E L E C T R O N I C S T R U C T U R E , M A G N E T I S M A N D D Y N A M I C S

S C I E N T I F I C H I G H L I G H T S

1 1 4 H I G H L I G H T S 2 0 2 1 I

Observation of novel charge ordering and spin reorientation in PbFeO3

High-quality PbFeO3 with a 2ap × 6ap × 2ap orthorhombic perovskite supercell and a Pb2+0.5Pb4+0.5Fe3+O3 charge combination was prepared. A novel A B B type charge ordering composed of Pb2+ and Pb4+ was observed, leading to a spin reorientation transition of Fe3+ at 418 K due to the competing anisotropic energies.

Charge degrees of freedom play an important role in the valence-skipping Pb-based perovskite oxides. In the PbMO3 (M = transition metal) family, as the d level on M ion becomes deeper, systematic charge distribution evolutions and interesting crystal structures and physical properties are shown. The 6s2 lone-pair active divalent lead appears in tetragonal PbTiO3 and PbVO3. PbNiO3 shows a pure Pb4+. In PbCrO3, the melting of Pb charge glass and Pb Cr charge transfer lead to a simultaneous insulator-to-metal transition and a large volume collapse under high pressure. The double charge disproportionation

and the ordered arrangement of four cations make PbCoO3 form an ingenious quadruple perovskite structure (Pb2+Pb4+3Co2+2Co3+2O12), and sequential spin-state transition and intermetallic charge transfer are further observed at high pressures. This work focuses on PbFeO3. Though the synthesis of PbFeO3 was reported in 2007, further study of its structure and physical properties has been hindered due to the difficulty of synthesising high- quality samples and resolving the crystal structure.

First, high-quality polycrystalline PbFeO3 samples were prepared at 8 GPa and 1423 K. Comprehensive investigations reveal that PbFeO3 crystallises into an unusual 2ap × 6ap × 2ap (ap refers to the pseudolattice parameter of a simple cubic perovskite ABO3 subcell) orthorhombic perovskite superlattice with the Cmcm symmetry (Figure 94a). Synchrotron X-ray absorption spectroscopy at beamline ID20 determined the proportion of Pb2+/Pb4+, and the bond valence analysis further confirmed the valence state of the cations at each Wyckoff site. Pb2+ and Pb4+ coexist at a ratio of 1:1 and construct a long-range layered charge ordering with an A B B

Fig. 94: Crystal structure and peculiar charge ordering

of PbFeO3. a) Schematic crystal structure. b) Left:

Illustration of Pb modulations; Right: HAADF image along

[001] pseudocubic zone axis. Distances for the bright spots,

which are the locations of Pb, indicate a modulation

with a shorter longer shorter pattern. c) Sketch of unique

-A-B-B-type of charge ordering composed of two types of differently charged layers.

Fig. 95: Various magnetic

properties of PbFeO3. a) Magnetic susceptibility.

b) Isothermal magnetisation loops. c) Magnetic structures

between TSR and TN (Γ4) as well as below 300 K (Γ1).

d) Magnetic-phase diagram. CAFM: collinear antiferromagnetism;

WFM: weak ferromagnetism; PM: paramagnetism.