Pressure-induced modifications of the magnetic order in the spin-chain compound $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$

The structural and magnetic properties of the $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$ spin-chain compound have been studied by means of neutron and x-ray powder diffraction at pressures up to 6.8 and 32 GPa, respectively. A suppression of the initial spin-density wave state $\left(T_{\mathrm{N}}=25\mathrm{K}\right)$ and stabilization of the collinear commensurate antiferromagnetic (AFM) state at high pressures $(T_{\mathrm{NC}}=26\mathrm{K}$ at $P=2.1$ GPa) were observed. The pressure behavior of the competing intra- and interchain magnetic interactions was analyzed on the basis of obtained structural data and their role in the formation of the magnetic phase diagram is discussed. The pressure behavior of the Néel temperature of the commensurate AFM phase was evaluated within the mean field theory approach and a good agreement with the experimental value $d{T}_{\mathrm{NC}}/dP=0.65\mathrm{K}/\mathrm{GPa}$ was obtained.

Figure 1

The rhombohedral crystal structure of $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$. The paths of the intra- $\left(J_1\right)$ and interchain $\left(J_2,J_3\right)$ interactions are shown.

Figure 2

X-ray-diffraction patterns of $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$ measured at selected pressures and ambient temperature, and refined by the Rietveld method. Experimental points and calculated profiles are shown. Tickmarks at the bottom represent the calculated positions of diffraction peaks at $P=0.41$ GPa. The weak peaks at $2\theta \approx 9.15$ and 9.50° are the background lines.

Figure 3

The pressure dependences of lattice parameters and unit-cell volume of $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$ and their interpolations using the Birch-Murnaghan equation of state.

Figure 4

Neutron-diffraction patterns of $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$, measured at selected pressures up to 6.8 GPa and low temperatures, and processed by the Rietveld method. The neighboring patterns are shifted along the vertical axis by 250 units. The experimental points and calculated profiles are shown. The ticks below represent the calculated positions of the structural peaks for $P=0$ GPa, $T=30\mathrm{K}$. The positions of the characteristic magnetic peaks of the SDW phase at ambient pressure and commensurate AFM phase at high pressure are marked as “M” and relevant peak indices are given.

Figure 5

The sections of the neutron-diffraction patterns in the $q$ space, demonstrating evolution of the peaks associated with the long-range SDW and short-range magnetic order (left) and temperature dependences of their intensities (right). The neighboring patterns are shifted along the vertical axis by 60 units.

Figure 6

Temperature dependencies of the ordered Co2 magnetic moment at selected pressures. For $P=0$ GPa, the line is a guide to the eye only. At high pressures, the solid lines represent interpolations by functions $m=m_0{[1-{\left(T/T_{\mathrm{N}}\right)}^{\alpha }]}^{\beta }$. The obtained coefficient values $\alpha \approx 3.5-3.0$ and $\beta \approx 0.43-0.56$ in the pressure range 2.1–6.8 GPa, respectively. Inset: Pressure dependence of the Néel temperature and its linear interpolation.

Figure 7

The pressure dependences of $\mathrm{Co}1\text{−}\mathrm{Co}2,\mathrm{Co}2\text{−}\mathrm{O}$, and O-O distances.

Figure 8

The pressure evolution of the absolute values of the $J_1$ and $J_{23}$ magnetic interaction parameters normalized by the ambient pressure values. The error bars do not exceed the symbol sizes (as evaluated from a combination of XRD and neutron-diffraction data).