Hydrothermal synthesis of lanthanide rhenium oxides: Structures and magnetism of Ln2Re2O7(OH) (Ln = Pr, Nd) and Ln4Re2O11 (Ln = Eu, Tb)

The reactions of Ln(2)O(3) (Ln = La-Lu) with ReO2 were examined in high temperature hydrothermal water (650 degrees C). In all cases the rhenium oxide was added in a three-fold excess. No external mineralizer was needed and the rhenate itself acted as a sufficient mineralizer to form high quality single crystals of several crystalline products. These form in good yield with compositions varying as a function of the size of the rare earth ion. The largest sesquioxide, La2O3, forms the well-known La4Re6O19 phase, but when Ln = Nd or Pr, a new product, Ln(2)Re(2)O(7)(OH), is isolated. Reactions with Ln= Sm-Tb form Ln(4)Re(2)O(11), and those with Dy-Lu form Ln(2)ReO(5). The new Ln(2)Re(2)O(7) (OH) series was characterized as a new structure type by single crystal X-ray diffraction. This structure features a tetrameric Re4O16 cluster embedded in a lanthanide oxide framework. Additional detailed structural data is also provided for Tb4Re2O11 and Eu4Re2O11, which were not previously reported for the Ln(4)Re(2)O(11) family. In Ln(2)Re(2)O(7)(OH), no direct Re-Re bond (Re-Re = 2.60 angstrom) is observed while Ln(4)Re(2)O(11) appears to possess a rhenium-rhenium double bond with a Re-Re distance of 2.42 angstrom. Single crystal Raman data supports both of these characterizations. Magnetic data is reported for Ln(2)Re(2)O(7)(OH) and Tb4Re2O11, and their behavior appears to be dominated by the f-element magnetic moments. At low temperatures Tb4Re2O11 displays possible canted antiferromagnetic coupling.