Vibrational spectra of hexaaqua complexes: VIII. The antisymmetric SO4 stretching bands in alums: LO–TO superior to correlation-field and site-group splitting
Introduction
Alums are double salts of the type MIMIII(RO4)2·12H2O, where MI is a univalent metal/group such as K, Na, Cs, Rb, NH4, CH3NH3, etc., MIII stands for a trivalent one: Al, In, Ga, Fe, V, Cr, etc., and R represents S or Se. All alums crystallize in the cubic system, space group Pa3 (Th6), with Z=4 ([1], [2], [3] and references therein). Half of the water molecules are coordinated to MIII, the other half being close to MI. The symmetry of the MI(H2O)6 and MIII(H2O)6 groups is S6 and that of the RO4 groups is C3.
From the structural point of view, alums are further divided into three types: α, β and γ. Alums of γ type differ from both α and β in the orientation of the tetrahedral anion along the C3 axes [2]. The differences between α and β types are best reflected through the geometry of the coordination polyhedron of water molecules around the univalent cation. In the case of α alums the coordination polyhedron is a distorted octahedron, while in β alums it is an almost regular hexagon. It is worth mentioning that a disorder of the sulfate group is found in many α alums, part of the SO4 groups adopting orientation as in γ alums ([4], [5], [6] and references therein).
Investigations concerning IR and Raman spectra have been reported many times ([6], [7], [8], [9], [10] and references therein), different parts of the spectrum being thoroughly analyzed. In the present work, attention is paid to the region of the antisymmetric stretching vibrations of the SO4 anion and, particularly, to the true nature of the doublet band which is clearly seen at LT. Reflection studies are also included, as these give information on the LO–TO splitting which is not predicted by unit-cell group theory.
Section snippets
Experimental
Single crystals of a number of alums were grown from aqueous solutions, of corresponding MI2RO4 and MIII2(RO4)3 salts. The spectra were recorded on a Perkin Elmer System 2000 FT-IR interferometer, using a Graceby Specac accessory for low temperature work and Perkin Elmer specular reflectance accessory for acquiring reflection spectra at near-normal incidence. All spectra were recorded with a 4 cm−1 resolution, taking 16 (32) background and 32 (64) sample scans. The OPD velocity was 0.2 cm s−1. The
Group-theoretical considerations
When analyzing the vibrational bands in solid phase samples, one may employ at least two levels of approximation. The first one is Halford’s site symmetry method [14] in the origin of which lies the oriented-gas model. The changes in the selection rules (band splitting, IR or Raman activity etc.) are governed only by the decrease in symmetry on going from the molecular point group to its subgroup (the site-group of the molecule in the crystal). The second method, somewhat more involved, is the
Results and discussion
Nothing but some band asymmetry is seen in the spectrum of RbAl(SO4)2·2H2O recorded at RT (Fig. 1(a)). However, band splitting is clearly observed in the LT spectrum (Fig. 1(b)).
What is the nature of this doublet? Is it because of Halford’s site-group splitting, or is it a collective effect? One way to answer this question is to employ isomorphous isolation, i.e., to study the IR spectra of sulfate ions isolated in a matrix of a selenate host. Under these circumstances, the sulfate ion retains
Acknowledgements
This work was sponsored by the Ministry of Science, Republic of Macedonia and the authors are grateful for the financial support.
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