Small crystal distortion and long-range antiferro-orbital ordering in the spinel oxide CoV2O4

In frustrated spinel oxide $\mathrm{Co}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, the knowledge of orbital, lattice, and spin structures has been fragmentary thus far. To investigate the structural and magnetic phase transitions in $\mathrm{Co}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, we performed high-resolution neutron powder diffraction, single crystal synchrotron radiation diffraction, and magnetization and specific heat measurements for high-quality samples. Extremely small crystal distortion from cubic to tetragonal phase ($1\ensuremath{-}c/al0.06%$) was observed using the high-resolution neutron diffraction measurement below ${T}^{*}\ensuremath{\sim}95\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where a cusp was observed in the temperature dependence of magnetization. The single crystal diffraction experiment revealed that the structural phase transition accompanied by a change in the space group from $I{4}_{1}/amd$ to $I{4}_{1}/a$ occurred at ${T}_{2}=59\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where the phase transition was observed in the specific heat measurement. Crystal and magnetic structure analysis was carried out with the neutron data, and the results suggest that the magnetic phase transition from collinear to noncollinear magnetic ordering occurs at ${T}^{*}$ and the antiferro-orbital ordering occurs below ${T}_{2}$. We discuss the observed small crystal distortion and orbital characteristics in the light of the boundary between localization and delocalization.