Characterization of a series of sodium molybdate structures by two-dimensional Raman correlation analysis

This paper demonstrates the use of two-dimensional (213) correlation spectroscopy in the characterization of the sodium molybdate structures that include and are intermediate between the hexagonal and orthorhombic forms; this series of sodium molybdates is produced by varying the preparation conditions, particularly the acid concentration and the heat-treatment temperature. In order to verify the assignments of the vibrational modes of hexagonal sodium molybdate, 2D correlation analysis was applied to the acid concentration-dependent Raman spectra. We can confirm from 2D Raman correlation analysis that this series of preparation conditions results in a series of sodium molybdates including and intermediate between the hexagonal and orthorhombic forms. Furthermore, 2D Raman correlation analysis elucidates the sequence of the structural transformation of hexagonal sodium molybdate into the orthorhombic form. With progression within the series from the hexagonal to the orthorhombic structures, the intensities of the oxygen-molybdenum stretching bands in sodium molybdate change in the following sequence: 973 cm(-1) (terminal OMo in the hexagonal structure, decrease in intensity) --> 996 cm(-1) (terminal OMo in the orthorhombic structure, increase in intensity) --> (897, 881) cm(-1) (corner OMo2 in the hexagonal structure, decrease in intensity) --> 664 cm(-1) (edge OMo3 in the orthorhombic structure, increase in intensity) --> 820 cm(-1) (comer OMo2 in the orthorhombic structure, increase in intensity) --> 692 cm(-1) (edge OMo3 in the hexagonal structure, decrease in intensity). This sequence implies that the changes in the comer OMo2 and edge OMo3 structures along the series of sodium molybdates are strongly correlated. Copyright (C) 2003 John Wiley Sons, Ltd.