Lanthanide rhenium oxide single crystals from hydrothermal fluids: Synthesis and Structures of Ln2ReO5 (Ln = Pr, Nd), Ln3ReO7 (Ln = Gd and Tb) and Ln6ReO12 (Ln = Yb, Lu)

The phase space involving reactions of rare earth oxides and ReO2 was examined under high temperature hydrothermal conditions (650 ​°C/200 ​MPa), focusing on reactions containing either excess (3:1) or equimolar ratios of lanthanide oxides to ReO2. The products were generally isolated as high quality single crystals and characterized by single crystal X-ray diffraction, in many cases for the first time. The systematic variation of the size of the lanthanide ions had a significant effect on the final product distribution, and a wide range of materials was obtained. In particular, the synthesis of new members of the Ln2ReO5 series based on the larger lanthanides Pr and Nd enabled the structural transition point to be pinpointed between structure types having eclipsed and staggered Re2O8 dimers with a Re–Re triple bond. In the moderately sized lanthanides, a new monoclinic polymorph of Ln3ReO7 (Ln ​= ​Gd, Tb) was obtained, featuring alternating long and short Re–Re bonds in edge-sharing rhenium oxide chains. Among the smallest lanthanides, the Ln6ReO12 composition was characterized for Yb and Lu. A variety of rhenium oxidation states were observed and these were supported by XPS characterization.