Low-dimensional magnetic properties of orthorhombic MnV2O6 : A nonstandard structure stabilized at high pressure

This paper presents the physical properties of a nonstandard orthorhombic form of ${\mathrm{MnV}}₂{\mathrm{O}}₆$, including a comparison with the isostructural orthorhombic niobate ${\mathrm{MnNb}}₂{\mathrm{O}}₆$, and with the usual ${\mathrm{MnV}}₂{\mathrm{O}}₆$ monoclinic polymorph. Orthorhombic ($Pbcn$) ${\mathrm{MnV}}₂{\mathrm{O}}₆$ is obtained under extreme conditions of high pressure (6.7 GPa) and high temperature ($800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$). A negative Curie-Weiss temperature ${\ensuremath{\theta}}_{\mathrm{CW}}$ is observed, implying dominant antiferromagnetic interactions at high temperatures, in contrast to the positive ${\ensuremath{\theta}}_{\mathrm{CW}}$ of the monoclinic form. Specific-heat measurements are reported down to 1.8 K for all three compounds, and corroborate the magnetic-transition temperatures obtained from susceptibility data. Orthorhombic ${\mathrm{MnV}}₂{\mathrm{O}}₆$ presents a transition to an ordered antiferromagnetic state at ${T}_N=4.7$ K. Its magnetic structure, determined by neutron diffraction, is unique among the columbite compounds, being characterized by a commensurate propagation vector $\mathbf{k}=\left(0,0,\frac{1}{2}\right)$. It presents antiferromagnetic chains running along the $c$ axis, but with a different spin pattern in comparison to the chains observed in ${\mathrm{MnNb}}₂{\mathrm{O}}₆$. By a comparative discussion of our observations in this three compounds, we are able to highlight the interplay between competing interactions and dimensionality that yield their magnetic properties.