Crystal Structure and Phase Transformations of Sodium Diarsenate, Na4As2O7

Abstract

Sodium diarsenate at room temperature is monoclinic with lattice parameters a = 5.969(1), b = 10.126(1), c = 12.272(3) Å and β = 92.87(2)°. There are four formula units per cell with space group C2/c. The structure was refined by full-matrix least-squares to a final R value of 0.027 utilizing 1157 reflections measured on a Syntex [Formula: see text] automatic diffractometer. The anion, consisting of two AsO4 tetrahedra sharing a common oxygen atom, with a bridging angle of 123.5° at that oxygen atom, lies on a two-fold axis. The bridging As—O bond lengths are 1.783 Å and the average terminal As—O bond length is 1.658 Å uncorrected for thermal motion. Both Na ions show fivefold coordination to oxygen atoms. One of these sodium ions has a long interaction (2.852 Å) with the bridging oxygen atom of the anion. Na4As2O7 undergoes a series of phase transformations between room temperature and 220 °C, and a further major transformation near 680 °C.