Phase transitions in langbeinites I: Crystal chemistry and structures of K-double sulfates of the langbeinite type M 2 + + K2(SO4)3, M + +=Mg,Ni, Co,Zn, Ca |
| |
Authors: | D. Speer E. Salje |
| |
Affiliation: | 1. Institut für Kristallographie und Petrographie und SFB 173, Universit?t Hannover, Welfengarten 1, 3000, Hannover 1, West Germany
|
| |
Abstract: | The crystal structures of the langbeinite type M 2 + + K2(SO4)3 with M + +=Mg, Ni, Co, Zn, Ca in their cubic phase (P 2 1 3) and Ca2K2(SO4)3 in its orthorhombic phase (P 2 1 2 1 2 1) are determined. Whereas the SO4-tetrahedra in these compounds are almost undistorted, the two symmetry-independent coordination polyhedra of M + + are highly distorted octahedra with trigonal site symmetry in P 2 1 3. The deformation of the oxygen octahedra and the off-centering of M + + along the trigonal axis show systematic dependences on the ionic radii and the electronegativities of the M + +-ions. The correlations are remarkably different for the two symmetry-independent M + +-ions indicating different M + + — O bonding. The octahedral deformations show also linear correlations with the phase transition temperatures (P 2 1 3 — P 2 1 2 1 2 1) of the different compounds. This observation leads to a new model for the phase transition mechanism which is based on thermal instabilities of the M + + — O and K — O polyhedral distortions. The cubic high temperature phase is characterized by high symmetric oxygen coordinations around M + + which distort with decreasing temperature. At T c the trigonal site symmetry is broken in such a way that the K — O coordination becomes denser at the expense of a wider and less symmetric M + + — O coordination. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|