Crystal structures and topological aspects of the high-temperature phases and decomposition products of the alkali-metal oxalates M2[C2O4] (M = K, Rb, Cs)

The high-temperature phases of the alkali-metal oxalates M-2[C2O4] (M=K, Rb, Cs), and their decomposition products M-2[CO3] (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): *alpha-K-2[C2O4], *alpha-Rb-2[C2O4], *alpha-Cs-2[C2O4], alpha-K-2[CO3], *alpha-Rb-2[CO3], and *alpha-Cs-2[CO3] in space group P6(3)/mmc; *beta-Rb-2[C2O4], *beta-Cs-2[C2O4], *beta-Rb-2[CO3], and *beta-Cs-2[CO3] in Pnma; gamma-Rb-2[C2O4], gamma-Cs[C2O4], gamma-Rb-2[CO3], and gamma-Cs-2[CO3] in P2(1)/c; and delta-K-2[C2O4] and delta-Rb-2[C2O4] 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 AlB2 family, and adopt either the AlB2 or the Ni2In 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[CO3] (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.