Lattice instabilities in Cs 2MFe(CN) 6 ( M = Mg 2+, Ca 2+, and Sr 2+): The crystal structure of Cs 2BaFe(CN) 6·2H 2O

Abstract

The room temperature Raman spectra of Cs 2 MFe(CN) 6 ( M = Mg 2+, Ca 2+, and Sr 2+) suggest that these salts undergo phase transformations similar to those found in Cs 2LiCr(CN) 6 where the distortion from the high-symmetry phase proceeds primarily along two modes of vibration. The distortion involves an antiferroelectric rotation of the hexacyanide moiety and a cesium translation. On the basis of the spectra a correlation has been made between the size of M and the apparent transition temperature. In going down the alkaline earths, the apparent transition temperature increases. The structure of the barium salt determined at room temperature shows the crystal latitce contains two waters of hydration. Many similarities have been found between Cs 2BaFe(CN) 6·2H 2O and the low-symmetry phase structure of Cs 2LiCr(CN) 6.