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

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, β = 116.58(1)°, V = 939.9(3) Å3, Z = 4, R1 = 0.027 for 2405 Fo > 4σ(Fo) and 78 variables). The structure is built up from ZnO4(H2O)2 and CrO4 groups forming kröhnkite-type infinite octahedral-tetrahedral chains which are linked by K+ cations to layers and further to a three-dimensional framework. Hydrogen bonds are located within the chains as well as further interlink adjacent layers. Among the large group of compounds containing kröhnkite-type chains, K2Zn(CrO4)2·2H2O represents a new structure type, designated ‘type H’, with closest relationships to the triclinic ‘type A’ compounds.Vibrational spectra (infrared and Raman) of K2Zn(CrO4)2·2H2O as well as infrared spectra of mixed crystals K2Zn(CrO4)2−x(SO4)x·2H2O (where x   is approximately 0.05, 0.15 and 0.30) are presented and discussed in the region of the Cr–O and S–O stretching modes, respectively. The SO42- guest ions matrix-isolated in K2Zn(CrO4)2·2H2O exhibit three infrared bands for ν3 and one band for ν1 as expected due to the low site symmetry C1 of the host CrO42- ions. When the larger CrO42- ions are replaced by the smaller SO42- ions the mean values of the asymmetric stretches of the guest ions are shifted to lower frequencies as compared to that of the same ions in K2Zn(SO4)2·2H2O (decreasing repulsion potential at 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 Δν3 (site-group splitting) and Δνmax (the difference between the highest and the lowest wavenumbered components of the stretching modes) decrease in the order Zn > Cd > Mg as a result of the decreasing covalency of the respective Me–O bonds in the same order (for example, Δν3 have values of 73, 58 and 36 cm−1, respectively). The infrared spectroscopic experiments show that the SO42- guest ions included in the chromates are considerably less distorted than in the selenates with kröhnkite-type chains Na2Me(SeO4)2·2H2O (Me = Cd, Zn) due to both the weaker static field caused by the larger K+ ions as compared to that caused by the smaller Na+ ions and the smaller unit-cell volumes of the chromate compounds (Δν3 and Δνmax of SO42- guest ions have values of 73 and 163 cm−1 in K2Zn(CrO4)2·2H2O and 116 and 207 cm−1 in Na2Zn(SeO4)2·2H2O).