Synthesis and Crystal Structure of the distorted perovskite Sr_0.97NbO₃, determined by high resolution neutron diffraction

A new perovskite related Sr0.97NbO3 phase has been synthesized. Although both powder X-ray diffraction and selected area electron diffraction studies suggest a primitive cubic perovskite structure with a(p)=4.023 Angstrom, high resolution powder neutron diffraction reveals a subtle lattice distortion from cubic symmetry. The detailed crystal structure has been refined with the orthorhombic space group P2(1)2(1)2 with a = 5.6881 Angstrom, b = 5.6821 Angstrom and c = 8.0566 Angstrom. The structure is built up from two types of NbO6 octahedra, one elongated, the other compressed along c. The lattice distortion from cubic symmetry has been found to mainly originate from tilting of NbO6 octahedra, whereas the root 2a(p) x root 2a(p) x 2a(p) superstructure arises from ordering of alternate elongated and compressed octahedra.