Low-dimensional magnetic properties of orthorhombic ${\mathrm{MnV}}_2{\mathrm{O}}_6$: 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 ($800{}^{\circ }\mathrm{C}$). A negative Curie-Weiss temperature ${\theta }_{\mathrm{CW}}$ is observed, implying dominant antiferromagnetic interactions at high temperatures, in contrast to the positive ${\theta }_{\mathrm{CW}}$ 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.

Figure 1

Rietveld refinement of the x-ray diffraction ($\lambda =1.540\AA$) pattern recorded at room temperature for orthorhombic ${\mathrm{MnV}}_2{\mathrm{O}}_6$ obtained at high pressure. The Bragg positions at the bottom correspond to the $Pbcn$ space group, as indicated.

Figure 2

Comparison of temperature dependence of the dc susceptibility recorded at ${\mu }_{0}H=0.5$ T for (a) ${\mathrm{MnV}}_2{\mathrm{O}}_6$ polymorphs, and (b) $Pbcn$ isostructural ${\mathrm{MnV}}_2{\mathrm{O}}_6$ and ${\mathrm{MnNb}}_2{\mathrm{O}}_6$ compounds.

Figure 3

Isothermal magnetization curves recorded at the indicated temperatures for the orthorhombic form of ${\mathrm{MnV}}_2{\mathrm{O}}_6$.

Figure 4

Low-temperature specific heat of orthorhombic ${\mathrm{MnV}}_2{\mathrm{O}}_6$ in comparison to its monoclinic polymorph, and with ${\mathrm{MnNb}}_2{\mathrm{O}}_6$.

Figure 5

Rietveld refinement of the ND pattern recorded at 1.5 K for ${\mathrm{MnV}}_2{\mathrm{O}}_6$, showing structural and magnetic Bragg positions.

Figure 6

Three views of the crystal structure of ${\mathrm{MnV}}_2{\mathrm{O}}_6$ orthorhombic compound, with special focus on the zigzag chains formed by oxygen (red spheres) octahedra surrounding the Mn (pink spheres) atoms along the $c$ axis. Vanadium is shown as blue spheres.

Figure 7

Temperature dependence of the Mn magnetic moment for ${\mathrm{MnV}}_2{\mathrm{O}}_6$ as obtained from a Rietveld refinement of the ND patterns. The zero-magnetic-moment result at 5.5 K confirms the absence of long-range order at this temperature.