KCo(H2O)2BP2O8.0.48H2O and K0.17Ca0.42Co(H2O)2BP2O8.H2O: two cobalt borophosphates with helical ribbons and disordered (K,Ca)/H2O schemes

The two title compounds, potassium diaquacobalt(II) borodiphosphate 0.48-hydrate and potassium-calcium(0.172/0.418) diaquacobalt(II) borodiphosphate monohydrate, were synthesized hydrothermally. They are new members of the borophosphate family characterized by infinity [BP2O8]3- helices running along [001] and constructed of boron (Wyckoff position 6b, twofold axis) and phosphorus tetrahedra. 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 octahedra (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 molecules 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 molecules. 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.