Structure of cobalt doped K2Cd2(SO4)3 langbeinite at three temperatures above the P213-P212121 phase transition,and a new trigger mechanism for the ferroelastic transformation

X-ray structure determinations of Langbeinite type K₂(Cd_1-xCo_x)₂(SO₄)₃, x≅0.02 at three temperatures (440, 540 and 640 K) above the P2 ₁3-P2 ₁2₁2₁ transition temperature (434 K) reveal that the M ²⁺ (M ²⁺=Cd) ion is displaced from the centre of the octahedron at all temperatures in the cubic phase. Simultaneously the distortion of the oxygen framework decreases with increasing temperature. The structural phase transition occurs when the bond lengths of the six bonds in each of the M ²⁺ octahedra are all equal, and it is proposed that this equalisation of bond lengths acts as the trigger for the phase transition. The structural deformation of the oxygen sublattice is such that rather regular octahedra around Cd occur at very high temperatures with Cd displaced from the centre. With decreasing temperature the octahedra distort under conservation of the triad, such that the differences between the various bond lengths Cd-O decrease. The phase transition occurs when all bond lengths around the Cd position become equal. The behaviour of the oxygen framework and the offcentring of the Cd/Co atom combine to produce an increasing distortion with increasing temperature as viewed by the central atom. Thus the interpretation of Optical Spectra, in which an increase in line splitting with temperature was observed, as being due to the off-centring of the Co, is confirmed.