Structure and real composition of undoped and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 single crystals

Abstract

Strontium barium niobate crystals with congruent melting composition Sr0.61Ba0.39Nb2O6 (SBN-61), both nominally pure and doped with Cr3+ и Ni3+ ions, have been investigated by neutron diffraction. Different strontium and barium contents as well as their different distribution over the Sr1, of Sr2 and Ba2 crystallographic sites of SBN-61 structure, caused by introduction of dopants, have been revealed. Coordination polyhedra of cations have been established based on the analysis of cation–anion internuclear distances together with the calculation of bond-valence sums for cations, which are equal to their formal charge. It was found that the Nb1 and Nb2 atoms are located in distorted octahedra with quadfurcated (the Nb1O6 polyhedron) or bifurcated (the Nb2O6 polyhedron) vertices, and the Sr1 atoms are located in a cuboctahedron with bifurcated vertices in the base plane. Different polyhedra have been revealed for the Sr2 and Ba2 atoms: Sr2 atoms are coordinated by 15 oxygen atoms to form a highly distorted five-capped pentagonal prism, whereas Ba2 atoms are located in a highly distorted three-capped trigonal prism with a coordination number 9. Comparison of interatomic and internuclear distances, determined by X-ray and neutron diffraction analyses, respectively, allowed to reveal a highly pronounced shift of electron density in Nb1 and Sr2 polyhedra, responsible for the covalent bond and properties of crystals. Location of Cr3+ и Ni3+ dopant ions in the SBN-61 structure as well as their formal charges has been discussed.