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 k = ( 1 2 , 1 2 , 1 2 ). Single crystal data confirmed the same k-vector and showed that the correct magnetic structure consists of antiferromagnetically-coupled chains running along the a or b-axes in adjacent Ni 2+ 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 tri-rutile phases, NiSb2O6 and NiTa2O6. µSR data finds a transition temperature of TN ∼ 15 K 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 gives 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 well-described by S = 1 spin chains, the interchain interactions in the β-polymorph are likely much larger.

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Single crystals of NiNb2O6 are grown in an optical floating zone furnace using a 1.2% molar excess of NiO in a sintered columbite rod as well as a pure O2 atmosphere and a growth rate of 25 mm/h.

Growing crystals of nickel niobate (NiNb 2 O 6 ), we noticed that changing growth 1 conditions allowed our material to enter different areas of the phase diagram. We also found that 2 excess material accumulated within and above the liquid zone. Analysis showed that this was an 3 excess of NiO. Changing the ratio of the constituent oxides -an excess of ∼4% of either NiO or 4 Nb 2 O 5 gave us the opportunity to investigate changes in zone stability, melting temperature and 5 quality of the resulting crystal. We found that a small excess of nickel oxide decreases the melting 6 temperature significantly, and created the best pseudo-rutile NiNb 2 O 6 crystal studied (Pbcn, space 7 group 60), while higher amounts of niobium oxide allowed us to stabilize the NiNb 2 O 6 columbite 8 phase (P4 2 /n, space group 80). This research reinforces the idea that self-flux as a travelling solvent 9 can significantly impact crystal growth parameters and quality.