Magnetically induced metal-insulator transition in ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure, ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ and ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$. Upon cooling below 80 K, ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ simultaneously undergoes a metal-to-insulator transition and develops antiferromagnetic order. ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structures. The Os atoms in ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ are arranged on an approximately face-centered cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ is heavily distorted up to at least 500 K including antiferroelectriclike displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which, in turn, drives the metal-insulator transition. Our results suggest that the phase transition in ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ is spin driven and could be a rare example of a Slater transition.

Figure 1

Bulk measurements on ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ and ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ as a function of temperature. (a) Resistivity, showing that ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ undergoes a MIT at 80 K, whereas ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ remains metallic down to 2 K. The resistivity curve for ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ is unaffected by an external magnetic field of 7 T. (b) Inverse susceptibility together with a fit to the Curie-Weiss law at high temperatures. At 80 K there is an anomaly in ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$, and at approximately the same temperature ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ starts to deviate from the Curie-Weiss law. (c) Heat capacity showing an anomaly at around 80 K in ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$. The inset shows the magnetic entropy of ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ calculated by subtracting a polynomial approximation of the lattice contribution to the heat capacity.

Figure 2

Examples of our neutron-diffraction data on (a) ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ and (b) ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ at 2 K. Data at low $Q$ (below dotted line) are from banks 2 and 9, data at higher $Q$ are from banks 5 and 6 of the WISH diffractometer. The inset in (b) shows the magnetic scattering found by subtracting the data at 105 K from the low-temperature data. In (a) the green lines mark Bragg peaks from ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ and vanadium, in (b) the lines mark Bragg peaks from ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$, the ${\mathrm{Pb}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ impurity, and the magnetic phase of ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$. (c) Synchrotron x-ray powder-diffraction data on ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ at high temperatures. The arrows point to some of the reflections from the structural distortion. The green (black) vertical lines indicate Bragg peaks from the distorted (undistorted) cells.

Figure 3

Temperature dependence of the lattice parameters for ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ determined from neutron powder-diffraction measurements. The statistical uncertainty on the parameters is smaller than the markers.

Figure 4

The unit cell of (a) ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ and (b) ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$. The distortion in ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ is mainly seen in the oxygen octahedra which are tilted and deformed.

Figure 5

Temperature dependence of the lattice constants and ordered magnetic moment of ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ determined from neutron powder-diffraction measurements. The vertical line marks the metal-insulator and magnetic ordering transitions, which coincide at $T_{\mathrm{c}}=T_{\mathrm{N}}=80\mathrm{K}$. In (d) the moment is shown for the noncollinear model.

Figure 6

The unit cell of ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ with arrows indicating magnetic moments on the Os sites for the noncollinear model. The different colors of the arrows represent the two different Os atoms with dark blue representing Os1 and teal representing Os2.

Figure 7

RIXS data on ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ at $\mathbf{Q}=(-2.5,7,0)$ at $T=20\mathrm{K}$. (a) shows a map with four distinct excitations. (b) A scan through the map at $E_{\mathrm{i}}=10.876\mathrm{keV}$. (c) A high-resolution scan through $E_{\mathrm{i}}=10.972\mathrm{keV}$ to look at the intra-$t_{2g}$ levels.

Figure 8

Magnetic susceptibility of the pyrochlore ${\mathrm{Pb}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$, showing paramagnetic behavior down to 2 K.

Figure 9

The local coordination of Pb in ${\mathrm{Pb}}_2{\mathrm{ZnOsO}}_6$ (left) and ${\mathrm{Pb}}_2{\mathrm{CaOsO}}_6$ (right). The labels of the oxygen atoms correspond with Table 6.