Unraveling structural, electronic, and magnetic ambiguities in P b 1 − δ Cr O 3 with an insulating charge-transfer band structure

As a recently identified Mott system, $\mathrm{PbCr}{\mathrm{O}}₃$ remains largely unexplored, especially for its band structure, leading to many contentious issues on its structural, electronic, and magnetic properties. Here we present a comprehensive study of two different $\mathrm{P}{{\mathrm{b}}_{1\text{\ensuremath{-}}}}_{\ensuremath{\delta}}\mathrm{Cr}{\mathrm{O}}₃$ (\ensuremath{\delta} = 0 and 0.15) samples involving atomic deficiency prepared under pressure. By means of state-of-the-art diffraction techniques, the crystal structure of $\mathrm{PbCr}{\mathrm{O}}₃$ is definitively determined to adopt the pristine $\mathit{Pm}\overline{3}m$ symmetry, rather than other previously misassigned structures of M2-Pm$\overline{3}m$ and Pmnm. The two materials exhibit a similar charge-transfer-type insulating band structure, and the charge-transfer effect splits both Cr $2p$ and Pb $4f$ orbitals, rationalizing doublet splitting of the associated spectral lines. Nearly identical nominal cationic valence states of $\mathrm{C}{\mathrm{r}}⁴⁺$ and $\mathrm{P}{\mathrm{b}}²⁺$ are identified for this oxide system, hence calling into question the validity of recently proposed charge disproportionation mechanisms. In addition, $\mathrm{P}{\mathrm{b}}_0.85\mathrm{Cr}{\mathrm{O}}₃$ exhibits an anomalously higher N\'eel temperature of \ensuremath{\sim}240 K than that of $\mathrm{PbCr}{\mathrm{O}}₃$ (i.e., \ensuremath{\sim}200 K), likely due to the deficiency-induced enhancements of $\text{Cr3}d\text{--}\mathrm{O}2p$ orbital overlap and magnetic exchange. These findings provide much solid evidence to look into the fundamental properties of this important material system.