Vibrational behavior of SO42- -ions included in K2Zn(CrO4)2×2H2O and crystal structure of K2Zn(CrO4)2×2H2O: A new structure type containing kröhnkite-type chains
- Author(s)
- Manfred Wildner, Mitko Georgiev, Donka Stoilova
- Abstract
The crystal structure of K2Zn(CrO4)2_2H2O was determined from single-crystal X-ray diffraction data(monoclinic, space group C2/c, a = 15.046(2), b = 5.664(1), c = 12.333(2) Å, b = 116.58(1)_, V = 939.9(3) Å3,Z = 4, R1 = 0.027 for 2405 Fo > 4r(Fo) and 78 variables). The structure is built up from ZnO4(H2O)2 and CrO4groups forming kröhnkite-type infinite octahedral-tetrahedral chains which are linked by K+ cations tolayers and further to a three-dimensional framework. Hydrogen bonds are located within the chains aswell as further interlink adjacent layers. Among the large group of compounds containing kröhnkite-typechains, K2Zn(CrO4)2_2H2O represents a new structure type, designated ¿type H¿, with closest relationshipsto the triclinic ¿type A¿ compounds.Vibrational spectra (infrared and Raman) of K2Zn(CrO4)2_2H2O as well as infrared spectra of mixedcrystals K2Zn(CrO4)2_x(SO4)x_2H2O (where x is approximately 0.05, 0.15 and 0.30) are presented anddiscussed in the region of the Cr-O and S-O stretching modes, respectively. The SO42_ guest ionsmatrix-isolated in K2Zn(CrO4)2_2H2O exhibit three infrared bands for m3 and one band for m1 as expecteddue to the low site symmetry C1 of the host CrO42_ ions. When the larger CrO42_ ions are replaced by thesmaller SO42_ ions the mean values of the asymmetric stretches of the guest ions are shifted to lowerfrequencies as compared to that of the same ions in K2Zn(SO4)2_2H2O (decreasing repulsion potentialat the lattice sites). The energetic distortion of the SO42_ guest ions included in K2Me(CrO4)2_2H2O(Me = Mg, Cd, Zn) as deduced from the values of Dm3 (site-group splitting) and Dmmax (the differencebetween the highest and the lowest wavenumbered components of the stretching modes) decrease inthe order Zn > Cd > Mg as a result of the decreasing covalency of the respective Me-O bonds in the sameorder (for example, Dm3 have values of 73, 58 and 36 cm_1, respectively). The infrared spectroscopicexperiments show that the SO42_ guest ions included in the chromates are considerably less distortedthan in the selenates with kröhnkite-type chains Na2Me(SeO4)2_2H2O (Me = Cd, Zn) due to both theweaker static field caused by the larger K+ ions as compared to that caused by the smaller Na+ ionsand the smaller unit-cell volumes of the chromate compounds (Dm3 and Dmmax of SO42_ guest ions havevalues of 73 and 163 cm_1 in K2Zn(CrO4)2_2H2O and 116 and 207 cm_1 in Na2Zn(SeO4)2_2H2O).
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Technical University of Sofia, Bulgarian Academy of Sciences (BAS)
- Journal
- Journal of Molecular Structure
- Volume
- 892
- Pages
- 239-245
- No. of pages
- 7
- ISSN
- 0022-2860
- Publication date
- 2008
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105113 Crystallography, 1030 Physics, Astronomy
- Portal url
- https://ucris.univie.ac.at/portal/en/publications/vibrational-behavior-of-so42-ions-included-in-k2zncro422h2o-and-crystal-structure-of-k2zncro422h2o-a-new-structure-type-containing-kroehnkitetype-chains(0b13ab69-ceb3-40b0-a6f7-f0f5cd103668).html