Polymorphism of Niobium Phosphate Bronze Na 4 Nb 8 P 4 O 32 Deduced by the Packings of Structural Units: A Centrosymmetric Phase.

Herein, a new centrosymmetric phase Na 4 Nb 8 P 4 O 32 (referred to as CS-Na 4 Nb 8 P 4 O 32 ) was obtained by a molten salt method, which is a polymorph of niobium phosphate bronze Na 4 Nb 8 P 4 O 32 . CS-Na 4 Nb 8 P 4 O 32 displays high structural similarity to the noncentrosymmetric Na 4 Nb 8 P 4 O 32 phase (referred to as NCS-Na 4 Nb 8 P 4 O 32 ): Distorted NbO 6 octahedra are corner-coordinated to form ReO 3 -type layers, which are further joined together by isolated PO 4 tetrahedra. However, two polymorphous phases adopt different packings of structural units, resulting in distinct symmetries. NbO 3 layers and PO 4 tetrahedra are reversely arranged along the crystallographic a direction in CS-Na 4 Nb 8 P 4 O 32 , thereby producing a centrosymmetric structure. The reverse packing cancels out all contributions of dipole moments originating from the distorted NbO 6 octahedra; NCS-Na 4 Nb 8 P 4 O 32 exhibits the C2-rotation distribution of NbO 3 layers and PO 4 tetrahedra, thus generating a noncentrosymmetric and polar structure. The C2-rotation packing of structural units brings a constructive addition of the dipole moments, further obtaining large calculated independent second harmonic generation susceptibilities. The study of structural evolution deduced by the packings of structural units in polymorphous Na 4 Nb 8 P 4 O 32 might provide valuable insights into polymorphism and structural regulation.