Metal-Insulator Transition and Orbital Order in ${\mathrm{PbRuO}}_3$

Anomalous low temperature electronic and structural behavior has been discovered in ${\mathrm{PbRuO}}_3$. The structure [space group $Pnma$, $a=5.56314(1)$, $b=7.86468(1)$, $c=5.61430(1)\AA$] and metallic conductivity at 290 K are similar to those of ${\mathrm{SrRuO}}_3$ and other ruthenate perovskites, but a sharp metal-insulator transition at which the resistivity increases by 4 orders of magnitude is discovered at 90 K. This is accompanied by a first-order structural transition to an $Imma$ phase [$a=5.56962(1)$, $b=7.74550(1)$, $c=5.66208(1)\AA$ at 25 K] that shows a coupling of ${\mathrm{Ru}}^{4+}$ $4d$ orbital order to distortions from ${\mathrm{Pb}}^{2+}$ $6s6p$ orbital hybridization. The $Pnma$ to $Imma$ transition is an unconventional reversal of the group-subgroup symmetry relationship. No long range magnetic order is evident down to 1.5 K. Calculations show that Pb $6s6p$ and Ru $4d$ orbital hybridization and strong spin-orbit coupling are significant.

Figure 1

Density of states (DOS) of (a) ${\mathrm{PbRuO}}_3$ and (b) ${\mathrm{SrRuO}}_3$ in the $Pnma$ structure calculated by the LDA in the ferromagnetic state. Up (down) arrows refers to spin up (down). The Ru $t_{2g}$ DOS is scaled by $\frac{1}{3}$, and the Sr DOS is scaled by $\frac{1}{2}$. The exchange splitting of the Ru $t_{2g}$-O $2p$ bands is significantly suppressed in ${\mathrm{PbRuO}}_3$.

Figure 2

Resistivity variation for ${\mathrm{PbRuO}}_3$; the inset shows magnetic susceptibility measured in a 1 T field with a modified Curie-Weiss fit as described in the text.

Figure 3

Fit of the $Imma$ model to the time-of-flight neutron diffraction profile of ${\mathrm{PbRuO}}_3$ at 1.5 K. The inset shows the disappearance of $Pnma$ x-ray superstructure reflections as the $Imma$ phase is formed on cooling.

Figure 4

Cell volume (top) and Ru-O distance (bottom) variations for ${\mathrm{PbRuO}}_3$ from fits to neutron diffraction data. The upper inset shows hysteresis in the lattice parameters, normalized to the cubic perovskite cell, from x-ray measurements while warming or cooling at $10\mathrm{K}/\min$. Idealized $d_{xy}$ orbital order in the $Imma$ phase is shown in the lower panel.