Revisiting the Crystal Structure of Rhombohedral Lead Metaniobate

Abstract

Lead metaniobate (PbNb₂O₆) can exist both as a stable rhombohedral and a metastable orthorhombic tungsten-bronze-type polymorph. Although the orthorhombic is a well-known ferroelectric material, the rhombohedral polymorph has been far less studied. The crystal structure and energetic stability of the stable rhombohedral polymorph of lead metaniobate is re-examined by powder X-ray diffraction and powder neutron diffraction in combination with ab initio calculations. We show that this structure is described by the polar space group <i>R</i>3, in contradiction to the previously reported space group <i>R</i>3<i>m</i>. The crystal structure is unusual, consisting of edge-sharing dimers of NbO_6/2 octahedra forming layers with 6- and 3-fold rings of octahedra and lead ions in channels formed by these rings. The layers are connected by corner-sharing between octahedra. Finally, the crystal structure is discussed in relation to other AB₂O₆ compounds with B = Nb, Ta