Observation and Isolation of Layered and Framework Ytterbium Hydroxide Phases Using In Situ Energy-Dispersive X-ray Diffraction

In situ energy-dispersive X-ray diffraction has been used to investigate the synthesis of the recently reported lanthanide hydroxynitrate phases, Ln2(OH)5NO3·1.5H2O (Ln = Y, Er, Yb). Through control of the reaction temperature, three distinct layered phases with differing degrees of hydration (x = 1 (1), 1.5 (2), and 2 (3)) were observed in the Yb synthesis and subsequently isolated. Only the phase with x = 1.5 was observed for the reactions with Y and Er. The crystal structures of Yb2(OH)5NO3·2H2O (2) and Yb2(OH)5NO3·H2O (3) were determined in a single-crystal diffraction study and represent the first crystal structures of the new lanthanide hydroxynitrate phases. Both Yb2(OH)5NO3·2H2O and Yb2(OH)5NO3·H2O adopt orthorhombic structures in which the nitrate anion is coordinated to the Yb3+ cations within the hydroxide layer and are largely resistant to anion exchange. This is in contrast to the Ln2(OH)5NO3·1.5H2O phases, which undergo facile anion exchange reactions at room temperature, indicating that the nitrate resides in the interlayer gallery. At temperatures above 200 °C, the three layered phases were determined to be intermediates in the synthesis of Yb4O(OH)9NO3 (4), which has a framework structure with the uncoordinated nitrate anions located in one-dimensional channels. The in situ diffraction technique has allowed for a detailed kinetic and mechanistic investigation of the synthesis and revealed that the layered ytterbium hydroxynitrate phases form via a two-dimensional phase boundary-controlled growth mechanism. It was also possible to estimate the activation energies for the hydrothermal synthesis of Yb2(OH)5NO3·H2O and Yb2(OH)5NO3·2H2O from basic solution, as +29 and +67 kJ/mol, respectively.