Neutron diffraction and 1H rigid lattice wide-line NMR studies of powder (K, Bi III, Bi V) pyrochlores

Abstract

The structure of an A cation-deficient pyrochlore (K, Bi) ⋍1.5Bi 2O ∼6(H 2O) ∼1 has been investigated by powder neutron diffraction using a deuterated sample. Main results of previous X-ray work are confirmed: 25% of A sites are empty; only a small amount of oxygen is found in 8(b) positions, a greater proportion being located in 32(e) positions which are displaced toward the A vacancy. The latter oxygen atoms belongs to a heavy water molecule, the deuterium atoms of which are distributed near 96(g) positions. Proton wide-line nuclear magnetic resonance experiments were performed at 4 K using an undeuterated sample. The experimental NMR spectrum has been simulated using three distinct models, all of them giving an acceptable chemical composition. Only one of these models gives the number of water molecules located in 32(e) sites in agreement with the neutron diffraction results. This same model reveals two other protonated species: water molecules, with short H-H distance, and OH groups belonging to the octahedral network.