Speetrochimioa Acts, Vol. 27A, pp. 967 to 960. Pergamon Press 1971. Printed in Northern Ireland

The Raman spectra of lKgMoO,*

P. J. MILLER Chemistry Department, University of Maryland

College Park, Maryland 20742, U.S.A.

(Received 26 Augwt 1970)

Abstract-The Raman spectra of a single crystal of a MgMoO, haa been recorded and the results are discussed.

THERE have been many Reman and infrared studies of compounds containing the molybdate ion, particulary those with the scheelite structure [l]. However, very little work has been reported on the metal molybdates (Mg, Mn, Zn, Co and Ni) which form monoclinic crystals. The oxygen atoms in these crystals are co-o&mated around the metal atoms in either regular or distorted octahedra or tetrahedra. The reported infrared spectra of mulls of some of these compounds [ 1, 21 were shown to differ markedly from those with the scheelite structure, but no assignments were attempted. In this report, the Reman spectra of a single crystal of MgMoO, are presented and analyzed.

The complete structure of MgMoO, has not been reported but it is known that this crystal, on the basis of similar diffraction patterns and cell parameters, is isomor- phous with MnMoO, [3,4]. The crystal is monoclinic with space group Ca,,,,(Ca2) and has 8 molecules per unit cell. It is of interest that the primitive cell contains 4

molybdenum atoms surrounded by slightly distorted oxygen tetrahedra in 2C, and 2C, sites. The optical spectra then should correspond to these two different molyb- denum-oxygen arrangements.

In using the standard group theory analysis to predict the correct number of active modes for each symmetry species of the crystal’s factor group, the analysis can be carried out by correlation of the MoOJ2- and Mg2+ ions, even though discrete MoOd2- ions probably do not exist in the lattice. The analysis, however, is more correctly done by correlating the site group of each atom in the unit cell individually to the factor group. In either case the same total number of modes is predicted for each symmetry species. The results of the analysis are listed in Table 1.

A single crystal of MgMoO, was cut into a cube with its edges parallel to the mutually perpendicular axes of its optical indicatrix. The polarizability tensor assignments where then made on the basis of these axes defined as X, Y and 2. The Z-axis (which contains the unique crystallographic ‘b’ axis) was easily dis- tinguished from the X and Y axes (which contains in their plane the ‘a’ and ‘c’ crystallographic axes) by the observation of the spectra containing the 2X, ZY, XY polarizability components. The XY spectrum contained the symmetric stretch

* Supported by a Grant from ARPA, Department of Defense. [l] R. G. BROWN, J. DENNINO, A. H&LETT and S. D. Ross, Spectrodim. Acta WA, 963 (1970). [2] G. M. CL~LRK and W. P. DOYLE, Spctrochim. Acta 2f& 1441 (1906). [3] A. W. SLEIQET, B. L. C~ERLAND and J. F. WE-R, Iwrg. Chem. 7, 1093 (1968). [4] S. C. AEZ~AMS and J. M. REDDY, J. Chem. Phye. 45, 2533 (1965).

967

958 P. J. MIUJZR

Table 1. The factor group analysis of MgMoO,

Internal tetrahedral vibrations

C, site C, site

Mode

1.r. R 1.r. R

Vl 44 4 % 4

% 244 24 4, % 48-B,

% A,, 2B, A,, 233, A,, 24, 24,,B,

VP A,, 2B, A,, 2B, A,, 2% 24, B,

External lattice modes

1.r. R

6A,, 9% SA,, lOB,

Total active modes

1.r. R

14A,, 19B, 19A,, 17B,

v1 of the MoO,~- ion, whereas the 2X and ZY spectra did not. In general, birefrin- gence makes it difficult to obtain completely polarized spectra of monoclinic crystals. However, by exciting this crystal with the laser beam converging by -2’ and the scattered light collected from a solid angle of ~5’, excellent polarization was ob- tained with the residual bands from other polarizations in each spectra kept below 3%. The Reman spectra (Fig. 1) of the crystal of MgMoO, were recorded on a Spex model 1400 double monochromator equipped with an argon laser (4880 A, -200

mW). Right angle scattering geometries were utilized in obtaining the spectra with about 2 cm-l resolution.

The total number of modes predicted by the factor group analysis for the A, and B, symmetry species is observed (Table 2). The number of bands in the stretch- ing region of the spectra correspond to the number of y1 and y3 modes that were predicted for the MOO az- tetrahedra on the C, and C, sites. These were assigned on the basis of previous Raman studies1 which have y1 > v3 in intensity and frequency for the MoOd2- ion. Although they can not be specifically resigned to a specific site, the strong bands at 970 cm-l and 959 cm-l in the A, spectra are the symmetric stretches, vl. However, these modes are more than 50 cm-l higher than the v1 modes of MoO,B- ion tetrahedra. The other 6 bands in this region of the A, and B,, spectra are similar to the two split triply degenerate v3 modes, but the splittings are unusually large, up to 150 cm-l. No similarity exists in the bending region of these spectra to those of the MoO,z- ion, because the v2 and vq modes are mixed with lattice modes of equal intensity making the spectra very complex.

In conclusion, the Raman spectra of MgMoO, reveal that the observed vibrations are not those of the MoO,~- ion. The bending vibrations can not be separated from the lattice modes because of the mixing of their motions, which is presumably due to the

960 P. J. MXLLER

Vl 970 959

906 912 866 874 764 774

427 403 386 371 371 361 346 322 339 308 332 290 324 280 308 276

206 203 179 168 166 121

Lattice 119 108 90 90

71 42

strong inters&ion between the tetrshedre and to the coupling with the msgnesium- oxygen stretching modes. The interactions between the stretching motions of the tetrahedra result in the higher frequencies for the symmetric r1 modes tend in the large splittings of the degenerate Ye modes. The frequencies of these modes compare favorably to those described by B~PRR~CLOUGH et aE. [5] in their studies of molyb- denum-oxygen bonds. They assigned bands at 985 cm-r to those of independent molybdenum~xygen bonds and bands st 8’70 cm-l and 813 cm-l as arising from two different cont~uous molybdenum~xy~e~ bonds. It is, therefore, doubtful that the assignments of bands to tetrahedral modes means anything in this complex crystal despite the fact that the oxygens are tetrahedrally coordinated about the molyb- denum atoms.

[6] G. BARXACLOUUH, J. LEWIS and R. S. NYHOYX, J. Chem. Sot. 3652 (1969).