Neutron diffraction study of ${\mathrm{YVO}}_3$, ${\mathrm{NdVO}}_3$, and ${\mathrm{TbVO}}_3$

The structural and magnetic properties of ${\mathrm{YVO}}_3$, ${\mathrm{NdVO}}_3$ and ${\mathrm{TbVO}}_3$ were investigated by single-crystal and powder neutron diffraction. ${\mathrm{YVO}}_3$ shows a structural phase transition at $200\mathrm{K}$ from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group $P{2}_1∕b$. But supplementary high-resolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium $t_{2g}$ orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below $77\mathrm{K}$. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of ${\mathrm{NdVO}}_3$ and ${\mathrm{TbVO}}_3$ are closely similar to that of ${\mathrm{YVO}}_3$. However, only a single magnetic phase transition was found in the vanadium sublattice down to $9.5\mathrm{K}$. Below $60\mathrm{K}$ the magnetic moments of the ${\mathrm{Nd}}^{3+}$- and ${\mathrm{Tb}}^{3+}$-ions are gradually polarized by the ordered vanadium moments. Below $11\mathrm{K}$, we found a noncollinear order of the terbium moments.

Figure 1

Projection of the orthorhombic structure of ${\mathrm{YVO}}_3$ along the $c$-axis. The large circles are yttrium, the medium circles oxygen and the small filled circles vanadium. The yttrium atoms are at $z=\frac{1}{4}$ and $z=\frac{3}{4}$. In the drawing the network of distorted corner-shared ${\mathrm{VO}}_6$-octahedra in the ab-plane is shown. The octahedra are rotated mostly around the $b$- and $c$-axes and slightly around the $a$-axis.

Figure 2

Lattice parameters of vanadates $R{\mathrm{VO}}_3$ with ${\mathrm{GdFeO}}_3$-type structure as obtained from x-ray diffraction at room temperature (Ref. 5). The values of the yttrium compound are placed between the dysprosium and holmium compounds with respect to their ionic radii.

Figure 3

Temperature dependence of some prominent reflections of ${\mathrm{YVO}}_3$. Below $200\mathrm{K}$ a structural phase transition occurs from the orthorhombic (Pbnm) into the monoclinic structure $(P{2}_1∕b)$. The weak change of the intensity of the reflection 101 at $116\mathrm{K}$ indicates the $G$-type ordering along the $c$-axis. Below the second structural phase transition at about $75\mathrm{K}$ returning to the orthorhombic phase (Pbnm), a pure $G_z$-type ordering is observed.

Figure 4

Rietveld refinements of the neutron diffraction data of ${\mathrm{YVO}}_3$ collected at $5\mathrm{K}$. The positions of the nuclear $\left(\mathsf{N}\right)$ and magnetic reflections $\left(\mathsf{M}\right)$ as well as the difference pattern $\left(\mathsf{D}\right)$ are shown. Due to the $G$-type ordering of the vanadium moments, magnetic intensity was found on the positions of the forbidden and allowed reflections 011 and 101, respectively.

Figure 5

Temperature dependence of the magnetic reflections 010, 100, 012, and 102 of ${\mathrm{NdVO}}_3$. Below about $135\mathrm{K}$ the magnetic ordering of the vanadium moments begins. The increase of the intensities of 010, 100, and 012, as well the decrease of that of the 102, indicates the additional induced ordering of the neodymium moments.

Figure 6

Temperature dependence of the magnetic reflections 010 and 100 of ${\mathrm{TbVO}}_3$. In the upper diagrams a logarithmic scale has been used. At $9.5\mathrm{K}$ the intensity of the reflection 010 strongly increases due to the spontaneous magnetic order of the terbium moments ($C_x$ type). Between 10 and $40\mathrm{K}$ the terbium moments are slightly induced by the ordered vanadium moments. The magnetic order of the vanadium moments disappears at about $110\mathrm{K}$.

Figure 7

Magnetic structure of ${\mathrm{NdVO}}_3$ and ${\mathrm{TbVO}}_3$ at $9.5\mathrm{K}$. The magnetic moments of the metal atoms are aligned in the ab-plane. The large circles are neodymium or terbium, the medium circles oxygen and the small filled circles vanadium. The lanthanide atoms are at $z=\frac{1}{4}$ and $z=\frac{3}{4}$. In the drawing the network of distorted corner-shared ${\mathrm{VO}}_6$-octahedra in the ab-plane is shown.