Disordered lithium niobate rock-salt materials prepared by hydrothermal synthesis

Abstract

An investigation of the one-step hydrothermal crystallisation of lithium niobates reveals that reaction between Nb₂O₅ and aqueous LiOH at 240 °C yields materials with a disordered rock-salt structure where the metals are statistically distributed over the cation sites. This contrasts with the well-studied reaction between Nb₂O₅ and NaOH or KOH that produces ANbO₃ (A = Na, K) perovskites. Powder neutron diffraction shows that materials prepared at short reaction times and lower LiOH concentration (2.5 M) are lithium deficient and have a slight excess of niobium, but that at longer periods of reaction in 5 M LiOH, close to the ideal, stoichiometric Li_0.75Nb_0.25O composition is produced. Upon annealing this phase cleanly transforms into the known ordered rock-salt material Li₃NbO₄, a process we have followed using thermodiffractometry, which indicates that transformation begins at ∼700 °C. Solid-state ⁹³Nb and ⁷Li NMR of the disordered and ordered rock-salt phases shows that both contain single metal sites but there is clear evidence for local disorder in the disordered samples. For the ordered material, NMR parameters derived from experiment are also compared to those calculated using density functional theory and are shown to be in good agreement.