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

Neutron diffraction and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) studies are presented for the newly characterized polymorph of ${\mathrm{NiNb}}_{2}{\mathrm{O}}_{6} (\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6})$ with space group ${P4}_{2}/n$ and $\ensuremath{\mu}\mathrm{SR}$ data only for the previously known columbite structure polymorph with space group $Pbcn$. The magnetic structure of the ${P4}_{2}/n$ form was determined from neutron diffraction using both powder and single-crystal data. Powder neutron diffraction determined an ordering wave vector $\stackrel{P\vec}{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$. Single-crystal data confirmed the same $\stackrel{P\vec}{k}$ vector and showed that the correct magnetic structure consists of antiferromagnetically coupled chains running along the $a$ or $b$ axis in adjacent ${\mathrm{Ni}}^{2+}$ 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 trirutile phases, ${\mathrm{NiSb}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{NiTa}}_{2}{\mathrm{O}}_{6}$. $\ensuremath{\mu}\mathrm{SR}$ data finds a transition temperature of ${T}_{N}\ensuremath{\sim}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for this system, while the columbite polymorph exhibits a lower ${T}_{N}=5.7(3)$ K. Our $\ensuremath{\mu}\mathrm{SR}$ measurements also allowed us to estimate the critical exponent of the order parameter $\ensuremath{\beta}$ for each polymorph. We found $\ensuremath{\beta} =0.25(3)$ and 0.16(2) for the $\ensuremath{\beta}$ and columbite polymorphs, respectively. The single-crystal neutron scattering data give a value for the critical exponent $\ensuremath{\beta} =0.28(3)$ for $\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6}$, in agreement with the $\ensuremath{\mu}\mathrm{SR}$ value. While both systems have $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ polymorph are likely much larger.