Stability of the Pb divalent state in insulating and metallic $\mathrm{PbCr}{\mathrm{O}}_3$

We have investigated the local electronic structure of $\mathrm{PbCr}{\mathrm{O}}_3$ by conducting x-ray absorption spectroscopy at the lead $L_3$ and chromium $L_{2,3}$ edges. We found that lead is divalent and chromium is charge-disproportionated into trivalent and hexavalent states at ambient pressures, explaining their insulating behavior. The high-resolution partial fluorescence yield of lead $L_3$ spectra as a function of pressure revealed that the divalent state of lead ions remains unchanged at high pressures, when $\mathrm{PbCr}{\mathrm{O}}_3$ changes into metal and its volume collapses by more than 10%. We infer that the insulator-to-metal transition does not involve a Pb-Cr charge transfer, but is associated with the conversion of ${\mathrm{Cr}}^{3+}/{\mathrm{Cr}}^{6+}$ charge-disproportionated states into a ${\mathrm{Cr}}^{4+}$ valence state.

Figure 1

Rietveld structural refinement of the XRD pattern for $\mathrm{PbCr}{\mathrm{O}}_3$ collected at 300 K using the ${\mathrm{Cr}}^{3+}/{\mathrm{Cr}}^{6+}$ charge disproportionation model. Observed (crosses), calculated (red), and difference (blue) are shown in the figure. The $R_{wp}, R_p$, and ${\chi }^2$ are 3.89%, 3.35%, and 4.276. The ticks indicate the allowed Bragg reflections of $\mathrm{PbCr}{\mathrm{O}}_3$ with the charge disproportionation model.

Figure 2

High-resolution XAS spectra at the lead $L_3$ edge of $\mathrm{PbCr}{\mathrm{O}}_3$ and $\mathrm{PbTi}{\mathrm{O}}_3$ (as ${\mathrm{Pb}}^{2+}$ reference) and $\mathrm{PbNi}{\mathrm{O}}_3$ (as ${\mathrm{Pb}}^{4+}$ reference).

Figure 3

(a) Chromium $L_{2,3}$ XAS experimental spectra of $\mathrm{PbCr}{\mathrm{O}}_3$ (black circles) and sum (blue line) of ${\mathrm{Cr}}^{3+}$ reference ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and ${\mathrm{Cr}}^{6+}$ reference ${\mathrm{Ag}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ in a 2:1 ratio. (b) Calculated chromium $L_{2,3}$ XAS of ${\mathrm{Cr}}^{3+}$ in octahedral coordination (red line), ${\mathrm{Cr}}^{6+}$ in tetrahedral coordination (green line), and the sum of them in a 2:1 ratio (blue line).

Figure 4

(a) High-resolution XAS spectra at the lead $L_3$ edge of $\mathrm{PbCr}{\mathrm{O}}_3$ as a function of pressure from 0 GPa to 14.1 GPa. The inset shows the simulation of the lead $L_3$ XAS spectrum at 0 GPa. (b) Pressure-dependent FWHM of shoulder A.