Synthesis and crystal structure of NaCsB5O8(OH)·H2O.

The solvothermal reaction of H3BO3, Cs2CO3, NaBO2·4H2O, formic acid and ethanol led to a new mixed alkali metal borate, NaCsB5O8(OH)·H2O, sodium caesium penta-borate hydroxide monohydrate. Its crystal structure contains penta-borate [B5O10(OH)]6- building units, and displays a layered structure containing nine-membered rings. The layers extend parallel to the ab plane and are stacked along the c axis. Na+ and Cs+ cations are situated between the layers and bond to oxygen atoms with coordination numbers of 7 and 10, respectively. The water mol-ecule likewise occupies the inter-layer space and is hydrogen-bonded to the layers, both as a donor and an acceptor.