<i>In‐situ</i> high temperature Raman spectroscopic study on the structural evolution of Na₂W₂O₇ from the crystalline to molten states

Abstract

In order to optimize the growth conditions for tungstate crystals, the structural evolution of ditungstate Na 2 W 2 O 7 from the crystalline to molten states during heating has been investigated by in‐situ high temperature Raman spectroscopic technique. The experimental temperature‐dependent Raman spectra showed that Na 2 W 2 O 7 crystal has not undergone any solid‐state phase transformations during heating process from 298 to 1013 K. In contrast to crystalline Na 2 W 2 O 7 , in which [WO 4 ] tetrahedra and [WO 6 ] octahedra coexist, the tungsten–oxygen groups in molten Na 2 W 2 O 7 have the form of (W 2 O 7 ) 2− anion composed of two [WO 4 ] by sharing their corner oxygen atoms. To validate the structural evolution of Na 2 W 2 O 7 above, Raman activity of vibrations of Na 2 W 2 O 7 crystal and its melt were calculated using density functional theory (DFT) and compared with the in‐situ Raman spectra of Na 2 W 2 O 7 . A non‐crystalline phase and an intermediate state of Na 2 W 2 O 7 were obtained by rapid quenching and relatively slow cooling of the homogeneous melt, respectively. Copyright © 2016 John Wiley &amp; Sons, Ltd.