Polarized Raman Spectra of

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