Synthesis and study of the crystallographic and magnetic structure ofSeCoO3

Abstract

We describe the preparation of $\mathrm{Se}\mathrm{Co}{\mathrm{O}}_{3}$ under moderate pressure conditions $(3.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, starting from reactive $\mathrm{Se}{\mathrm{O}}_{3}{\mathrm{H}}_{2}$ and MO mixtures, contained in sealed gold capsules under the reaction conditions ($850\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for $1\phantom{\rule{0.3em}{0ex}}\mathrm{h}$). The sample has been studied by neutron powder diffraction (NPD) data, specific heat, and magnetization measurements. $\mathrm{Se}\mathrm{Co}{\mathrm{O}}_{3}$ is an orthorhombically distorted perovskite (space group $Pnma$), with unit cell parameters $a=5.9300(2)$, $b=7.5961(2)$, $c=5.0287(2)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Below ${T}_{N}\ensuremath{\approx}54\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ it experiences an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. Above the N\'eel temperature, a paramagnetic moment of $5.3\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{f.u.}$ (expected is 5.2 for ${\mathrm{Co}}^{2+}$ in $^{4}T_{1g}$) and ${\ensuremath{\theta}}_{\text{Weiss}}=\ensuremath{-}86.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are obtained from the reciprocal susceptibility. Below ${T}_{N}$, the magnetic reflections observed in the neutron patterns can be indexed with a propagation vector $\mathbf{k}=0$, thus the magnetic unit cell coincides with the chemical one. The magnetic structure is given by the basis vectors $({\mathbf{A}}_{\mathbf{x}},{\mathbf{G}}_{\mathbf{y}},\mathbf{0})$ and it remains stable down to $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The ${\mathrm{Co}}^{2+}$ moments are lying on the $(a,b)$ plane of the perovskite; the structure is noncollinear but noncanted. There exists perfect AFM interactions (180\ifmmode^\circ\else\textdegree\fi{}) between moments within the $(c,a)$ plane (via O2 oxygens), whereas the magnetic moments exhibit angles of about 90\ifmmode^\circ\else\textdegree\fi{} along the $\mathbf{b}$ axis (via O1 oxygens). At low temperatures $(2\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ the ${\mathrm{Co}}^{2+}$ ordered saturation moment is of $3.4\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$, which is consistent with high spin ${\mathrm{Co}}^{2+}$ $(3{t}_{2g}^{5}{e}_{g}^{2})$.