Structural characterization of (Sm,Tb)PO4 solid solutions and pressure-induced phase transitions

Abstract Sm 1- x Tb x PO 4 solid solutions were synthesized and extensively characterized by powder X-ray diffraction, vibrational spectroscopy, and X-ray absorption spectroscopy. At ambient conditions solid solutions up to x  = 0.75 crystallize in the monazite structure, whereas TbPO 4 is isostructural to xenotime. For x  = 0.8 a mixture of both polymorphs was obtained. Moreover, a phase with anhydrite structure was observed coexisting with xenotime, which was formed due to mechanical stress. Selected solid solutions were investigated at pressures up to ∼40 GPa using in situ high pressure synchrotron X-ray diffraction and in situ high pressure Raman spectroscopy. SmPO 4 and Sm 0.5 Tb 0.5 PO 4 monazites are (meta)stable up to the highest pressures studied here. TbPO 4 xenotime was found to transform into the monazite structure at a pressure of about 10 GPa. The transformation of Sm 0.2 Tb 0.8 PO 4 xenotime into the monazite polymorph commences already at about 3 GPa. This study describes the reversibility of the pressure-induced (Sm,Tb)PO 4 xenotime-monazite transformation.