Low-temperature phases of dicalcium barium hexakis(propanoate)

The structure determinations of phases (II) and (III) of barium dicalcium hexakis(propanoate) {or poly[hexa-μ₄-propanoato-bariumdicalcium], [BaCa₂(C₃H₅O₂)₆]_n} are reported at 240 and 130 K, respectively [phase (I) was determined previously by Stadnicka & Glazer (1980). Acta Cryst. B36, 2977-2985; our structure determination of phase (I) at room temperature is included in the supporting information]. In the high-temperature phase, the Ba²⁺ cation is surrounded by six carboxylate groups in bidentate bridging modes. In the low-temperature phases, five carboxylate groups act in bidentate bridging modes and one acts in a monodentate bridging mode around Ba²⁺. The Ca²⁺ cations are surrounded by six carboxylate O atoms in a trigonal antiprism in all the structures. The Ba²⁺ and Ca²⁺ cations are underbonded and significantly overbonded, respectively, in all the phases. The bonding of the Ba²⁺ cation increases slightly at the cost of the bonding of Ca²⁺ cations during cooling to the low-temperature phases. The phase transitions during cooling are accompanied by ordering of the ethyl chains. In room-temperature phase (I), all six ethyl chains are positionally disordered over two positions in the crossed mode, with additional splitting of the ethyl α- and β-C atoms. In phase (II), on the other hand, there are three disordered ethyl chains, one with positionally disordered ethyl α- and β-C atoms, and the other two with positionally disordered ethyl β-C atoms only, and in the lowest-temperature phase (III) there are four ordered ethyl chains and two disordered ethyl chains with positionally disordered ethyl β-C atoms only.