Neutron Diffraction and $\hat{I}¼$SR Studies of Two Polymorphs of Nickel Niobate (NiNb${}_2$O${}_6$)

Abstract

Neutron diffraction and muon spin relaxation ($\hat{I}¼$SR) studies are presented for the newly characterized polymorph of NiNb${}_2$O${}_6$ (${\hat{I}}^2$-NiNb${}_2$O${}_6$) with space group P4${}_2$/n and $\hat{I}¼$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 $\vec{k}$ = ($\frac{1}{2},\frac{1}{2},\frac{1}{2}$). Single crystal data confirmed the same $\vec{k}$-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, NiSb${}_2$O${}_6$ and NiTa${}_2$O${}_6$. $\hat{I}¼$SR data finds a transition temperature of $T_N\sim$ 15 K for this system, while the columbite polymorph exhibits a lower $T_N=$ 5.7(3) K. Our $\hat{I}¼$SR measurements also allowed us to estimate the critical exponent of the order parameter ${\hat{I}}^2$ for each polymorph. We found ${\hat{I}}^2=$ 0.25(3) and 0.16(2) for the ${\hat{I}}^2$ and columbite polymorphs respectively. The single crystal neutron scattering data gives a value for the critical exponent ${\hat{I}}^2=$~0.28(3) for ${\hat{I}}^2$-NiNb${}_2$O${}_6$, in agreement with the $\hat{I}¼$SR value. While both systems have ${\hat{I}}^2$ 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 ${\hat{I}}^2$-polymorph are likely much larger.

10 pages, 12 figures