Structural design of alkali-metal titanates: electrochemical growth of KxTi8O16, Na2+xTi6O13, and Li2+xTi3O7 single crystals with one-dimensional tunnel structures

Abstract

Single crystals of lithium, sodium, and potassium titanates were grown with high-temperature constant-voltage electrolysis of TiO₂ with A₂MoO₄ (A = Li, Na, and K) as fluxes. Using the potassium-containing flux, needle-like crystals of hollandite-type K_xTi₈O₁₆ with a maximum length of ∼1 mm were successfully obtained. On the other hand, needle-like crystals of Na_2+xTi₆O₁₃ and ramsdellite-type Li_2+xTi₃O₇ with a maximum length of ∼3 mm were grown when the sodium- and lithium-containing fluxes were employed, respectively. This report demonstrates that the electrochemical crystal growth is favorable to obtain titanates with various one-dimensional tunnel structures upon varying the alkali-metal elements. The diversity in structural types of the resultant single crystals is reasonably explained in terms of the ionic size of the alkali metals incorporated.