Charge transfer transitions stemming from 6s0 and 6s2 charge degree of freedom in valence skipping Bi and Pb ions

100 %

1 / 1

100 %

1 / 1

Chemical Science Research Frontiers 2020 74 Charge transfer transitions stemming from 6 s 0 and 6 s 2 charge degree of freedom in valence skipping Bi and Pb ions Bi and Pb are main group elements but they have charge degrees of freedom that depend on the 6 s 2 (Bi 3+ , Pb 2+ ) and 6 s 0 (Bi 5+ , Pb 4+ ) electronic configurations. The 6 s 1 configuration is prohibited. Bi and Pb are therefore called valence skipping elements. Since the energy levels of Bi and Pb 6 s , transition metal 3 d and oxygen 2 p are close to each other, intermetallic charge transfer is observed in perovskite oxides composed of these elements [1]. The composition-, temperature- and pressure- induced changes in local charge distributions were investigated comprehensively by synchrotron X-ray diffraction, total scattering and photoemission studies. T h e p e r o v s k i t e B i N i O 3 h a s a n u n u s u a l Bi 3+ 0.5 Bi 5+ 0.5 Ni 2+ O 3 valence distribution at atmospheric pressure. Distinct Bi 3+ /Bi 5+ crystallographic sites exist in the √ ⎯ 2 a × √ ⎯ 2 a × 2 a unit cell, where a is the lattice parameter of a simple cubic perovskite. The simultaneous melting of charge disproportionation and charge transfer between Bi 5+ and Ni 2+ ions accompanied by a triclinic ( P -1) to an orthorhombic GdFeO 3 -type ( Pbnm ) structural transition occur under high-pressure conditions. This structural transition results in a contraction of the unit cell volume by ~3%, because of the shrinkage of the Ni–O bond owing to the oxidation of Ni from 2+ to 3 + . F e 3 + s u b s t i t u t i o n f o r N i 2 + s t a b i l i z e s t h e Bi 3+ (Ni, Fe) 3+ O 3 phase, and a charge transfer transition accompanied by volume shrinkage occurs in BiNi 1– x Fe x O 3 upon heating at atmospheric pressure [2]. This transition is of first order, but because large low-temperature (LT) triclinic and small high-temperature (HT) orthorhombic phases coexist and their fractions change upon heating, the weighted average unit cell volume linearly decreases, leading to negative thermal expansion (NTE). The crystal structure of the LT phase investigated by powder diffraction at SPring-8 BL02B2 changed to the polar R 3 c BiFeO 3 -type with a unique Bi site, shown in Fig. 1(d), at x = 0.3. On the other hand, hard X-ray photoemission spectroscopy at SPring-8 BL09XU indicated the presence of Bi 3+ and Bi 5+ , as shown in Fig. 1(a). The PDF analysis of total scattering data collected at SPring-8 BL22XU (Figs. 1(b) and 1(c)) revealed that the local structure had R 3 symmetry with two Bi sites (Fig. 1(e)). These results mean that the ordering of Bi 3+ /Bi 5+ is short-ranged, probably because of the deviation from the 1:1 ratio. Since the Fe ion has a +3 charge, as confirmed by Mössbauer s p e c t r o s c o p y, t h e a m o u n t s o f B i 5 + a n d N i 2 + contributing to the charge transfer transition decrease with increasing x . The amount of unit cell volume reduction during the NTE therefore decreases with x . However, the volume reductions of BiNi 1– x Fe x O 3 with x ≥ 0.3 are found to be independent of x , as shown in Fig. 2(a). It is well known that ferroelectric to the Fig. 1. (a) Observed hard X-ray photoemission spectroscopy (HAXPES) spectra (black points), fitting results (green line), Bi 3+ components (blue) and Bi 5+ components (red) for BiNi 1- x Fe x O 3 measured at 300 K. Observed (blue points) and simulated (red line) pair distribution functions (PDFs) for BiNi 0.7 Fe 0.3 O 3 using (b) R 3 c and (c) R 3 models. Lower green lines indicate the difference between the observed and simulated data. Average R 3 c (d) and local R 3 (e) structures. [3] (a) (b) (c) (d) (e) Intensity (arb. units) G ( r ) G ( r ) Binding Energy (eV) r (Å) –168 –164 –160 –156 –8 –4 0 4 8 –8 –4 0 4 8 5 10 15 BiFeO 3 R 3c observed calculated difference observed calculated difference R wp =13.27 R wp =12.47 R 3 Bi 3+ Bi 3+ BiNiO 3 x = 0.5 x = 0.4 x = 0.3 x = 0.15 Bi 3+ Bi 3+ Bi 5+ Bi 5+ Research Frontiers 2020 75 paraelectric change in materials such as PbTiO 3 is accompanied by NTE. The spontaneous polarization of the polar rhombohedral LT phase of BiNi 1– x Fe x O 3 increases with x (Fig. 2(b)). These indicate that the decrease in the amount of volume reduction due to charge transfer is compensated by the NTE induced by the polar–nonpolar transition [3]. PbCoO 3 prepared at 12 GPa has a Pb 2+ 0.25 Pb 4+ 0.75 Co 2+ 0.5 Co 3+ 0.5 O 3 charge distribution. Pb 2+ /Pb 4+ and Co 2+ /Co 3+ are respectively ordered at A- and B-sites of perovskite ABO 3 , leading to the quadruple p e r o v s k i t e P b 2 + P b 4 + 3 C o 2 + 2 C o 4 + 2 O 1 2 s t r u c t u r e illustrated in Fig. 3(a) [4]. Similarly to BiNiO 3 , a pressure-induced charge transfer transition leading to Pb 2+ 0.5 Pb 4+ 0.5 Co 3+ O 3 is expected to occur. Synchrotron X-ray diffraction (SXRD) data collected at BL22XU indicated 1.76 and 1.12% reductions in unit cell volume at 20 and 24 GPa (Fig. 3(b)). An X-ray emission study at SPring-8 BL12XU revealed that the first reduction was due to the high-spin to low-spin transition of Co 2+ . The second reduction was attributed to the charge transfer transition [5]. Masaki Azuma a,b, *, Takumi Nishikubo a and Yuki Sakai a,b a Laboratory for Materials and Structures, Tokyo Institute of Technology b Kanagawa Institute of Industrial Science and Technology *Email: mazuma@msl.titech.ac.jp References [1] M. Azuma et al. : Dalton Transactions 47 (2018) 1371. [2] K. Nabetani et al. : Appl. Phys. Lett. 106 (2015) 061912. [3] T. Nishikubo, Y. Sakai, K. Oka, T. Watanuki, A. Machida, M. Mizumaki, K. Maebayashi, T. Imai, T. Ogata, K. Yokoyama, Y. Okimoto, S. Koshihara, H. Hojo, T. Mizokawa and M. Azuma: J. Am. Chem. Soc. 141 (2019) 19397. [4] Y. Sakai et al. : J. Am. Chem. Soc. 139 (2017) 4574. [5] Z. Liu, Y. Sakai, J. Yang, W. Li, Y. Liu, X. Ye, S. Qin, J. Chen, S. Agrestini, K. Chen, S.-C. Liao, S.-C. Haw, F. Baudelet, H. Ishii, T. Nishikubo, H. Ishizaki, T. Yamamoto, Z. Pan, M. Fukuda, K. Ohashi, K. Matsuno, A. Machida, T. Watanuki, S. I. Kawaguchi, A. M. Arevalo-Lopez, C. Jin, Z. Hu, J. P. Attfield, M. Azuma and Y. Long: J. Am. Chem. Soc. 142 (2020) 5731. Fig. 3. (a) Crystal structure of PbCoO 3 with Pb 2+ Pb 4+ 3 Co 2+ 2 Co 4+ 2 O 12 charge distribution. (b) Pressure evolution of unit cell volume of PbCoO 3. [5] Fig. 2. Negative thermal expansion (NTE) induced by simultaneous charge transfer and polar– nonpolar transitions in BiNi 1- x Fe x O 3 . (a) Fe concentration dependence of the volume difference between low-temperature (LT) phases (triclinic or rhombohedral) and high-temperature (HT) orthorhombic phase. Blue and green triangles indicate transitions from triclinic to orthorhombic phases and from rhombohedral to orthorhombic phases, respectively. (b) Calculated P S from structural parameters refined by Rietveld analyses of synchrotron X-ray diffraction (SXRD) patterns. [3] (a) (b) V / V ortho (%) Δ Δ P s ( C/cm 2 ) μ μ x in BiNi 1– x Fe x O 3 x in BiNi 1 – x Fe x O 3 0.0 0.0 50 60 70 80 –1.0 –2.0 –3.0 0.2 triclinic to orthorhombic rhombohedral to orthorhombic 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 (a) (b) P (GPa) V p (Å 3 ) 0 44 46 48 50 52 54 20 1.76% 1.12% 40 60 Co 2+ Pb 2+ Pb 4+ Co 3+ AP Phase B 0 = 164(3) GPa HP-I Phase B 0 = 187(8) GPa HP-II Phase B 0 = 203(3) GPa