Hydrothermal synthesis and structural characterization of several complex rare earth tantalates: LnTaO(OH) (Ln = La, Pr) and LnTaO(OH) (Ln = Pr, Nd).

Reactions are reported of early rare earth oxides, REO (RE = La, Pr, Nd) with TaO under hydrothermal conditions (650 °C, 1.5 kbar) in concentrated aqueous hydroxide (20-30 M KOH) as a mineralizer. Under various stoichiometries several members of two new structure types were isolated, LnTaO(OH) (Ln = La, Pr) and LnTaO(OH) (Ln = Pr, Nd). The analogous niobate LaNbO(OH) was also obtained. Both structure types were characterized by single crystal X-ray diffraction and contain pentavalent tantalatum oxide octahedra and complex rare earth oxide frameworks. The LnTaO(OH) structure type contains Ln-O and Ln-O building blocks and TaO octahedra in a 3-D framework. It contains a 3-D rare earth oxide framework formed by from zig-zag chains of rare earth oxides linking sheets of rare earth oxides. The tantalates form edge-shared TaO dimers occupying gaps in the rare earth oxide frameworks. The structure of LnTaO(OH) contains two types of 2-D rare earth oxide slabs built of seven and eight coordinate rare earth metals. The tantalate units form 2-D slabs through a multiple corner-sharing scheme of TaO octahedra. The LnTaO(OH) structure type has an interesting close structural relationship to the previously reported rare earth titanate LaTiO(OH), which is discussed. The presence of hydroxide in the lattice is confirmed by IR spectroscopy and the H atom locations are assigned unambiguously using bond valence sums.