A new form of NaMnAsO(4)

A new form of NaMnAsO(4), sodium manganese(II) orthoarsenate, has been obtained under hydro­thermal conditions, and is referred to as the β-polymorph. In contrast to the previously reported ortho­rhom­bic α-polymorph that crystallizes in the olivine-type of structure and has one manganese(II) cation in a distorted octa­hedral coordination, the current β-polymorph contains two manganese(II) cations in [5]-coordination, inter­mediate between a square-pyramid and a trigonal bipyramid. In the crystal structure of β-NaMnAsO(4), four [MnO(5)] polyhedra are linked through vertex- and edge-sharing into finite {Mn(4)O(16)} units strung into rows parallel to [100]. These units are linked through two distinct orthoarsenate groups into a framework structure with channels propagating parallel to the manganese oxide rows. Both unique sodium cations are situated inside the channels and exhibit coordination numbers of six and seven. β-NaMnAsO(4) is isotypic with one form of NaCoPO(4) and with NaCuAsO(4).