Infra R Ed Specular Reflectance Spectra of Pressed Polycrystalline Samples of Alums − Comparison with Single Crystal Spectra

356 Geologica Macedonica, Vol. 34, No. 1, pp. 5–14 (2020) GEOME 2 In print: ISSN 0352 – 1206 Manuscript received: December 5, 2019 On line:-ISSN 1857 – 8586 Accepted: April 20, 2020 UDC: 548.75:[548.2:687.53.04 Original scientific paper INFRA R ED SPECULAR REFLECTANCE SPECTRA OF PRESSED POLYCRYSTALLINE SAMPLES OF ALUMS − COMPARISON WITH SINGLE CRYSTAL SPECTRA Miha Bukleski Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University in Skopje, Arhimedova 5, 1000 Skopje, Republic of North Macedonia [email protected], [email protected] A b s t r a c t: Vibrational properties of several alums were analyzed by employing polarized specular reflectance IR spectroscopy. A polycrystalline sample in a form of pellet was used for that purpose. Due to the cubic crystal system in which these crystals crystallize, the same equations for the reflectance and dielectric function were employed when conducting the dispersion analysis as those used for single crystals. Very good fit results were obtained for the investigated polycrystalline samples. Apart from this, dispersion analysis on single crystal reflectance spectra was also performed. Vibrational parameters, like the oscillator strength S, attenuation constant γ and the frequency of the transversal phonons ωt obtained from the fit of the polycrystalline samples, were compared with the corresponding data obtained from the dispersion analysis performed on single crystal specimens. The dielectric and optical properties of the alums were obtained and the LO–TO splitting for each of the modeled bands was calculated. Key words: alums; specular reflection spectroscopy; dispersion analysis; polycrystalline pressed pellets; optical and dielectric properties INTRODUCTION 2– Alums are a large group of compounds that O atom from the tetrahedral SO4 anion lying along have been studied extensively. Alums occur natural- the C3 symmetry axis (Cromer et al., 1967). ly in various minerals. Potassium alum, is found in The main reason for using the alums in this the minerals kalinite, alunite and leucite. These salts study is that their characteristics are adequate for have particular ability to form a vast family of this kind of analysis. The alums are relatively soft isomorphous compounds with a common formula (as all other crystals obtained from water solution) M'M'''(RY4)2∙12H2O, where M' stands for a uni- and they can be easily sintered without applying + + + + + + + valent ion: Li , Na , K , Rb , Cs , Tl , NH4 , temperatures higher than ambient. The main disad- + + + + CH3NH3 , N2H5 , NH3OH or C(NH2)3 ; M''' for: vantage for the reflection technique is that it be- 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ Al , Ga , In , Sc , Ti , V , Cr , Fe , Co , comes destructive (through preparation of a pallet) 3+ 3+ 3+ 3+ Mo , Ru , Rh and Ir ; R = S or Se when Y is O, from nondestructive (by using of a single crystal). and R = Be when Y = F. Alums crystallize in the The reflectance spectra of the alum single cubic crystal system adopting the space group 푃푎3̄ crystals have already been studied thoroughly and and having four formula units (Z = 4) per unit cell. the vibrational characteristics discussed Ivanovski There are three types of alums: α, β and γ (Lipson, et al., 1999a, 1999b; Petruševski et al., 2001. The 1935). In the structure of the alums, half of the water optical and dielectric properties have also been molecules are coordinated to the M' ion, and the obtained in the past (Ivanovski et al., 1999) by using other half are coordinated to the M''' ion, thus Kramers–Kronig method. However, this method making the metal ions octahedrally coordinated. In has its drawbacks (Kuzmany, 2009; Palik, 1985) the α alums the M' octahedron is being distorted and has to be applied with caution. while in the β alums the octahedron is almost regu- lar (Cromer et al., 1966). The γ type alums differ The results presented in this work are a good from both α and β in terms of the orientation of the base to extend the analysis for polycrystalline 6 M. Bukleski samples prepared in a certain way, enabling to ob- performing dispersion analysis. The alums used in tain the optical, dielectric and vibrational properties this work are listed in Table 1. out of polycrystalline samples. It is important to emphasize that this approach is just an approximation without taking into account the rigorous (and T a b l e 1 thus more complicated) theories of effective med- Corresponding abbreviations used for the twelve ium (Levy and Stroud, 1997). The concept of the synthesized single crystals and polycrystalline effective medium theories lies on the self-consistent samples used in this work, together procedure according to which particles from one of with the type of the alum the components are assumed to have usually spherical or ellipsoidal shape and are embedded in an Alum Abbreviation Type effective medium (EM). The properties of this so- NH4Al(SO4)2·12H2O АAlSD Α called effective medium are self-consistent (Brug- geman, 1937). According to the EM approximation, NH4Cr(SO4)2·12H2O ACrSD Α the particles have either spherical or ellipsoidal NH4Fe(SO4)2·12H2O AFeSD Α shape and have a random orientation in the matrix CsCr(SeO4)2·12H2O CsCrSeD Α that has effective optical and dielectric properties. CsFe(SO4)2·12H2O CsFeSD Β EM theories assume that the field around each particle inside the heterogeneous material is cons- CsGa(SeO4)2·12H2O CsGaSeD Α tant and both the matrix and the inclusions are KAl(SO4)2·12H2O KAlSD Α treated symmetrically or asymmetrically, according KCr(SO4)2·12H2O KCrSD Α to the approach, and they are both considered as particles (Pecharromán and Iglesias, 1994). RbAl(SO4)2·12H2O RbAlSD Α RbCr(SO4)2·12H2O RbCrSD Α In this work, polarized IR specular reflectance spectra from twelve single crystals and polycrys- RbFe(SO4)2·12H2O RbFeSD Α talline alums have been recorded and analyzed by TlAl(SO4)2·12H2O TlAlSD Α 2. EXPERIMENTAL One part of the investigated alums were com- mercially available, therefore the polycrystalline samples were prepared by recrystallization from their saturated water solutios. The other crystal samples were synthesized via slow evaporation out of water solutions from the corresponding salts (M'2SO4∙nH2O and M'''2(SO4)3∙mH2O) taken in stoichiometric ratio. The possibility for this kind of synthesis lays in the fact that double salts have a) lower solubility in water than the corresponding salts from which they are obtained. To avoid the possible efflorescence (since many alums are known to be efflorescent), the crystallization was done in refrigerator at temperature of about 5 °C. b) The morphology of the grown single crystals Fig. 1. Pellet made from polycrystalline sample of RbFeSD. resembled truncated octahedra with (111) planes All pellets were transparent (a) with glossy and reflective best developed for α and (210) for β alums. The surface (b). crystals were powdered in a vibrating mill for time ranging from 30 s to 45 s. Afterwards, the powder The reflectance spectra of single crystal alums was pressed at a pressure of nearly 1 GPa for 5 were previously recorded using Perkin-Elmer fixed minutes to form a pellet. After pressing, the pellets angle specular reflectance accessory with an inci- were transparent (Figure 1a) with glossy and highly dence angle of 6.5 and non-polarized incident ra- reflective surface (Figure 1b). diation in the region from 6000 to 370 cm–1. The Geologica Macedonica, 34, 1, 5–14 (2020) Infra red specular reflectance spectra of pressed polycrystalline samples of alums − comparison with single crystal spectra 7 pellets were used to obtain the IR specular reflec- spectra using the s-polarized radiation in the region tance spectra. These spectra were recorded using from 6000 to 400 cm–1. The s-polarization of the Perkin-Elmer fixed angle specular reflectance light employed in the investigations was obtained accessory with an incidence angle of 16 and s-po- by the use of a KRS-5 grid polarizer. larized radiation. The same technique (specular ref- Using incidence angle near normal (Ivanovski, lectance IR spectroscopy) was applied in both cases. et al., 1999a, 199b) have previously done, enables All spectra were recorded with 4 cm–1 resolution applying K–K relations for determination the using a Perkin-Elmer System 2000 FTIR spectro- optical and dielectric parameters. The already meter. obtained results for single crystals (as previously The main modification of the specular reflec- described) were used for comparison with the tance IR spectroscopy technique is about the way parameters obtained via dispersion analysis. The the sample is prepared for analysis. Instead of using reason for using the accessory with incidence angle single crystal, reflectance spectra from polycrystal- of 16 instead of the accessory with 6.5° (as for the line samples were recorded by placing the pellet in single crystals) when recording the polycrystalline the Perkin-Elmer specular reflecting accessory samples, was the lower quality of the spectra under an incidence angle of 16 and recording the obtained with the accessory having 6.5° angle. 3. RESULTS AND DISCUSSION The reflectance spectra of alums (being cubic number of oscillators present in the dielectric func- ̄ 6 and crystallizing in 푃푎3(푇ℎ ) space group, thus tion [given with Eq. (4)], was gradually increased optically isotropic), can be easily recorded, since the until a satisfactory fit was achieved (Figure 2). spectrum does not depend on the crystal surface The “good fit” results were also proven by the from which it is obtained. In order to calculate the standard deviation as a statistical parameter. The phase angle θ, K–K relations can be used. These number of oscillators that were used in the fitting relations are valid for normal incidence angles and procedure changes, depending on whether the alum they can be used when the incidence is near normal contains metallic or nonmetallic cation or whether it (Nussenzveig, 1972), as in the case of the single has sulphate or selenate anion.

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