Crystal Structures and Topological Aspects of the High‐Temperature Phases and Decomposition Products of the Alkali‐Metal Oxalates M2[C2O4] (M=K, Rb, Cs)

Abstract The high‐temperature phases of the alkali‐metal oxalates M 2 [C 2 O 4 ] (M=K, Rb, Cs), and their decomposition products M 2 [CO 3 ] (M=K, Rb, Cs), were investigated by fast, angle‐dispersive X‐ray powder diffraction with an image‐plate detector, and also by simultaneous differential thermal analysis (DTA)/thermogravimetric analysis (TGA)/mass spectrometry (MS) and differential scanning calorimetry (DSC) techniques. The following phases, in order of decreasing temperature, were observed and crystallographically characterized (an asterisk denotes a previously unknown modification): *α‐K 2 [C 2 O 4 ], *α‐Rb 2 [C 2 O 4 ], *α‐Cs 2 [C 2 O 4 ], α‐K 2 [CO 3 ], *α‐Rb 2 [CO 3 ], and *α‐Cs 2 [CO 3 ] in space group P 6 3 /mmc ; *β‐Rb 2 [C 2 O 4 ], *β‐Cs 2 [C 2 O 4 ], *β‐Rb 2 [CO 3 ], and *β‐Cs 2 [CO 3 ] in Pnma ; γ‐Rb 2 [C 2 O 4 ], γ‐Cs[C 2 O 4 ], γ‐Rb 2 [CO 3 ], and γ‐Cs 2 [CO 3 ] in P2 1 /c ; and δ‐K 2 [C 2 O 4 ] and δ‐Rb 2 [C 2 O 4 ] in Pbam . With respect to the centers of gravity of the oxalate and carbonate anions, respectively, the crystal structures of all known alkali‐metal oxalates and carbonates belong to the AlB 2 family, and adopt either the AlB 2 or the Ni 2 In arrangement depending on the size of the cation and the temperature. Despite the different sizes and constitutions of the carbonate and oxalate anions, the high‐temperature phases of the alkali‐metal carbonates M 2 [CO 3 ] (M=K, Rb, Cs), exhibit the same sequence of basic structures as the corresponding alkali‐metal oxalates. The topological aspects and order–disorder phenomena at elevated temperature are discussed.