KCo(H2O)2BP2O8·0. 48H2O and K0.17Ca0.42Co(H2O) 2BP2O8·H2O: Two cobalt borophosphates with helical ribbons and disordered (K,Ca)/H2O schemes

Abstract

The two title compounds, potassium diaqua-cobalt(II) borodiphosphate 0.48-hydrate and potassium-calcium(0.172/0.418) diaqua-cobalt(II) borodiphosphate monohydrate, were synthesized hydro-thermally. They are new members of the borophos-phate family characterized by ∞[BP 2O8]3- helices running along [001] and constructed of boron (Wyckoff position 6b, twofold axis) and phospho-rus tetra-hedra. The [CoBP2O8]- anionic frameworks in the two materials are structurally similar and result from a connection in the ab plane between the CoO4(H2O)2 coordination octa-hedra (6b position) and the helical ribbons. Nevertheless, the two structures differ in the disorder schemes of the K,Ca and H2O species. The alkali cations in the structure of the pure potassium compound are disordered over three independent positions, one of them located on a 6b site. Its framework is characterized by double occupation of the tunnels by water mol-ecules located on twofold rotation axes (6b) and a fraction of alkali cations; its cell parameters, compared with those for the mixed K,Ca compound, show abnormal changes, presumably due to the disorder. For the K,Ca compound, the K and Ca cations are on twofold axes (6b) and the channels are occupied only by disordered solvent water mol-ecules. This shows that it is possible, due to the flexibility of the helices, to replace the alkali and alkaline earth cations while retaining the crystal framework. © 2012 International Union of Crystallography.