Structures, formation mechanisms, and ion exchange properties of α-, β-, and γ-Na2TiO3

alpha-, beta-, and gamma-Na2TiO3 were prepared from rutile TiO2 and molten NaOH. Three models of beta-Na2TiO3 with space groups of R (3) over bar, P (1) over bar, and P (3) over bar were proposed, and the R (3) over bar model was refined from the experimental data by using the Rietveld method. The structure of beta-Na2TiO3 is a superstructure of alpha-Na2TiO3 and supposedly contains Ti6O19 clusters. The structures of Na2TiO3 were mainly determined by the particle sizes of rutile and the reaction temperatures. alpha-Na2TiO3 could be prepared from fine rutile particles (D(50) < 25.8 mu m) and molten NaOH at 500 degrees C or quenching the melt of Na2TiO3 at 1000 degrees C quickly. gamma- and beta-Na2TiO3 were the thermodynamically stable phases of Na2TiO3 at around 500 degrees C and above 800 degrees C, respectively. alpha-Na2TiO3 was formed far beyond the thermodynamically stable state. The Na+ in alpha-Na2TiO3 was easier to exchange with H+ in water than that in beta or gamma phases. They all converted to amorphous phases after the 2nd, 6th, and 4th water washings at 25 degrees C, respectively. beta-Na2TiO3 followed similar paths of ion exchange as alpha-Na2TiO3, which was different from that of gamma-Na2TiO3.