Local structure of LuBO3 prepared via sol-gel process and its transformation with temperature: B-NMR and L-edge XAS studies

The present work deals with the temperature effect on the stability range of LuBO₃ prepared by sol-gel synthesis. Structural modifications from vaterite to calcite form versus the thermal treatments have been measured by X-ray diffraction and infrared spectroscopy. Several samples derived from the xerogel annealed at 400, 800 and 1200 °C were studied by ¹¹B solid state NMR and at the lutetium L_I-L_III edges by X-ray absorption spectroscopy to determine the local atomic arrangement around the rare earth and the boron ions. The predominance of BO₄ groups and the eightfold coordination of lutetium atoms in the LuBO₃ xerogel in samples treated up to 800 °C indicate an evolution of the xerogel to vaterite form after thermal treatment instead of the expected calcite form as established for samples prepared by solid state reaction.