Abstract
${\mathrm{BiMn}}_{2}{\mathrm{O}}_{5}$ has been prepared in polycrystalline form by a citrate technique, and studied by neutron powder diffraction (NPD), specific-heat, and magnetization measurements. High-resolution NPD data show that ${\mathrm{BiMn}}_{2}{\mathrm{O}}_{5}$ is orthorhombic (space group $Pbam,$ $Z=4)$ and contains infinite chains of ${\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ octahedra sharing edges, linked together by ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{5}$ pyramids and ${\mathrm{BiO}}_{8}$ units. These units are strongly distorted with respect to those observed in other members of the $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ family $(R$ denotes rare earths), due to the presence of the electronic lone pair on ${\mathrm{Bi}}^{3+}.$ ${\mathrm{BiMn}}_{2}{\mathrm{O}}_{5}$ becomes magnetically ordered below ${T}_{\mathrm{N}}=39 \mathrm{K}.$ The magnetic structure, investigated from low-temperature NPD data, is defined by the propagation vector $\mathbf{k}=(1/2,0,1/2).$ A group theory analysis has been carried out to determine the possible solutions for the magnetic structure. For the ${\mathrm{Mn}}^{3+}$ ions located at the $4h$ site, the magnetic moments are ordered according to the basis vectors ${(F}_{x}{,C}_{y},0);$ for the ${\mathrm{Mn}}^{4+}$ ions placed at the $4f$ site, the basis vectors are ${(G}_{x}^{\ensuremath{'}}{,A}_{y}^{\ensuremath{'}},0).$ At $T=1.6 \mathrm{K},$ the magnetic moments for ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ cations are $3.23(6){\ensuremath{\mu}}_{\mathrm{B}}$ and $2.51(7){\ensuremath{\mu}}_{\mathrm{B}},$ respectively. The commensurate character of the magnetic structure of ${\mathrm{BiMn}}_{2}{\mathrm{O}}_{5},$ in contrast with the incommensurate structure observed for other compounds of the $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ family, for which $\mathbf{k}{=(1/2,0,k}_{z}),$ is discussed in terms of competing antiferromagnetic interactions, depending on the observed variation of crucial Mn-O-Mn distances.
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