Polarized Raman Spectra of NH_𝟒Al(SO_𝟒)_𝟐⋅𝟏𝟐H_𝟐O

Abstract

The study by spectroscopie Raman relave to compound NH₄Al(SO₄)₂⋅12H₂O 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 NH₄Al(SO₄)₂⋅12H₂O 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 NH₄Al(SO₄)₂⋅12H₂O crystals were prepared by the slow evaporation of an aqueous solutions containing stoichiometric amounts of (NH₄)₂SO₄ and Al₂(SO₄)₃⋅𝑥H₂O salts.

1.2. Polarised Raman Spectra of NH₄Al(SO₄)₂⋅12H₂O

1.2.1. Factor Group Analysis

The ammonium alum, NH₄Al(SO₄)₂⋅12H₂O, belongs to a family of compounds with general formula AM(XO₄)₂⋅12H₂O (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 (T⁶_h), 𝑍=4. 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 NH₄⁺ 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 NH₄⁺ 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: Γ_optic=42A_g(R)+42E_g(R)+50F_g(R)+46A_u(I)+46E_u(I)+51F_u(IR). (1) The A_g,E_g,andF_g modes are Raman active, and F_u mode is infrared active.

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

The contribution of the different groupments (SO₄, NH₄, and H₂O) and the Al³⁺ 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).

(a)

(b)

Figure 1 

Photograph showing (a) face (001) and (b) face (111) things of NH4Al(SO4)2⋅12H2O.

Analysis of the tensor components shows that, on (001) face, the F_g modes were obtained in the 𝑧(𝑥𝑦)𝑧 geometry, the 𝑧(𝑥𝑥)𝑧 geometry gives both the A_g and the E_g modes. E_g mode can be observed when the crystal is oriented as in (111) face and incident and scattered beams are perpendicular −𝑍(𝑋′𝑌′)𝑍, and in polarizations parallel −𝑍(𝑋′𝑋′)𝑍 geometry the A_g, E_g, and F_g 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 NH₄Al(SO₄)₂⋅12H₂O.

Table 2 

Correlation table of the Al(H₂O)₆³⁺ octahedral.

Figure 2 

Raman spectra of NH4Al(SO4)2⋅12H2O. The geometries and the corresponding mode symmetries are given in each diagram.

1.2.2. Results and Discussions

NH₄⁺ Vibrations
The internal modes of vibration of free NH₄⁺ ion with T_d symmetry have average frequencies at 3033, 1685, 3134, and 1397 cm⁻¹ for 𝜈₁(𝐴₁), 𝜈₂(E), 𝜈₃(F₂), and 𝜈₄(F₂) modes, respectively, [6].
The stretching vibrations NH₄⁺ 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 NH₄Al(SO₄)₂⋅12H₂O by comparison with the RbAl(SO₄)₂⋅12H₂O sample. The differences observed between the two spectra are most likely due to the NH₄⁺ vibrations. Indeed, the bands observed in the second at 1444 cm⁻¹ in the𝑍(𝑋𝑋)𝑍 polarisation and at 1453 in the 𝑍(𝑋′𝑌′)𝑍 polarisation, and not observed in RbAl(SO₄)₂⋅12H₂O, are most likely assignable to the asymmetric bending (𝜈₄) mode of NH₄⁺. By similar reasoning we can deduce that the band observed in NH₄Al(SO₄)₂⋅12H₂O at 2442/2482 and at 2210/2473 cm⁻¹ in the −𝑧(𝑥𝑥)𝑧 and −𝑍(𝑋′𝑋′)𝑍 polarisation, respectively, corresponds to the combinations of bending modes 𝜈₂ with the internal rotation mode 𝜈₆. Appearance of these bands indicates that the NH₄⁺ ion does not rotate freely in the crystalline lattice, thus the ammonium ions form hydrogen bonds in the crystals [7–10]. The bands observed in the 𝑧(𝑥𝑥)𝑧 polarisation at 3134 cm⁻¹ can be assigned to the asymmetric stretching 𝜈₃(A_g) vibrations.

H₂O Vibration
The vibrational frequencies of a free water molecule are at 3756 (𝜈₃), 3652 (𝜈₁), and 1595 cm⁻¹(𝜈₂) [11]. By comparison with RbAl(SO₄)₂⋅12D₂O, and (CH₃)₂NH₂Al(SO₄)₂⋅6H₂O, it was possible to identify the vibrational frequencies of water molecule in NH₄Al(SO₄)₂⋅12H₂O.
From group theoretical predictions, 18 lines (6A_g+6F_g+6E_g) are expected in all orientation (Table 1).
In the 𝑧(𝑥𝑥)𝑧 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], RbAl(SO₄)₂⋅12H₂O for RbAl(SO₄)₂⋅12H₂O. In the 𝑧(𝑥𝑦)𝑧 geometrics (F_g symmetry) five bands are observed at 1617, 1738, 2936, 3365, and 3376 cm⁻¹. Their analogues in RbAl(SO₄)₂⋅12H₂O appear at 1605, 1632, 2985, 3120, 3330, and 3380 cm⁻¹. In the 𝑍(𝑋′𝑌′)𝑍 geometry (E_g 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 𝜈₂(H₂O). 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 NH₄Al(SO₄)₂⋅12H₂O. 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 𝜈₄(SO₄) and 𝜈₁(Al(H₂O)₆), and the 𝜈₃(Al(H₂O)₆) 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 RbAl(SO₄)₂⋅12H₂O [5], CsAl(SO₄)₂⋅12H₂O [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 [5–17].

Al(H₂O)₆ Vibrations
The metal cation Al³⁺ forms an octahedron with surrounding six water molecules. Under O_h symmetry, the ion has six modes of vibration 𝜈₁(A_1g), 𝜈₂(E_g), 𝜈₃(F_1u), 𝜈₄(F_1u), 𝜈₅(F_2g), and𝜈₆(F_2u). Out of these 𝜈₁, 𝜈₂, and 𝜈₅ are Raman active, 𝜈₃ and 𝜈₄ IR active, and 𝜈₆ inactive in both IR and Raman. Table 2 shows the correlation diagram between the free Al(H₂O)₆ group vibrations in O_h symmetry and the Al(H₂O)₆ internal vibrations in T_h 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 Al(H₂O)₆³⁺ complex in RbAl(SO₄)₂⋅12H₂O [5] and in similar compounds in (CH₃)₂NH₂Al(SO₄)₂⋅6H₂O [13]. The medium bands around 530 cm⁻¹ are due to the 𝜈₁ mode of the ion. 𝜈₂ modes are observed as very weak bands around 400 cm⁻¹. The triply degenerate symmetric bending mode (𝜈₅) 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].

SO₄²⁻ Vibration
The four internal modes of a free SO₄²⁻ ion with tetrahedral, T_d symmetry are at 𝜈₁(A₁) 981 cm⁻¹, 𝜈₂(E) 451 cm⁻¹, 𝜈₃(F₂) 1104 cm⁻¹, and 𝜈₄(F₂) 613 cm⁻¹ [19]. Table 2 shows resultant of correlation diagram between the free SO₄ group vibrations in T_d symmetry and the SO₄ internal vibrations in T_h factor group symmetry through the C₃ one in the crystal. In an ordered structure 15 Raman lines are predicted:(i)7 stretching modes: 2𝜈₁(1A_g, 1F_g); 5𝜈₃(1A_g, 1E_g, 3F_g);(ii)8 deformation modes: 3𝜈₂(1E_g,2F_g); 5𝜈₄(1A_g,1E_g,3F_g).In the ordered lattice, group theory analysis (Table 1) predicts two bands arising from the symmetric stretching mode 𝜈₁(A_g and F_g 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 𝑧(𝑥𝑥)𝑧 geometry (A_g and F_g symmetry), in the 𝑧(𝑥𝑦)𝑧 geometry (E_g symmetry), and in 𝑍(𝑋′𝑋′)𝑍 geometry (A_g, E_g, and F_g symmetry). The two bands should be attributed to A_g component to two types of sulfate anion, as done in all previous assignments (see [5–17]. The two bands with different intensity are also observed in the single crystal Raman spectrum of potassium alum for 𝑥(𝑧𝑧)𝑦 and 𝑥(𝑥𝑦)𝑥 orientations at 973 and 989 cm⁻¹ [20]. These last authors assert that the two bands have some A_g symmetry, and there is no indication of a band with F_g 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 𝜈₁(991 cm⁻¹), the weak intensity in the 𝑧(𝑥𝑦)𝑧 orientation was attributed to spillover of the A_g intensity because the peak positions were identical in both orientations. Intensity ratio of the bands due to the two sites gave a higher value (𝑝₁/𝑝₂∼9.9) than (𝑝₁/𝑝₂∼5) calculated from X-Ray studies [20]. In the 𝑍(𝑋′𝑌′)𝑍 orientation where only the E_g 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 𝜈₁(F_g) confirms the hypothesis of a disordered structure. The two F_g modes are observed at 985 and 968 cm⁻¹ in the single crystal Raman spectrum of K-Alum [21]. For the symmetric bending mode 𝜈₂, three Raman components (1E_g+2F_g) are predicted in all polarization geometries. In the 𝑧(𝑥𝑥)𝑧 and 𝑍(𝑋′𝑌′)𝑍 orientations where only the F_g modes are expected, three bands are observed at 440, 460, and at 471 cm⁻¹. Their analogues in RbAl(SO₄)₂⋅12H₂O and KAl(SO₄)₂⋅12H₂O appear at (441, 458 and 480 cm⁻¹) and (441, 458, and 475 cm⁻¹), respectively. Two E_g modes (𝑧(𝑥𝑦)𝑧 geometry) are observed at 439 and 460 cm⁻¹. Thus the number of deformation mode 𝜈₂ of SO₄ shows very clearly the two different orientations of SO₄²⁻ groupments in this structure.
From group theoretical predictions, five lines (1A_g+1E_g+3F_g) are expected in all orientation for the asymmetric bending mode 𝜈₄. In the 𝑍(𝑋′𝑌′)𝑍 geometry (E_g symmetry) two bands are observed at 614 and 634 cm⁻¹. In the 𝑧(𝑥𝑥)𝑧 geometry (A_g+E_g 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 A_g symmetry. The splitting of the A_g and E_g modes confirms the existence of two types of tetrahedral. In the 𝑧(𝑥𝑦)𝑧 geometry (F_g symmetry) three bands are observed at 610, 621, and 632 cm⁻¹.
The asymmetric stretching mode 𝜈₃ is found in the 1000–1160 cm⁻¹ region. Group theory analysis predicts five bands for this mode (1A_g+1E_g+3F_g). In the 𝑥(𝑧𝑧)𝑥 geometry (A_g+E_g symmetry) as in the 𝑥(𝑧𝑦)𝑥 geometry (F_g symmetry) and in the 𝑍(𝑋′𝑌′)𝑍 geometry (E_g 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 A_g mode, and those at 1134 and 1189 cm⁻¹ are attributed to the E_g mode. The four peaks observed in the second orientation are attributed to the F_g mode. In the 𝑍(𝑋′𝑋′)𝑍 geometry (A_g+E_g+F_g) 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 𝜈₃(SO₄) modes, in the IR spectra of various alums [21], where 3𝜈₃(SO₄) 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 𝜈₃(SO₄) modes of the mirror sulfates [22]. Thus the number of 𝜈₃(SO₄) 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 NH₄⁺ 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 A_g, 10 E_g, and 21 F_g lines can be expected in the Raman low-frequency range, corresponding to external modes. Experimentally, 11 lines in the 𝑧(𝑥𝑥)𝑧 geometry (A_g+E_g symmetry), 5 lines in the 𝑍(𝑋^′𝑌^′)𝑍 geometry (E_g symmetry), and 10 lines in the 𝑥(𝑧𝑦)𝑥 geometry (F_g 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 RbAl(SO₄)₂⋅12H₂O and by considering the fact that the translator and libratory modes of NH₄⁺ ion occur at higher wave numbers than those of the SO₄²⁻ ions. The librations of both NH₄⁺ and SO₄²⁻ 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 NH₄⁺ 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 [5–18]. 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 SO₄²⁻ [5–21].

Table 3 

Raman band positions and assignments of NH₄Al(SO₄)₂⋅12H₂O.