THE HIGH-PRESSURE BEHAVIOR OF XENOTIME AND RARE-EARTH PHOSPHATES

Rare-earth elements (REEs) have been in high demand since the twentieth century given their value to the technology on which our modern infrastructure relies. Consequently, minerals which host REEs, such as rare-earth phosphates, have high potential in the materials engineering industry for REE extraction. In addition, rare-earth phosphates enable geologic dating in the earth sciences as a result of their structural hosting of U and Th atoms. Therefore, it is critical to fully characterize the crystal structure and structural properties of rare-earth phosphates, and how they change with the hosted REE; however, there has been very little work done to constrain the high-pressure structural behavior of any rare-earth phosphate. Xenotime, which has the chemical formula YPO₄, is a mineral of great interest in the materials industry given its high molar content of yttrium, yet much is still unknown regarding its structural and material properties.

This study demonstrates the effect of pressure on the structural and lattice parameters of Y-hosted xenotime synthesized by L. Boatner of Oak Ridge National Laboratory. Xenotime crystallizes in space group I4₁/amd (#141) and consists of edge-sharing YO8 dodecahedra and PO4 tetrahedra forming chains that run parallel to [001]. A third-order Birch-Murnaghan equation of state fit to 15 P-V data points from 1 bar to 9.126(7) GPa yielded a bulk modulus K=143.5(6) GPa and dK/dP=7.0(2). Axial compressibilities are anisotropic with a approximately 2.4 times more compressible than c. Volume reduction of the structure is accommodated by bond length shortening in both the PO₄ tetrahedra and YO₈ dodecahedra. These results are compared with other rare-earth phosphates which host larger REEs (such as La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd) and crystallize in a monoclinic, monazite-type structure with space group P2₁/n. We show a linear relationship between the major-element substitution of rare-earth phosphates in both monazite- and xenotime-type rare-earth phosphate structures and their elastic properties.