Optical absorption spectroscopy of the P213-P212121 transformation in K2Co2(SO4)3 langbeinite

Crystal field spectra have been obtained from K₂Co₂(SO₄)₃ at a number of temperatures from 20 K to room temperature. The transition to the ⁴ T _1g(P) excited state is found to be split at all temperatures owing to the trigonal distortion of the Co site. Below the P2₁3-P2₁2₁2₁ phase transition temperature an excess splitting is observed owing to the additional distortion of the site in the orthorhombic phase. This excess distortion is found to be a linear function of temperature with no first order step at T _c. Thus on a local scale the transition appears to be second order. The trigonal splitting increases with increasing temperature above T_c; this is not expected from the Speer-Salje model of the transition mechanism, which predicted decreasing distortion of the oxygen octahedra with increasing temperature. We propose that the apparent increase of asymmetry is due to the off centring of the Co atom in the high temperature phase in a more regular environment.