A Raman spectroscopic and TEM study on the structural evolution of Na2Ti3O7 during the transition to Na2Ti6O13

Abstract

The synthesis of sodium hexatitanate from sodium trititanatewas characterized by Raman spectroscopy, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The structural evolution from trititanate to hexatitanate was studiedusingRamanspectra,XRDandHRTEMtechniques. Itwas found that theRamanbands at 279cm−1 corresponding to very long Ti–Obonds and at 883 cm−1 corresponding to the very short Ti–Obonds decrease in intensity and finally disappear during the transition from sodium trititanate to sodium hexatitanate. The band at 922 cm−1 corresponding to an intermediate-length Ti–O bond was observed to become stronger with the increase in temperature, indicating that there is no terminal oxygen atom in the crystal structure of Na2Ti6O13 and that all the oxygen atoms become linearly coordinated by two titanium atoms. Furthermore, the TiO6 octahedron in Na2Ti6O13 aremore regular because the very long (2.2 Å) or very short (1.7 Å) Ti–O bonds disappear. It is revealed that the phase transition from trititanate to hexatitanate is a step-by-step slipping process of the TiO6 octahedral slabs with the loss of sodium cations, and a new phase with formula Na1.5H0.5Ti3O7 has been discovered as an intermediate phase to interlink Na2Ti3O7 and Na2Ti6O13.