Chemical Science Research Frontiers 2015 82 Melting of Pb charge glass and simultaneous Pb-Cr charge transfer in PbCrO 3 as the origin of volume collapse Charge degrees of freedom of transition metal ions gives rise to various fascinating properties of transition- metal compounds, such as superconductivity or magnetoresistance. The ordering or disproportionation of charges in systems with integer or half-integer charge number per atom, such as Pr 0.5 Ca 0.5 MnO 3 or CaFeO 3 , causes metal-insulator transitions. These can be regarded as the crystallization of charges. The insulating state tends to have a glassy nature when randomly located dopants are introduced. A charge cluster glass state owing to geometric frustration arising from a triangular arrangement without randomness was recently observed in an organic, compound θ -(BEDT-TTF) 2 RbZn(SCN) 4 , and is attracting significant attention. Bi and Pb are the main group elements, but they have a charge degree of freedom depending on 6 s 0 and 6 s 2 electronic configurations. These are so-called valence skippers because the 6 s 1 state is prohibited and charge disproportionation of Bi is found in perovskites Ba 2+ Bi 3+ 0.5 Bi 5+ 0.5 O 3 and Bi 3+ 0.5 Bi 5+ 0.5 Ni 2+ O 3 . PbCrO 3 has long been regarded as a perovskite compound with a Pb 2+ Cr 4+ O 3 oxidation state, like metallic SrCrO 3 [1]. However, despite the fact that Sr 2+ and Pb 2+ have similar ionic radii and that PbCrO 3 and SrCrO 3 have the same cubic structure with Pm-3m symmetry, PbCrO 3 is an antiferromagnetic insulator with an enhanced lattice constant of 4.01 Å, which is 4.8% larger than that of SrCrO 3 . Using synchrotron X-ray scattering and electronic microscopes, we have unraveled a mystery that has eluded researchers for 50 years. PbCrO 3 was found to be Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 where Pb 2+ and Pb 4+ ions forms a distorted “charge glass” state [2]. The Pb-4 f hard X-ray photoemission (HAXPES) spectrum for PbCrO 3 collected at beamline BL47XU is shown in Fig. 1. Both the 4 f 5/2 and 4 f 7/2 peaks, which are split by the spin-orbit interaction, are doubles and asymmetric with shoulders at higher binding energies, indicating the coexistence of two valence states, namely, Pb 2+ and Pb 4+ . The valence state of PbCrO 3 is thus determined to be Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 . Cr 3+ with the t 2g 3 electronic configuration, which accounts for explaining the large lattice constant and insulating nature of PbCrO 3 . Next the local structure of PbCrO 3 was investigated. We performed the pair distribution function (PDF) analysis of synchrotron X-ray total scattering data collected at BL22XU , as shown in Fig. 2(a), assuming an ordered model with the 3 a 0 × 3 a 0 × 3 a 0 superstructure suggested by the diffuse scattering in electron diffraction and synchrotron X-ray powder diffraction data collected at BL02B2 . The 27 Pb sites are divided into 2 groups, A and B with the rock-salt-type arrangement, as shown in Fig. 2(b), corresponding to Pb 2+ and Pb 4+ or vice versa, as clarified by HAXPES. Then, longitudinal wave- type shift of the Pb positions was applied to both sublattices. The Pb positions were constrained so that these form sine waves in a , b , and c directions. Large shifts of the Pb positions were indeed observed in the high-angle annular dark-field (HAADF) STEM image (Fig. 2(c)). The distribution of the Pb-Pb lengths, 3.4 – 4.2 Å, agrees quite well with each other. It should be noted that the numbers of Pb 2+ and Pb 4+ are different in this model, 14:13 or vice versa. Furthermore, there are A-A or B-B arrangements at the interface between 3 a 0 × 3 a 0 × 3 a 0 cells. The long-range ordering of Pb 2+ and Pb 4+ is therefore prohibited. Taking into account the above results, one can conclude that Pb 2+ and Pb 4+ form a glassy structure, namely, charge glass. Finally, we investigated the pressure effect on the structure and resistivity of PbCrO 3 . The unit cell volume calculated from the peak positions of energy- dispersive SXRD (collected at BL14B1 ) is plotted in Fig. 3 as a function of pressure. It decreases by 2.7%, corresponding to a volume collapse of 7.8% at 2.5 GPa, as reported earlier [3]. The electrical resistivity suddenly drops at the corresponding pressure. Fig. 1. Pb-4 f core-level XPS spectrum of PbCrO 3 with the fitted results. The black and red lines are observed spectra and total fitted spectrum, respectively. 146 144 Pb( IV )4 f 5/2 Pb( IV )4 f 7/2 Pb( II )4 f 5/2 Pb( II )4 f 7/2 142 140 138 136 134 Binding Energy (eV) Intensity (arb. units) Research Frontiers 2015 83 The observed volume collapse and simultaneous insulator-to-metal transition can be explained by assuming a pressure-induced intermetallic charge transfer between Pb 4+ and Cr 3+ ions and a Pb 2+ Cr 4+ O 3 valence state for the high-pressure phase (see insets of Fig. 3). In conclusion, we showed that PbCrO 3 has a Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 valence state with a glassy distribution of Pb 2+ and Pb 4+ ions and that intermetallic charge transfer between Pb 4+ and Cr 3+ induced by the application of pressure leads to the volume collapse and the insulator-to-metal transition. References [1] W.L. Roth et al .: J. Appl. Phys. 38 (1967) 951. [2] R. Yu, H. Hojo, T. Watanuki, M. Mizumaki, T. Mizokawa, K. Oka, H.J. Kim, A. Machida, K. Sakaki, Y. Nakamura, A. Agui, D. Mori, Y. Inaguma, M. Schlipf, K. Rushchanskii, M. Ležaić, M. Matsuda, J. Ma, S. Calder, M. Isobe, Y. Ikuhara and M. Azuma: J. Am. Chem. Soc. 137 (2015) 12719 . [3] W.S. Xiao et al .: Proc. Natl. Acad. Sci. 107 (2010) 14026. Fig. 2. (a) Observed and calculated PDF of an ordered model with 3 a 0 × 3 a 0 × 3 a 0 superstructure. (b) The refined crystal structure of the ordered model with 3 a 0 × 3 a 0 × 3 a 0 superstructure. The pink atoms are Cr and the other atoms are Pb. (c) HAADF STEM image viewed along the perovskite [001] zone axis. The lateral shift of Pb positions is evident. Fig. 3. Pressure evolutions of crystal structure, electrical resistivity and lattice constant of PbCrO 3 . Abrupt decreases in the lattice constant and resistivity owing to the transition from Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 to Pb 2+ Cr 4+ O 3 are evident. Runze Yu* and Masaki Azuma Materials and Structures Laboratory, Tokyo Institute of Technology *E-mail: physyu@hotmail.com –10 –5 0 5 20 18 16 14 12 10 8 6 4 2 Pb 3 a × 3 a × 3 a modulation Experimental fit3a3a3ar1_diff Distance, r (Å) PDF, G(r) (Å –2 ) (a) (b) (c) 0 10 –2 10 –1 10 0 10 1 10 2 10 3 1 2 3 4 5 3.85 3.90 3.95 Pb 2+ Cr 4+ Cr 3+ Pb 2+ Cr 4+ O 3 Pb 2+ 0.5 Pb 4+ 0.5 Cr 3+ O 3 Pb 2+ /Pb 4+ 4.00 Resistivity Unit Cell Pressure (GPa) Resistivity (Ω·cm) Unit Cell (Å)
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