${\mathrm{BiMn}}_2{\mathrm{O}}_5$ 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 $T_{\mathrm{N}}=39\mathrm{}\hspace{0.5em}\mathrm{K}.$ 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 ${(G}_x^{\prime }{,A}_y^{\prime },0).\mathrm{}$ At $T=1.6\mathrm{}\hspace{0.5em}\mathrm{K},$ the magnetic moments for ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ cations are $3.23\left(6\right)\mathrm{}{\mu }_{\mathrm{B}}$ and $2.51\left(7\right)\mathrm{}{\mu }_{\mathrm{B}},$ 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.

• Received 8 November 2001

DOI:https://doi.org/10.1103/PhysRevB.65.144423