In-situhigh temperature Raman spectroscopic study on the structural evolution of Na₂W₂O₇from 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 ) 2À 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.

In-Situ Studies of Structure Transformation and Al Coordination of KAl(MoO4)2 during Heating by High Temperature Raman and 27Al NMR Spectroscopies

“…Hence, more complete data on melt structure and properties will benefit possible future applications of M I M III (M VI O 4 ) 2 compounds with layered structure. The basic structure units and the form of cluster structures present in molten alkali metal molybdates/tungstates K 2 M n O 3 n +1 ( n = 1, 2, 3, 4) have been determined, in which Mo and W were found to be present only in tetrahedral coordination [ 11 , 12 , 13 , 14 ]. Although Voron’ko et al [ 15 , 16 ] pointed out earlier that the [WO 4 ] 2− group is more likely to be coordinated to rare earth ions than to alkali metal ions in mixed molten tungstates, the coordination chemistry and structural role of trivalent cations in molten M I M III (M VI O 4 ) 2 double molybdates/tungstates remain unclear, let alone the more complex situation of rare earth ions.…”

High Oxide Ion Conduction in Molten Na2W2O7

“…Thus, the simple tetrahedral dimer model is not able to satisfactorily describe Raman spectrum for the melt, and the conformation of 1D W 2 O 7 chains in the melt cannot been completely excluded either given the limited calculations did not fully considered the complex configurations that are randomly composed of disordered 1D W 2 O 7 chains. Therefore, the simple tetrahedral dimer model proposed by Wang et al is treated with cautions here. Here with both results of Miyake and ours in the present work taken into consideration, it is not too flimsy to propose that the conformation of the infinite W 2 O 7 chains are preserved in the molten Na 2 W 2 O 7 but highly activated owing to the disordered orientation showing part condensation with neighboring chains via sharing the oxygen between WO 4 tetrahedra; this transfers some WO 4 to WO 5 or WO 6 , and therefore increases the average coordination number for W atoms; the interchain condensation further enhances the disordering of mixed WO 4+ x polyhedra in the molten Na 2 W 2 O 7 .…”

Raman Spectroscopic Study of Coal Samples during Heating

“…This cooling allowed for the cleaning of the silica glass prior to measurement. Figure 6, it is clear that the peak position of the D band related to the low molecular weight carbon organics, while the intensity of the D band changed negligibly with the increasing temperature [26,27]. This implies an increase in the graphitization degree along with the increase in temperature.…”