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International Journal of Spectroscopy
Volume 2011 (2011), Article ID 128401, 6 pages
Review Article

Polarized Raman Spectra of

Laboratoire de l'État Solide, Faculté des Sciences de Sfax, B.P. 1171.3000, Sfax, Tunisia

Received 16 December 2010; Accepted 23 February 2011

Academic Editor: Rolf W. Berg

Copyright © 2011 M. Boujelbene and T. Mhiri. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The study by spectroscopie Raman relave to compound was interpreted and attributed one being based Theory of group and by comparison with others composed. The current studies of the polarised Raman spectra of give evidence that the disorder is indeed present in the ammonium alum. This is best manifested through the appearance of surplus bands in the spectral regions of vibrations of the sulphate anion.

1. Experimental

1.1. Preparation

Alum crystals were prepared by the slow evaporation of an aqueous solutions containing stoichiometric amounts of and salts.

1.2. Polarised Raman Spectra of
1.2.1. Factor Group Analysis

The ammonium alum, , belongs to a family of compounds with general formula (A is a univalent metal such as Na, K, Rb, and Cs; M is a trivalent metal: Al, Ce, In, Fe, Cr, Ir etc.; X is S or Se).

According to the crystallographic data [1], the ammonium alum crystallizes in the cubic structure Pa-3 (T6h), . The site symmetry of sulfate is C3, the trivalent cations occupy S6 sites, and the site symmetry of the two crystallographically distinct types of water molecules is C1. In the model the site symmetry of is S6. However, X-ray diffraction studies of ammonium alum by with C3 symmetry. Abdeen et al. [2] indicated the unusual situation of two possible orientations of the sulfate ion, and the site symmetry of is C3. Similar suggestions were deduced from neutron diffraction date by Abdeen et al. [3]. Factor group analysis using standard correlation method [4] has been carried out to determine the symmetries of the vibrations and to predict the IR and Raman active modes.

Excluding the acoustic modes, 227 normal modes are predicted.

These are distributed as follows: The and modes are Raman active, and Fu mode is infrared active.

The Au and Eu modes exposed by Suresh et al. [5] as being IR active are really inactive.

The contribution of the different groupments (SO4, NH4, and ) and the ion to the Raman active vibrations are given in Table 2.

Our polarized measurements were conducted on the (001) and (111) faces of the single crystal (Figure 1).

Figure 1: Photograph showing (a) face (001) and (b) face (111) things of .

Analysis of the tensor components shows that, on (001) face, the Fg modes were obtained in the geometry, the geometry gives both the Ag and the Eg modes. Eg mode can be observed when the crystal is oriented as in (111) face and incident and scattered beams are perpendicular , and in polarizations parallel geometry the Ag, Eg, and Fg modes can be observed.

The polarized Raman spectra recorded for these different geometries are shown in Figure 2, and the frequencies of the observed bands and their vibrational assignments are given in Tables 1 and 2 (the spectra were collected from 5 to 4000 cm−1).

Table 1: Active modes of .
Table 2: Correlation table of the octahedral.
Figure 2: Raman spectra of . The geometries and the corresponding mode symmetries are given in each diagram.
1.2.2. Results and Discussions

The internal modes of vibration of free ion with Td symmetry have average frequencies at 3033, 1685, 3134, and 1397 cm−1 for , , , and modes, respectively, [6].
The stretching vibrations ion and the water molecule were observed in the 2900 to 3500 cm−1. It was possible to separate the absorption bands observed in the Raman spectra of by comparison with the sample. The differences observed between the two spectra are most likely due to the vibrations. Indeed, the bands observed in the second at 1444 cm−1 in the polarisation and at 1453 in the polarisation, and not observed in , are most likely assignable to the asymmetric bending mode of . By similar reasoning we can deduce that the band observed in at 2442/2482 and at 2210/2473 cm−1 in the and polarisation, respectively, corresponds to the combinations of bending modes with the internal rotation mode . Appearance of these bands indicates that the ion does not rotate freely in the crystalline lattice, thus the ammonium ions form hydrogen bonds in the crystals [710]. The bands observed in the polarisation at 3134 cm−1 can be assigned to the asymmetric stretching (Ag) vibrations.

The vibrational frequencies of a free water molecule are at 3756 (), 3652 (), and 1595 cm−1 [11]. By comparison with , and , it was possible to identify the vibrational frequencies of water molecule in .
From group theoretical predictions, 18 lines () are expected in all orientation (Table 1).
In the orientation six bands are observed at 1620, 1726, 2856, 3103, 3356, and 3400 and are close to those (1678, 1735, 2900, 3085, 3360, and 3412 cm−1) reported by Abdeen et al. [3], for . In the geometrics (Fg symmetry) five bands are observed at 1617, 1738, 2936, 3365, and 3376 cm−1. Their analogues in appear at 1605, 1632, 2985, 3120, 3330, and 3380 cm−1. In the geometry (Eg symmetry) only three bands (from six predicted) are observed at 2908, 3072, and 3338 cm−1. The bands observed in each orientation at ~1700 cm−1 can be assigned to the HOH bending vibration . The bands observed in the 2800–3500 cm−1 region are assigned to the symmetric and asymmetric stretching modes of water molecules. The observed bands in the stretching and bending regions show the presence of two distinct water molecules in . The significant decrease in the frequencies of the stretching modes and the increase in frequencies of the bending modes from the free state values show that water molecules form hydrogen bonds of medium and strong strength [12, 13].
The librational modes of water are characteristic of coordinated water and fall in the 500–900 cm−1 region [14, 15]. This region is complicated by presence of strong absorptions of and , and the is Raman inactive Table 2. The low polarizability of water molecules makes these bands appear weak. Librational modes of water are assigned by comparison with [5], [15], and its partly deuterated analogues. There is no doublet that the bands observed in the 700–900 cm−1 region should be attributed to librations of water molecules, as done in all previous assignments [517].

The metal cation forms an octahedron with surrounding six water molecules. Under Oh symmetry, the ion has six modes of vibration , , , , , and. Out of these , , and are Raman active, and IR active, and inactive in both IR and Raman. Table 2 shows the correlation diagram between the free Al(H2O)6 group vibrations in Oh symmetry and the internal vibrations in Th factor group symmetry through the S6 one in the crystal.
Unambiguous assignments of these modes are difficult as they fall in the region of the anion bending modes, librational modes of water molecules, and external modes. However, a few of these modes are assigned by comparison with the bands observed for the complex in [5] and in similar compounds in [13]. The medium bands around 530 cm−1 are due to the mode of the ion. modes are observed as very weak bands around 400 cm−1. The triply degenerate symmetric bending mode () is observed with medium intensity in the Raman spectra, around 330 cm−1, being observed at similar frequencies (517, 416 and 386 cm−1) in the spectrum of potassium alum [18].

The four internal modes of a free ion with tetrahedral, Td symmetry are at 981 cm−1, 451 cm−1, 1104 cm−1, and 613 cm−1 [19]. Table 2 shows resultant of correlation diagram between the free SO4 group vibrations in Td symmetry and the SO4 internal vibrations in Th factor group symmetry through the C3 one in the crystal. In an ordered structure 15 Raman lines are predicted:(i)7 stretching modes: , ; , , ;(ii)8 deformation modes: ; .In the ordered lattice, group theory analysis (Table 1) predicts two bands arising from the symmetric stretching mode (Ag and Fg modes). This vibration that involves large changes in polarizability is observed as very intense bands at 990 cm−1 and medium bands at 975 cm−1: in the geometry (Ag and Fg symmetry), in the geometry (Eg symmetry), and in geometry (Ag, Eg, and Fg symmetry). The two bands should be attributed to Ag component to two types of sulfate anion, as done in all previous assignments (see [517]. The two bands with different intensity are also observed in the single crystal Raman spectrum of potassium alum for and orientations at 973 and 989 cm−1 [20]. These last authors assert that the two bands have some Ag symmetry, and there is no indication of a band with Fg symmetry. Although in the studied Raman spectrum of polycrystalline K-alum anly one very strong band is observed below 1000 cm−1 originating from the SO4 symmetry stretching mode (991 cm−1), the weak intensity in the orientation was attributed to spillover of the Ag intensity because the peak positions were identical in both orientations. Intensity ratio of the bands due to the two sites gave a higher value () than () calculated from X-Ray studies [20]. In the orientation where only the Eg modes are expected, two very weak bands, with equal intensity, are observed at 986 and 993 cm−1. The presence of two bands for the symmetric stretching mode (Fg) confirms the hypothesis of a disordered structure. The two Fg modes are observed at 985 and 968 cm−1 in the single crystal Raman spectrum of K-Alum [21]. For the symmetric bending mode , three Raman components are predicted in all polarization geometries. In the and orientations where only the Fg modes are expected, three bands are observed at 440, 460, and at 471 cm−1. Their analogues in and appear at (441, 458 and 480 cm−1) and (441, 458, and 475 cm−1), respectively. Two Eg modes ( geometry) are observed at 439 and 460 cm−1. Thus the number of deformation mode of SO4 shows very clearly the two different orientations of groupments in this structure.
From group theoretical predictions, five lines are expected in all orientation for the asymmetric bending mode . In the geometry (Eg symmetry) two bands are observed at 614 and 634 cm−1. In the geometry ( symmetry) three bands are observed at 615, 634, and 648 cm−1. The two bands appearing at 615 and 648 cm−1 are assigned to the Ag symmetry. The splitting of the Ag and Eg modes confirms the existence of two types of tetrahedral. In the geometry (Fg symmetry) three bands are observed at 610, 621, and 632 cm−1.
The asymmetric stretching mode is found in the 1000–1160 cm−1 region. Group theory analysis predicts five bands for this mode . In the geometry ( symmetry) as in the geometry (Fg symmetry) and in the geometry (Eg symmetry), three (1107, 1131, and 1189 cm−1), four (1034, 1092, 1102, and 1131 cm−1), and two (1134 and 1189 cm−1) bands are obtained, respectively. The bands observed at 1107 and 1131 cm−1, for the first orientation, can be associated to the Ag mode, and those at 1134 and 1189 cm−1 are attributed to the Eg mode. The four peaks observed in the second orientation are attributed to the Fg mode. In the geometry symmetry five bands are observed at 1099, 1101, 1131, 1134, and 1191 cm−1. In the polycrystalline spectrum of Raman are observed in the region of the modes, in the IR spectra of various alums [21], where modes predict [19] two intense doublet at 1100–1090 cm−1 and two less intense bands at 1070–1200 cm−1. These last were assigned to modes of the mirror sulfates [22]. Thus the number of modes observed in spectra gives evidence that the disorder is indeed in NH4-alum.
In conclusion, X-Ray diffraction studies indicated that the sulphate groups in alum is orientationally disordered. The current studies of the polarised Raman spectra give evidence that the disorder is indeed present in the ammonium alum. This is best manifested through the appearance of surplus bands in the spectral regions of vibrations of the sulphate anion.

External Modes
10 Ag, 10 Eg, and 21 Fg lines can be expected in the Raman low-frequency range, corresponding to external modes. Experimentally, 11 lines in the geometry ( symmetry), 5 lines in the geometry (Eg symmetry), and 10 lines in the geometry (Fg symmetry) indeed exist (Table 3). Each external mode may be dominated by the contributions from all cations. However, in certain frequency ranges the external modes may be dominated by contribution from one kind of cation (translations) or groups of atoms (rotation). The external modes are assigned by comparison with attribution proposed in and by considering the fact that the translator and libratory modes of ion occur at higher wave numbers than those of the ions. The librations of both and ions are found to be stronger in intensity than the corresponding translational modes. By relative data comparison to the ammonium and rubidium alum, the bands observed in the 53–61 cm−1 region, at 167, 186, and at ~245 cm−1, can be attributed to the translations and librations of ion, since these bands are not observed in the rubidium alum. By similar reasoning we can confirm the attribution of the band observed at 238 cm−1 in rubidium alum to the Rb–O stretching mode [518]. Thus, the other bands observed in the 90–150 cm−1 region can be assigned to translational modes and those in the 150–200 cm−1 region to rotational modes of [521].

Table 3: Raman band positions and assignments of .


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