In situ synchrotron diffraction of pressure-induced phase transition in ${\mathrm{DyPO}}_4$ under variable hydrostaticity

In situ synchrotron x-ray diffraction was conducted on polycrystalline ${\mathrm{DyPO}}_4$ to elucidate the details of the pressure-induced transition from the xenotime polymorph to the monazite polymorph. We used three different pressure-transmitting media (neon, a 16:3:1 methanol-ethanol-water mixture, and potassium chloride) to investigate the effect of hydrostaticity on the phase behavior. Specifically, our data clearly show a hydrostatic onset pressure of the xenotime-monazite transition of 9.1 GPa, considerably lower than the 15.3 GPa previously determined by Raman spectroscopy. Based on (quasi)hydrostatic data taken in a neon environment, third-order Birch-Murnaghan equation-of-state fits give a xenotime bulk modulus of 144 GPa and a monazite bulk modulus of 180 GPa (both with pressure derivatives of 4.0). Structural data and axial compressibilities show that ${\mathrm{DyPO}}_4$ is sensitive to shear and has an anisotropic response to pressure. More highly deviatoric conditions cause the onset of the transition to shift to pressures at least as low as 7.0 GPa. We attribute early transition to shear-induced distortion of the ${\mathrm{PO}}_4$ tetrahedra. Our characterization of the high-pressure behavior of ${\mathrm{DyPO}}_4$ under variable hydrostaticity is critical for advancing rare earth orthophosphate fiber coating applications in ceramic matrix composites and may inform future tailoring of phase composition for controlled shear and pressure applications.

Figure 1

[001] views of the (a) xenotime and (b) monazite phases of dysprosium orthophosphate ${\mathrm{DyPO}}_4$. Violet spheres represent ${\mathrm{Dy}}^{3+}$ cations, gray tetrahedra represent $\mathrm{P}{{\mathrm{O}}_4}^{3\text{–}}$ groups, and the boxes represent one unit cell. The apparent 90 ° rotation of monazite with respect to xenotime is merely a result of the monoclinic cell setting of monazite.

Figure 2

${\mathrm{DyPO}}_4$ unit cell contraction during compression under neon, mixture, and KCl pressure-transmitting media (PTM). Volumes are normalized by the xenotime volume at 0 GPa, $V_{0,X}$ (in which $X$ denotes xenotime). Vertical dashed lines indicate $P_{\mathrm{onset}}$. For each PTM, the cluster at the top of the dashed line represents xenotime, while the cluster at the bottom of the dashed line represents monazite. Error bars represent standard deviation. In mixture and KCl datasets, pressure was recorded as a nominal value. Volume error bars for all datasets are within the symbols. Black dashed curves represent equation of state (EoS) fits of neon data performed with a fixed bulk modulus pressure derivative (see Table 2).

Figure 3

Neon dataset. X ticks show a low-pressure xenotime reference pattern [30], while the M ticks show a monazite fit. (a) Contour plot showing all x-ray diffraction (XRD) patterns. Monazite peaks emerge at 9.1(1) GPa. Circles and triangles denote the gold and neon peaks, respectively. (b) Select XRD patterns (initial, transition onset, final) and their LeBail fits.

Figure 4

Mixture dataset. X ticks show a low-pressure xenotime reference pattern [30], while the M ticks show a monazite fit. (a) Contour plot showing all x-ray diffraction (XRD) patterns in ramp. Monazite peaks emerge at 9.3 GPa. (b) Select XRD patterns (initial, transition onset, final) and their LeBail fits.

Figure 5

KCl dataset. X ticks show a low-pressure xenotime reference pattern [30], while the M ticks show a monazite fit. (a) Contour plot showing all x-ray diffraction (XRD) patterns in ramp. Inverted and upright triangles represent the B1 and B2 phases of KCl, respectively. Monazite peaks emerge at 7.0 GPa. (b) Select XRD patterns (initial, transition onset, final) and their LeBail fits.

Figure 6

Pressure dependence of xenotime and monazite lattice parameters from the (a) neon, (b) mixture, and (c) KCl datasets. Dashed lines indicate $P_{\mathrm{onset}}$. Lattice parameter error bars are within the symbol size. Insets show the monazite beta angle with standard deviation error bars. Across pressure-transmitting media (PTM), consistent $y$-axis ranges for the panels and for the insets enable slope comparison.