Powder neutron diffraction study of the nasicon-related phases NaxMIIxMIII2 –x(SO4)3 –y(SeO4)y: MII= Mg, MIII= Fe, In

Abstract

The structures of Na-containing nasicon-type phases of the form Na_xM^II_xM^III_{2 –x}(SO₄)_{3 –y}(SeO₄)_y(0 ⩽x⩽ 1.5, 0 ⩽y⩽ 3) at 25 °C have been refined using powder neutron diffraction. The data are consistent with a rhombohedral structure similar to that of Na_{1 +x}Zr₂Si_xP_{3 –x}O₁₂, but a reduction in symmetry (R compared to Rc) results in a division of the octahedral sites for both Na and M^II/M^III into two non-equivalent sets. For x⩽ 0.5, preferential occupation of one of the split Na sites is observed, and partial ordering of the M^II and M^III cations within the two available octahedral sites is indicated in some materials. At temperatures of 150 °C and above, subtle structural changes occur in NaMgFe(SO₄)₃. These changes imply monoclinic, C2/c, symmetry and appear responsible for the previously reported change in activation energy for ionic conductivity.