Hydrothermal synthesis and structural characterization of several complex rare earth tantalates: Ln2TaO5(OH) (Ln = La, Pr) and Ln3Ta2O9(OH) (Ln = Pr, Nd)

Abstract

Reactions are reported of early rare earth oxides, RE₂O₃ (RE = La, Pr, Nd) with Ta₂O₅ 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, Ln₂TaO₅(OH) (Ln = La, Pr) and Ln₃Ta₂O₉(OH) (Ln = Pr, Nd). The analogous niobate La₂NbO₅(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 Ln₂TaO₅(OH) structure type contains Ln–O₈ and Ln–O₉ building blocks and TaO₆ 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 Ta₂O₁₀ dimers occupying gaps in the rare earth oxide frameworks. The structure of Ln₃Ta₂O₉(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 TaO₆ octahedra. The Ln₃Ta₂O₉(OH) structure type has an interesting close structural relationship to the previously reported rare earth titanate La₅Ti₄O₁₅(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.