Crystal Structures of and Topological Aspects on the High Temperature Phases and the Decomposition Products of M₂C₂O₄ M=[K, Rb,Cs]

Robert E. Dinnebier¹, Sascha Vensky¹, Martin Jansen¹, Jonathan Hanson²

¹Max-Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
²Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, U.S.A.

The high temperature phases of the higher homologues of the alkali oxalates M₂C₂O₄ M=[K, Rb, Cs] and their decomposition products M₂CO₃ were investigated using fast angle dispersive X-ray powder diffraction with an image plate detector and DTA/DSC measurements. The following phases in order of decreasing temperature have been observed and crystallographicaly characterized. A (*) denotes a previously unknown modification: α-K₂[C₂O₄]*, α-Rb₂[C₂O₄]* (Fig. 1), α-Cs₂[C₂O₄]*, α-K₂[CO₃], α-Rb₂[CO₃]* (Fig. 1), α-Cs₂[CO₃]*: P6₃/mmc; β -K₂[C₂O₄]*, β -Rb₂[C₂O₄]* (Fig. 1), β-Cs₂[C₂O₄]*, β-Rb₂[CO₃]* (Fig. 1), β-Cs₂[CO₃]*: Pnma; β-K₂[CO₃]: C2/c; γ -K₂[C₂O₄], γ -Rb₂[C₂O₄] (Fig. 1), γ -Cs[C₂O₄], γ -K₂[CO₃], γ -Rb₂[CO₃] (Fig. 1), γ -Cs₂[CO₃]: P2₁/c; δ-K₂[C₂O₄], δ-Rb₂[C₂O₄] (Fig. 1): Pbam.

Figure 1

With respect to the center of gravity locations of the oxalate respectively carbonate dianions, the crystal structures of all known alkali- oxalates and – carbonates belong to the AlB₂ structure family, crystallizing either in the AlB₂ or in the Ni₂In structure type. Despite the different sizes of the carbonate- and oxalate dianions the high temperature phases of the alkali carbonates M₂CO₃ M=[K, Rb, Cs] show the same sequence crystal structures as the corresponding (isotypic) alkali oxalates. Topological aspects and order-disorder phenomena at elevated temperature are discussed. For data reduction and graphical visualization of large sets of powder patterns, a new software POWDER3D has been developed which is subject of another abstract.

Acknowledgments: This work was supported by the MPG, the FCI and the BMBF.