Strong magnetoelastic coupling in VOCl: Neutron and synchrotron powder x-ray diffraction study

We present a combined neutron and synchrotron powder-diffraction study of the crystal and magnetic structures of VOCl. The occurrence of antiferromagnetic order in VOCl is accompanied by a monoclinic distortion. The high sensitivity of the magnetic interaction parameters on V-O-V bond angles and V-V distances yields strong magnetoelastic coupling which implies the structural distortion below $T_N$ as well as anomalous structural effects in the paramagnetic phase.

Figure 1

(Color online) Synchrotron-radiation powder x-ray diffraction patterns. (a) Rietveld fit (gray) to the data at 15 K (black) with an enlargement of the diffraction pattern around the $\left(\pm 212\right)$ profiles in the inset. (b) Temperature dependency of the $\left(\pm 212\right)$ and $\left(\pm 121\right)$ profiles.

Figure 2

(Color online) Nuclear and magnetic structures of VOCl. Yellow: Cl, blue: O, magenta: V ions, black: the structural unit cell. The directions of the magnetic moments at each V site is indicated by magenta arrows. Green (red) dotted lines indicate the antiferromagnetically (ferromagnetically) coupled V-V pairs in the AFM1 structure.

Figure 3

Results of synchrotron-radiation powder x-ray diffraction measurements: [(a)–(c)] lattice parameter, (d) monoclinic angle, (e) unit-cell volume, and (f) orthorhombic splitting $\epsilon =\left(a-b\right)/\left(a+b\right)$. (Lines are guide to the eyes.)

Figure 4

(a) Powder neutron-diffraction patterns measured at 2 K (D20 diffractometer). Circles: data points, lines: Rietveld fits to the data with the AFM1-AFM4 structure models. The area below the AFM1 fit (moment $\parallel a$) is filled (gray). (b) Fit of the D1A measurement at 10 K; black: data points, gray: Rietveld fit to the data, light gray: difference ${\text{I}}_{\text{obs}}{\text{-I}}_{\text{calc}}$, arrows indicate the magnetic peaks.

Figure 5

(Color online) (a)–(d) The possible irreducible representations in space group $P112/n$ for $k=\left(1/21/21/2\right)$ with magnetic atoms in the primitive unit cell at ($-0.25$, 0.25, $z_0$). For (a) and (b) the presented spin direction along $a$ indicates the direction of the ordered moment found in VOCl, which exhibits magnetic order according to the representation 1.

Figure 6

Results of powder neutron diffraction (black circles) and synchrotron measurements (gray triangles). (a) V-O distances for bonds in $a$ and $b$ directions. (b) V-Cl distance. (c) V-V distances for V ions within the same V bilayer in diagonal direction (red and green bonds in Fig. 1). (d) V-O-V bond angles for V ions (with the same value of $z$) in $a$ direction ${\left(\text{V-O-V}\right)}_a$ and in $b$ direction ${\left(\text{V-O-V}\right)}_b$. (e) Cl-V-Cl angle and V-O-V angle in diagonal direction. (f) $\Delta d{\left(\text{V-V}\right)}_{\text{diag}}$ denotes the difference of the two diagonal V-V distances. (g) $\sqrt{{\left\{d{\left(V-V\right)}_{\text{diag}\mathrm{,}1}\right\}}^2-{\left\{d{\left(V-V\right)}_{\text{diag}\mathrm{,}2}\right\}}^2}$ plotted with the monoclinic angle $\gamma$ (gray). (h) The antiferromagnetic moment (black) together with $\gamma$ (gray). (Lines are guide to the eyes.)