Spin reorientation of Fe3+ induced by peculiar Pb charge ordering in PbFeO3

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Chemical Science Research Frontiers 2021 74 Spin reorientation of Fe 3+ induced by peculiar Pb charge ordering in PbFeO 3 The cross interplays and correlations among lattices, charges, spins, and orbital degrees of freedom in transition metal perovskite oxides give rise to various fascinating electronic and magnetic properties, such as high-temperature superconductivity, colossal magnetoresistance, metal-insulator transition, multiferroicity, electrocatalysis, and negative thermal expansion. Such functions generally originate from transition metal ions, but Pb also has a charge degree of freedom stemming from the possibility of having either the 6 s 2 (Pb 2+ ) or 6 s 0 (Pb 4+ ) electronic configuration. Perovskite oxide Pb M O 3 ( M : 3 d transition metal) shows systematic charge distribution changes depending on the depth of the 3 d level of M [1]. PbVO 3 is Pb 2+ V 4+ O 3 , similar to Pb 2+ Ti 4+ O 3 [2], but PbCrO 3 has been found to be Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 with charge disproportionated Pb 2+ and Pb 4+ [3]. PbCoO 3 has been found to be Pb 2+ Pb 4+ 3 Co 2+ 2 Co 3+ 2 O 12 [4]. PbNiO 3 has a valence distribution of Pb 4+ Ni 2+ O 3 [5]. That is, Pb M O 3 changes from Pb 2+ M 4+ O 3 to Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 (average valence state of P b 3 + M 3 + O 3 ) t o P b 2 + 0 . 2 5 P b 4 + 0 . 7 5 C o 2 + 0 . 5 C o 3 + 0 . 5 O 3 (Pb 3.5+ Co 2.5+ O 3 ) and finally to Pb 4+ M 2+ O 3 following the order of M in the periodic table corresponding to the depth of the M d level. However, the charge distribution of PbFeO 3 remains a mystery. Our hard X-ray photoemission spectroscopy (HAXPES) measurement performed at SPring-8 BL09XU revealed the Pb 2+ 0.5 Pb 4+ 0.5 Fe 3+ O 3 charge distribution [6]. Figure 1 shows the HAXPES results for PbFeO 3 and other Pb M O 3 compounds with M = Ti, Cr, Co, and Ni used as standard references. Two components appeared in both the Pb 4 f 7/2 and Pb 4 f 5/2 peaks for PbCrO 3 (Pb 2+ 0.5 Pb 4+ 0.5 CrO 3 ), PbCoO 3 (Pb 2+ 0.25 Pb 4+ 0.75 CoO 3 ), and PbFeO 3 . Each peak can be deconvoluted into two Gaussians, as reported previously. The 6 s 0 electronic configuration in Pb resulted in a binding energy lower than that of 6 s 2 because of a strong screening effect; hence, the components at lower binding energies are attributable to Pb 4+ ions. The peak energies of PbFeO 3 were close to those of Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 , indicating the coexistence of Pb 2+ and Pb 4+ ions. We estimated the fractions of Pb 2+ and Pb 4+ from their area ratios using PbCrO 3 data as the standard for Pb 2+ 0.5 Pb 4+ 0.5 and concluded that PbFeO 3 also had the Pb 2+ 0.5 Pb 4+ 0.5 Fe 3+ O 3 charge distribution. On the bases of the above-estimated valence distribution, the crystal structure was determined by comprehensive scanning transmission electron microscopy, synchrotron X-ray powder diffraction (SPring-8 BL02B2 ) and neutron powder diffraction analyses. PbFeO 3 crystallized into a unique charge- ordered state in which a layer of Pb 2+ ions was interleaved by two layers each made up of a mixture of Pb 2+ and Pb 4+ ions in a 1: 3 ratio, along Fig. 1. Determination of the charge distribution of PbFeO 3 by HAXPES. (a) Pb-4 f HAXPES results for PbTiO 3, PbCrO 3, PbFeO 3, PbCoO 3 , and PbNiO 3 at RT. Predominant Pb 4+ and Pb 2+ are evident in the spectrum for PbFeO 3 . (b) Average Pb valence state calculated from area ratios of Pb 2+ and Pb 4+ components. PbTiO 3 , PbCrO 3 , and PbNiO 3 are standards for Pb 2+ , Pb 3+ (Pb 2+ 0.5 Pb 4+ 0.5 ) and Pb 4+ , respectively. (a) (b) Intensity (arb. units) Pb Valence Binding Energy (eV) M in Pb M O 3 150 145 140 135 Ti 2.0 2.5 3.0 3.5 4.0 V Cr Mn Fe Co Ni Pb 4 f PbNiO 3 PbCoO 3 Pb 2+ Pb 4+ PbFeO 3 PbCrO 3 PbTiO 3 Research Frontiers 2021 75 the direction of layer stacking (Fig. 2). Two distinct Fe sites with different types of Pb coordination are therefore generated. Upon cooling the sample from high temperature, two distinct magnetic phase transitions were observed, as shown in Fig. 3: a weak ferromagnetic transition occurring at 600 K owing to canted antiferromagnetic spin ordering and a continuous spin reorientation (SR) transition at 418 K despite the absence of magnetic rare-earth ions that are believed to be necessary for the appearance of spin reorientation in RFeO 3 (R: rare-earth element). Our DFT calculations revealed that the unique charge ordering in PbFeO 3 led to the formation of two Fe 3+ sublattices with competing energies that, in turn, caused the peculiar SR transition [6]. This finding provides a new avenue for studying the charge ordering phase and distinctive SR transition with potential applications in spintronic devices because of the high transition temperature and possibility of tuning. Masaki Azuma a,b, *, Youwen Long c and Takumi Nishikubo b,a a Laboratory for Materials and Structures, Tokyo Institute of Technology b Kanagawa Institute of Industrial Science and Technology c Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS, China *Email: mazuma@msl.titech.ac.jp References [1] M. Azuma et al. : Annu. Rev. Mater. Res. 51 (2021) 329. [2] A. Belik et al. : Chem. Mater. 17 (2005) 269. [3] R.Z. Yu et al. : J. Am. Chem. Soc. 137 (2015) 12719. [4] Y. Sakai et al. : J. Am. Chem. Soc. 139 (2017) 4574. [5] Y. Inaguma et al. : J. Am. Chem. Soc. 133 (2011) 16920. [6] X. Ye, J. Zhao, H. Das, D. Sheptyakov, J. Yang, Y. Sakai, H. Hojo, Z. Liu, L. Zhou, L. Cao, T. Nishikubo, S. Wakazaki, C. Dong, X. Wang, Z. Hu, H.-J. Lin, C.-T. Chen, C. Sahle, A. Efiminko, H. Cao, S. Calder, K. Mibu , M. Kenzelmann, L.H. Tjeng, R. Yu, M. Azuma, C. Jin and Y. Long: Nat. Commun. 12 (2021) 1917. Fig. 3. Spin reorientation observed in PbFeO 3. (a) Temperature dependence of magnetic susceptibility of PbFeO 3 measured at 0.01 T. (b) Magnetic structures of PbFeO 3 between T SR and T N ( Γ 4 ) as well as below 300 K ( Γ 1 ) determined by neutron diffraction studies. Fig. 2. Crystal structure of PbFeO 3 . Left part: illustration of structure revealing the Pb 2+ /Pb 4+ ordering and presence of two Fe 3+ sites. Right part: HAADF image of PbFeO 3 . Distances for the bright spots, which are the locations of Pb, indicate a modulation with a narrow–wide–narrow pattern. Pb 2+ Pb ion Narrow Narrow Narrow Narrow HAADF – STEM image Wide Wide Pb 4+ Fe 3+ 1 Fe 3+ 2 (a) (b) χ χ (emu·mol –1 · Oe –1 ) T (K) T SR T N 0 0.0 0.1 0.2 0.3 0.4 0.5 150 300 600 b c a Γ Γ 1 Γ Γ 4 450