Vibrational behavior of [formula omitted] ions included in K 2Zn(CrO 4) 2·2H 2O and crystal structure of K 2Zn(CrO 4) 2·2H 2O: A new structure type containing kröhnkite-type chains

Abstract

The crystal structure of K 2Zn(CrO 4) 2·2H 2O 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) Å 3, Z = 4, R 1 = 0.027 for 2405 F o > 4 σ( F o) and 78 variables). The structure is built up from ZnO 4(H 2O) 2 and CrO 4 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, K 2Zn(CrO 4) 2·2H 2O represents a new structure type, designated ‘type H’, with closest relationships to the triclinic ‘type A’ compounds. Vibrational spectra (infrared and Raman) of K 2Zn(CrO 4) 2·2H 2O as well as infrared spectra of mixed crystals K 2Zn(CrO 4) 2− x (SO 4) x ·2H 2O (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 SO 4 2 - guest ions matrix-isolated in K 2Zn(CrO 4) 2·2H 2O exhibit three infrared bands for ν 3 and one band for ν 1 as expected due to the low site symmetry C 1 of the host CrO 4 2 - ions. When the larger CrO 4 2 - ions are replaced by the smaller SO 4 2 - 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 K 2Zn(SO 4) 2·2H 2O (decreasing repulsion potential at the lattice sites). The energetic distortion of the SO 4 2 - guest ions included in K 2 Me(CrO 4) 2·2H 2O ( Me = Mg, Cd, Zn) as deduced from the values of Δ ν 3 (site-group splitting) and Δ ν max (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, Δ ν 3 have values of 73, 58 and 36 cm −1, respectively). The infrared spectroscopic experiments show that the SO 4 2 - guest ions included in the chromates are considerably less distorted than in the selenates with kröhnkite-type chains Na 2 Me(SeO 4) 2·2H 2O ( 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 (Δ ν 3 and Δ ν max of SO 4 2 - guest ions have values of 73 and 163 cm −1 in K 2Zn(CrO 4) 2·2H 2O and 116 and 207 cm −1 in Na 2Zn(SeO 4) 2·2H 2O).