High-pressure stability and compressibility of $A{\text{PO}}_4$ ($A=\text{La}$, Nd, Eu, Gd, Er, and Y) orthophosphates: An x-ray diffraction study using synchrotron radiation

Room-temperature angle-dispersive x-ray diffraction measurements on zircon-type ${\text{YPO}}_4$ and ${\text{ErPO}}_4$, and monazite-type ${\text{GdPO}}_4$, ${\text{EuPO}}_4$, ${\text{NdPO}}_4$, and ${\text{LaPO}}_4$ were performed in a diamond-anvil cell up to 30 GPa using neon as pressure-transmitting medium. In the zircon-structured oxides we found evidence of a reversible pressure-induced structural phase transformation from zircon to a monazite-type structure. The onset of the transition is at 19.7 GPa in ${\text{YPO}}_4$ and 17.3 GPa in ${\text{ErPO}}_4$. In ${\text{LaPO}}_4$ a nonreversible transition is found at 26.1 GPa and a barite-type structure is proposed for the high-pressure phase. For the other three monazites studied, their structures were found to be stable up to 30 GPa. Evidence for additional phase transitions or chemical decomposition of the materials was not found in the experiments. The equations of state and axial compressibility for the different phases are also determined. In particular, we found that in a given compound the monazite structure is less compressible than the zircon structure. This fact is attributed to the higher packing efficiency of monazite versus zircon. The differential bond compressibility of different polyhedra is also reported and related to the anisotropic compressibility of both structures. Finally, the sequence of structural transitions and compressibilities are discussed in comparison with other orthophosphates.

Figure 1

Selected x-ray diffraction patterns of ${\text{LaPO}}_4$ at different pressures; (r) indicates the pattern collected on pressure release. The inset shows the low-angle section of the spectrum collected at 26.1 GPa enlarged. The peaks appearing at low angles can be more clearly seen there.

Figure 2

Unit-cell parameters versus pressure for ${\text{LaPO}}_4$. Circles: low-pressure phase. Triangles: high-pressure phase. The empty symbols correspond to data obtained after pressure release. The dashed lines are a guide to the eye and the solid lines represent the fits shown in Table . The inset shows the pressure evolution of the $\beta$ angle of the monazite structure.

Figure 3

Pressure-volume relation in ${\text{LaPO}}_4$, ${\text{NdPO}}_4$, ${\text{EuPO}}_4$, and ${\text{GdPO}}_4$. Solid symbols: experiments. Empty circles: data points collected after pressure release. Empty square: literature data (Ref. 1). Lines: EOS fit.

Figure 4

Selected x-ray diffraction patterns of ${\text{YPO}}_4$ at different pressures; (r) indicates the pattern collected on pressure release.

Figure 5

Unit-cell parameters versus pressure for ${\text{YPO}}_4$. Squares: low-pressure phase. Circles: high-pressure phase. The dashed lines are a guide to the eye and the solid lines represent the fits shown in Table . The inset shows the pressure evolution of the $\beta$ angle of the monazite structure.

Figure 6

Pressure-volume relation in ${\text{YPO}}_4$ and ${\text{ErPO}}_4$. Solid squares: experiments for the zircon phase. Solid circles: data points obtained for the high-pressure phase. Empty triangles: literature data (Ref. 1). Empty squares: data obtained after pressure release. Lines: EOS fit. In ${\text{YPO}}_4$ the empty square overlaps the empty triangle.

Figure 7

Pressure dependence of bond distances in monazite ${\text{LaPO}}_4$ (solid symbols: experiments. Lines: fits) and zircon ${\text{YPO}}_4$ (empty symbols: experiments. Lines: fits). The P-O distance of ${\text{YPO}}_4$ has been shifted up $0.1\text{Å}$ to avoid the overlap with the P-O distances of ${\text{LaPO}}_4$.