the background). Vertical pink (black) bars identify the Bragg reflections of Fe_0.9Al_0.1VO₄ (copper).

5

Rietveld refinement of the XRD pattern of the monoclinic phase IV structure at 9.15 GPa (λ = 0.4956 Å). Data are shown as black crosses (×), while the red solid line represents the result of the refinement. The blue (green) line represents the residuals of the fitting (the background). Vertical pink (black) bars identify the Bragg reflections of Fe_0.9Al_0.1VO₄ (copper).

6

Pressure dependence of unit-cell parameters of Fe_0.9Al_0.1VO₄. Black, red, and blue circles are used for phases I, I′, and IV. Solid symbols are from compression experiments and empty symbols from decompression experiments. The symbols used for different angles are identified in the legend.

7

Volume versus pressure results for Fe_0.9Al_0.1VO₄. Black, red, and blue circles are used for phases I, I′, and IV, respectively. Solid symbols are from compression experiments and empty symbols from decompression experiments. Solid lines represent Birch–Murnaghan fits to the data. The unit-cell volume of phase IV was multiplied by three to facilitate the comparison.