Neutron diffraction and μSR studies of two polymorphs of nickel niobate NiNb2O6

Neutron diffraction and muon spin relaxation (μSR) studies are presented for the newly characterized polymorph of NiNb2O6 (β-NiNb2O6) with space group P42/n and μSR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P42/n form was determined from neutron diffraction using both powder and single-crystal data. Powder neutron diffraction determined an ordering wave vector k⃗=(12,12,12). Single-crystal data confirmed the same k⃗ vector and showed that the correct magnetic structure consists of antiferromagnetically coupled chains running along the a or b axis in adjacent Ni2+ layers perpendicular to the c axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related trirutile phases, NiSb2O6 and NiTa2O6. μSR data finds a transition temperature of TN∼15K for this system, while the columbite polymorph exhibits a lower TN=5.7(3) K. Our μSR measurements also allowed us to estimate the critical exponent of the order parameter β for each polymorph. We found β =0.25(3) and 0.16(2) for the β and columbite polymorphs, respectively. The single-crystal neutron scattering data give a value for the critical exponent β =0.28(3) for β-NiNb2O6, in agreement with the μSR value. While both systems have β values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to be well described by S=1 spin chains, the interchain interactions in the β polymorph are likely much larger.