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Aug 4, 2016 - Glassy Distribution of Bi3+/Bi5+ in Bi1−x. Pbx. NiO3 and Negative Thermal. Expansion Induced by Intermetallic Charge Transfer. Kiho Na...
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Glassy Distribution of Bi3+/Bi5+ in Bi1−xPbxNiO3 and Negative Thermal Expansion Induced by Intermetallic Charge Transfer Kiho Nakano,† Kengo Oka,‡,† Tetsu Watanuki,§ Masaichiro Mizumaki,∥ Akihiko Machida,§ Akane Agui,§ Hyunjeong Kim,⊗ Jun Komiyama,⊥ Takashi Mizokawa,∇ Takumi Nishikubo,† Yuichiro Hattori,† Shigenori Ueda,¶,& Yuki Sakai,*,# and Masaki Azuma*,† †

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan § Quantum Beam Science Center, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan ∥ Japan Synchrotron Radiation Research Institute, SPring-8, Sayo-gun, Hyogo 679-5198, Japan ⊗ National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ⊥ Department of Complexity Science and Engineering, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ∇ Department of Applied Physics, School of Advanced Science and Engineering, Waseda Unversity, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ¶ Quantum Beam Unit, National Institute for Materials Science, Sengen, Tsukuba 305-0047, Japan & Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, Sayo, Hyogo 679-5148, Japan # Kanagawa Academy of Science and Technology, KSP, 3-2-1 Sakado, Takatsu-ku, Kawasaki City, Kanagawa 213-0012, Japan ‡

S Supporting Information *

ABSTRACT: The valence distribution and local structure of Bi1−xPbxNiO3 (x ≤ 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi3+ and Bi5+ was observed for all the samples, but it was a longranged one with distinct crystallographic sites in the P1̅ triclinic structure for x ≤ 0.15, while the ordering was short-ranged for x = 0.20 and 0.25. An intermetallic charge transfer between Bi5+ and Ni2+, leading to large volume shrinkage, was observed for all the samples upon heating at ∼500 K.

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charge transfer, resulting in a Bi3+Ni3+O3 high-pressure (HP) phase that crystallizes in an orthorhombic GdFeO3-type structure with a Pbnm space group and unique Bi site accompanied by 2.9% volume shrinkage, is induced by applying pressure.5,6 This large change results from the dominant contraction of the Ni−O perovskite framework, as Ni2+ is oxidized to the smaller Ni3+ at the transition, which outweighs the lattice expanding effects of reducing Bi5+ to Bi3+ and increases in the Ni−O−Ni angles. PbCrO3 also shows a pressure-induced intermetallic charge transfer transition to Pb3+Cr3+O3 HP phase accompanied by a pronounced volume collapse of 9.8%.7 Note that, for other scenarios, CD values of the Cr ion and ferroelectricity for the ambient pressure phase

INTRODUCTION Charge degrees of freedom in transition-metal elements give rise to various fascinating properties, such as metal−insulator transition, high-temperature (HT) superconductivity, and magnetoresistance. Bi and Pb are the main group elements, but these have charge degrees of freedom, depending on the 6s2 (Bi3+, Pb2+) and 6s0 (Bi5+, Pb4+) electronic configurations, typically found in BaBiO3 (BaBi3+0.5Bi5+0.5O3).1 These are socalled valence skippers or negative U ions, where 6s1 (Bi4+, Pb3+) configurations are prohibited.2 BiNiO3 and PbCrO3 have characteristic charge distributions of Bi3+0.5Bi5+0.5Ni2+O3 and Pb2+0.5Pb4+0.5Cr3+O3 with a Bi (Pb) charge disproportionation (CD) in the A-site of perovskite ABO3.3,4 In the former compound, Bi3+ and Bi5+ occupy distinct crystallographic sites in the 2 a × 2 a × 2a unit cell with space group P1,̅ where a is the lattice parameter of a cubic perovskite. Since Bi 6s and Ni 3d states are close to each other in energy, an intermetallic © XXXX American Chemical Society

Received: March 22, 2016 Revised: August 3, 2016

A

DOI: 10.1021/acs.chemmater.6b01160 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

at 300 K by a total electron yield method. The energy resolution ΔE at Ni L-edges was set to 160 meV. The incident photon was calibrated by measuring the energy of the Ti L3,2-edges of TiO2 and Ni L3,2-edges of NiO. A powder sample was pasted uniformly on a sample holder with carbon tape. Pb 4f and Bi 4f HAXPES measurements were performed at 300 K with E = 5953.4 eV at the undulator beamlines BL15XU of SPring-8.19,20 For these measurements, a hemispherical photoelectron analyzer (VG Scienta R4000) was used. The polycrystalline samples were fractured in situ for the HAXPES measurements. The binding energy was calibrated using the Au 4f7/2 peak (84.0 eV) and the Fermi edge of gold reference samples. The total-energy resolution was set to 240 meV. The total scattering data were collected using the diffractometer at beamline BL22XU of SPring-8.21 A powder sample of Bi1−xPbxNiO3 was loaded into a cylindrical Kapton capillary that had a diameter of 1 mm. Monochromatized synchrotron X-rays of 60.21 keV (λ = 0.2059 Å) were irradiated in the sample at RT. Scattering data were obtained up to the maximum momentum transfer Qmax of 23 Å−1 (2θmax = 45°). The scattering signal from the Kapton capillary was removed by subtracting the empty capillary data. Various other corrections were made as shown in the reference.22 The pair distribution function (PDF) information was obtained by using the PDFgetX2 program.23 Local structural analysis was performed by using the PDFgui program.24

also are proposed.8−10 Lanthanide (denoted by Ln, where Ln = La, Nd, Eu, Dy) substitution for Bi destabilizes the Bi3+/Bi5+ disproportionated state, so the intermetallic charge transfer occurs upon heating, resulting in an orthorhombic (Bi,Ln)3+Ni3+O3 high-temperature (HT) phase with a unique Bi site.6,11,12 The advantages of this compound are a negative thermal expansion (NTE) coefficient three times as large as those of other materials, including antiperovskite manganese nitrates, and also a tunable temperature range of NTE.13,14 On the other hand, temperature hysteresis resulting from the firstorder nature of the transition can be a problem for practical applications. Fe substitution for Ni also induces NTE.15 Compared with Bi1−xLnxNiO3, the thermal hysteresis of BiNi1−xFexO3 is suppressed because the random distribution of Fe in the Ni site changes the first-order transition to secondorder-like transition. Bi1−xPbxNiO3 was also investigated in 2007.16 The room-temperature (RT) structure of Bi1−xPbxNiO3 reportedly changed from triclinic to orthorhombic at x = 0.20, and the valence state of the orthorhombic phase was (Bi0.8Pb0.2)4+Ni2+O3, despite the aforementioned instability in the Bi4+ state. We have recently found the disproportionation of Pb valence into Pb2+ and Pb4+ in PbCrO3 without long-range ordering.4 This finding suggests that Bi in Bi0.8Pb0.2NiO3 is also disproportionated to Bi3+ and Bi5+ without long-range ordering and macroscopic symmetry lowering, and it is detected as 4+ in the investigation of the average structure. In this study, we reinvestigated the valence distribution and the local structure of Bi1−xPbxNiO3 by means of Ni L-edge Xray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. The samples with 0.05 ≤ x ≤ 0.15 showed triclinic to orthorhombic transition accompanied by Bi5+ and Ni2+ charge transfer upon heating, as observed in Bi1−xLnxNiO3 and BiNi1−xFexO3. On the other hand, the charge distributions of the x = 0.2 and 0.25 samples were not (Bi1−xPbx) 4+Ni2+O3 , but these were (Bi3+0.5Bi5+0.5)1−xPb4+xNi2+O3 with short-range ordering of Bi3+/Bi5+, with a correlation length of ∼15 Å. These showed an orthorhombic-to-orthorhombic phase transition, accompanied by Bi to Ni charge transfer and volume contraction, because of the oxidation on Ni ions.

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RESULTS AND DISCUSSION Figure 1 shows the results of Rietveld refinements of synchrotron X-ray diffraction (SXRD) patterns of

Figure 1. Observed (points), calculated (line), and difference (lower line) patterns from the Rietveld analysis of the synchrotron X-ray diffraction (SXRD) data for Bi1−xPbxNiO3 powder with x = 0.05 and 0.25 at RT and 500 K (λ = 0.42002 and 0.4199632 Å for x = 0.05 and 0.25, respectively). The tick marks correspond to the positions of Bragg reflections of the triclinic phases (top) and the orthorhombic phases (bottom).

EXPERIMENTS

Polycrystalline samples of Bi1−xPbxNiO3 (x = 0.05, 0.1, 0.15, 0.2, and 0.25) were prepared by high-pressure (HP) synthesis, as reported in ref 16. Bi2O3, Ni, and PbO2 (stoichiometric amounts for metal elements) were dissolved in nitric acid, stirred, and then heated at 993 K in the air for 12 h. The obtained fine powders were mixed with KClO4 at a weight ratio of 5:1 and then sealed in gold capsules. The samples were then treated at 8 GPa and 1273 K for 30 min in a cubic anvil-type HP apparatus. The remaining KCl was removed by washing with distilled water (see Figures S1 and S2 in the Supporting Information). No traces of residual C, N, Cl and K were detected by energy-dispersive spectroscopy (EDS) analysis with a JEOL Model JSM-6610LA system, as shown in Figure S3 in the Supporting Information. The XRD patterns were collected at various temperatures with a diffractometer (Model D8 Avance, Bruker) using Cu Kα radiation. The lattice parameters and fractions of low-temperature (LT) triclinic and high-temperature (HT) orthorhombic phases were refined by Rietveld analysis using TOPAS software. The synchrotron X-ray diffraction (SXRD) patterns were collected with a large Debye− Scherrer camera installed at the BL02B2 beamline of SPring-8,17 with λ = 0.42002 and 0.4199632 Å, and analyzed using RIETAN-FP programs.18 The XAS measurements around the Ni L-edges were carried out at BL27SU of SPring-8 Japan. The XAS data were collected

Bi1−xPbxNiO3 with x = 0.05 and 0.25 samples at RT and at 500 K with the refined lattice parameters, crystallographic parameters, and bond valence sums (BVSs) summarized in Tables 1 and 2. These indicate that the crystal structure at RT changed from a triclinic one with Bi3+/Bi5+ ordering for x = 0.05 to an orthorhombic one with (Bi,Pb)4+Ni2+O3 for x = 0.25, in accordance with the previous report.16 The valence distributions at 500 K are (Bi,Pb)3+Ni3+O3. The Ni L-edge XAS data shown in Figure 2 support the divalent nature of Ni ion at RT. The spectra for x = 0, 0.05, 0.10, 0.20, and 0.25 are identical, indicating the absence of a distinct valence change in Ni. The increase of peak intensities at 853 and 871 eV, indicating the appearance of Ni3+ (see ref 15), is not present. To further investigate the valence state of Bi and Pb, we performed HAXPES measurements. Figure 3 shows the HAXPES spectra for Bi0.75Pb0.25NiO3 with the data for Bi and B

DOI: 10.1021/acs.chemmater.6b01160 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Table 1. Crystallographic Parameters of Bi1−xPbxNiO3 (x = 0.05 and 0.25) at RT and 550 K atom

g

site

x

y

z

x = 0.05 at RT, Triclinic Phase (Space Group P1̅) 0.95 0.0101(3) 0.0492(2) 0.05 0.0101(3) 0.0492(2) 0.95 0.5117(3) 0.4427(2) 0.05 0.5117(3) 0.4427(2) 1 0.5 0 1 0 0.5 1 0.5 0 1 0 0.5 1 0.850(4) 0.455(3) 1 0.416(3) 0.080(3) 1 0.841(4) 0.172(3) 1 0.312(4) 0.339(3) 1 0.211(3) 0.789(3) 1 0.677(3) 0.688(3) x = 0.05 at 550 K, Orthorhombic Phase (Space Group, Pbnm)b 0.95 0.5076(2) 0.05368(8) 0.05 0.5076(2) 0.05368(8) 1 0 0 1 0.406(1) 0.467(1) 1 0.196(1) 0.698(1) x = 0.25 at RT, Orthorhombic Phase (Space Group, Pbnm)c 0.75 0.5169(3) 0.0536(1) 0.25 0.5169(3) 0.0536(1) 1 0 0 1 0.376(2) 0.429(2) 1 0.187(2) 0.680(2) x = 0.25 at 550 K, Orthorhombic Phase (Space Group, Pbnm)d 0.75 0.5126(3) 0.0542(2) 0.25 0.5126(3) 0.0542(2) 1 0 0 1 0.386(2) 0.450(3) 1 0.189(2) 0.692(2)

Uiso(Å2)

a

Bi1 Pb1 Bi2 Pb2 Ni1 Ni2 Ni3 Ni4 O1 O2 O3 O4 O5 O6

2i 2i 2i 2i 1d 1c 1f 1g 2i 2i 2i 2i 2i 2i

Bi Pb Ni O1 O2

4c 4c 4a 4c 8d

Bi Pb Ni O1 O2

4c 4c 4a 4c 8d

Bi Pb Ni O1 O2

4c 4c 4a 4c 8d

0.2352 (2) 0.2352 (2) 0.7274(2) 0.7274(2) 0 0 0.5 0.5 0.250 (3) 0.760(2) 0.969(2) 0.078(2) 0.411(2) 0.549(3)

0.0050(2) 0.0050(2) 0.0050(2) 0.0050(2) 0.0054(5) 0.0054(5) 0.0054(5) 0.0054(5) 0.005 (2) 0.005 (2) 0.005 (2) 0.005 (2) 0.005 (2) 0.005 (2)

0.25 0.25 0 0.25 −0.0470(7)

0.0128(1) 0.0128(1) 0.0080(3) 0.018(1) 0.018(1)

0.25 0.25 0 0.25 −0.065(1)

0.0098(3) 0.0098(3) 0.0090(6) 0.010(2) 0.010(2)

0.25 0.25 0 0.25 −0.051(1)

0.0118(3) 0.0118(3) 0.0111(6) 0.019(3) 0.019(3)

a Space group P1̅ (No. 2), Z = 1, a = 5.37863(6) Å, b = 5.63433(6) Å, c = 7.71334(7) Å, α = 91.9710(8)°, β = 89.8568(7)°, γ = 91.4221(7)°, ρcalc = 8.975480 g/cm3, V = 233.5419(45) Å3. R values: Rwp = 4.7%, Rp = 3.2%. bSpace group Pbnm (No. 62), Z = 4, a = 5.35717(4) Å, b = 5.57200(5) Å, c = 7.67695(6) Å, ρcalc = 9.147182 g/cm3, V = 229.1581(32) Å3. R values: Rwp = 3.2%, Rp = 2.2%. cSpace group Pbnm (No. 62), Z = 4, a = 5.31599(23) Å, b = 5.58141(24) Å, c = 7.73020(33) Å, ρcalc = 9.128795 g/cm3, V = 229.3606(172) Å3. R values: Rwp = 5.8%, Rp = 3.6%. dSpace group Pbnm (No. 62), Z = 4, a = 5.33394(10) Å, b = 5.57880(10) Å, c = 7.70380(15) Å, ρcalc = 9.133518 g/cm3, V = 229.2420(75) Å3. R values: Rwp = 6.3%, Rp = 3.9%. Occupation factors of all sites were fixed.

Table 2. Bond Valence Sumsa for Bi1−xPbxNiO3 (x = 0.05 and 0.25) at RT and 550 K (Bi,Pb)1

(Bi,Pb)2

Ni1

Ni2

Ni3

Ni4

x = 0.05 (RT) x = 0.05 (550 K)

3.28 3.07

4.67

1.89 3.03

2.05

1.97

1.83

x = 0.25 (RT) x = 0.25 (550 K)

4.04 2.68

1.99 2.83

Vi = ∑jSij, where Sij = exp(r0 − rij/0.37). Values calculated using rij = 2.094 for Bi3+, 2.06 for Bi5+, 2.042 for Pb4+, 1.654 for Ni2+, and 1.75 for Ni3+. a

Figure 2. Ni L-edge XAS spectra of Bi1‑xPbxNiO3 with x = 0, 0.05, 0.10, 0.2, and 0.25 at RT.

Pb standard materials. The intense peak at 158 eV in the Bi 4f data for both BiNiO3 that is absent for the data for Bi3+Fe3+O3 indicates the presence of Bi5+ in addition to Bi3+. The Bi 4f spectrum for Bi0.75Pb0.25NiO3 is the same as that of BiNiO3, indicating that the valence state of Bi is Bi3+/Bi5+, not Bi4+. On the other hand, the Pb 4f spectrum for Bi0.75Pb0.25NiO3 is rather closer to that of Pb4+Ni2+O325 than that of Pb2+Ti4+O3, indicating that the valence state of Pb is Pb4+. The present

results show that the Bi(Pb) 4f binding energy increases as the Bi(Pb) 6s electrons decrease. The same trend also has been reported for PbO and PbO2.26,27 Usually, since the valence electrons contribute to the screening of the core hole charge, the binding energy decreases as the number of valence C

DOI: 10.1021/acs.chemmater.6b01160 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 3. (a) Bi 4f HAXPES spectra and (b) Pb 4f HAXPES spectra of Bi0.75Pb0.25NiO3 at RT with those of BiFeO3 and BiNiO3 as standard materials for Bi3+ and Bi3+/5+ and with those of PbTiO3 and PbNiO3 as the standard materials for Pb2+ and Pb4+. Fitting parameters are summarized in Tables S1−S6 in the Supporting Information.

electrons increases. The well-known example is the transitionmetal 2p binding energy of the 3d transition-metal oxides. The 2p binding energy decreases as the 3d electrons increase or as the valence number decreases. The increase of the Bi(Pb) 4f binding energy from Bi5+(Pb4+) to Bi3+(Pb2+), that is apparently inconsistent with the screening effect of the valence electrons, can be understood considering the strong hybridization between the Bi(Pb) 6s orbitals and the O 2p orbitals. The screening effect of the Bi(Pb) 6s electron should be much smaller than that of the transition-metal 3d electrons, because of the strong Bi(Pb) 6s−O 2p hybridization. In addition, the Bi(Pb) 6s band hybridized with O 2p is fully occupied for Bi3+(Pb2+), shifting the energy levels downward and increasing the Bi(Pb) 4f binding energy. Next, PDF analysis of the synchrotron X-ray total scattering data was performed to investigate the local distribution of Bi3+ and Bi5+. The initial analysis, assuming the orthorhombic structure with a unique Bi/Pb site, gave an R-factor of 15.0. The R-factor significantly decreased to 11.3 by employing a triclinic structural model with distinct Bi3+ and Bi5+ sites. The fitting results are shown in Figures 4a and 4b. The fit below r = 15 Å improves in the latter model, indicating that the local structure is triclinic with Bi3+/Bi5+ ordering, but the correlation length is only 15 Å, which is approximately twice the length of the c-axis. The absence of long-range ordering was confirmed by performing Rietveld analysis, assuming the triclinic model shown in Figure 4c. The characteristic (011), (102), and (102) reflection characteristics for the P1̅ model appear only in the simulation and are absent in the observed diffraction data, indicating that the average structure is an orthorhombic Pbnm one. Next, we consider the origin of a glassy distribution of Bi3+ and Bi5+. Suppose that Pb valence is 4+ and that the system is subjected to the HP and HT synthesis conditions; the crystal structure is the Pbnm one with the valence state of Bi3+1−xPb4+xNi(3−x)+O3. Partial ordering of Bi3+ and Pb4+ occurs because of the differences in the valence and the ionic radius, as schematically illustrated in Figure 5a. When the sample is cooled and the pressure is released, the valence state changes to (Bi3+0.5Bi5+0.5)1−xPb4+xNi2+O3, where Bi3+/Bi5+ disproportionation occurs. However, when x exceeds the critical concentration of 0.2, the development of Bi3+/Bi5+ long-range ordering is hindered by the presence of small Pb4+ at the large Bi3+ site, as shown in Figure 5b, and the system freezes in a glassy, shortrange ordered state. The valence distributions at 500 K are different from those at RT. BVS calculations show that the HT

Figure 4. Observed (red points), calculated (blue line), and difference (green line) PDF of Bi0.75Pb0.25NiO3 with the (a) orthorhombic and (b) triclinic structural models. The isotropic displacement parameters of each element were fixed at Uiso(Bi) = 0.01 Å2, Uiso(Pb) = 0.01 Å2, Uiso(Ni) = 0.01 Å2, and Uiso(O) = 0.02 Å2. (c) Magnified view of the fitting results of Rietveld analysis of the SXRD data for Bi0.75Pb0.25NiO3 at RT (λ = 0.4199632 Å) with the triclinic model. Observed (red line) and calculated (blue line) patterns are shown.

Figure 5. Illustration of the Bi/Pb layer of Bi0.75Pb0.25NiO3: (a) HPHT orthorhombic phase and (b) AP-LT triclinic phase. The yellow circles represent Bi3+, the red circles represent Pb4+, and the blue circles represent Bi5+.

orthorhombic phases of both x = 0.05 and 0.25 samples are (Bi,Pb)3+Ni3+O3. Accordingly, the unit-cell volumes of the HT phases are smaller than those of the RT phases, indicating the presence of NTE. Figure 6 shows the temperature dependence of the SXRD patterns of Bi1−xPbxNiO3 with x = 0.05, 0.10, 0.15, 0.20, and 0.25 upon heating. The x = 0.05, 0.10, and 0.15 samples showed a triclinic-to-orthorhombic transition via two-phase coexistence, as observed in Bi1−xLnxNiO3 and BiNi1−xFexO3. On the other hand, the x = 0.25 sample showed an orthorhombic-to-orthorhombic phase transition at 500 K. D

DOI: 10.1021/acs.chemmater.6b01160 Chem. Mater. XXXX, XXX, XXX−XXX

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Table 3. NTE Parameters of Bi1−xPbxNiO3 (x = 0.05, 0.10, 0.15, 0.20, and 0.25)

a

x

TNTEa (K)

transition temperature range (K)

coefficient of linear thermal expansion, α (ppm/K)

0.05 0.10 0.15 0.20 0.25

530 520 511 500 500

20 20 30 30 50

−398 −250 −124 −46 −42

TNTE is the temperature at which NTE is observed.

determined to be 4+ by HAXEPS, the formal valence state should be (Bi3+0.5Bi5+0.5)1−xPb4+xNi2+O3. Furthermore, as clearly indicated in Figure 2, the Ni valence at RT was independent of x. The average ionic radius of Bi3+ and Bi5+ is (1.03 + 0.76)/2 = 0.895 Å, and that of Pb4+ is 0.78 Å. The volume contraction is induced by the substitution of large Bi ions with small Pb ions.

Figure 6. Temperature variation of the magnified SXRD patterns around the prominent peaks of Bi1‑xPbxNiO3 (x = 0.05, 0.10, 0.15, 0.20, and 0.25) upon heating.

The x = 0.20 sample was the mixture of triclinic and orthorhombic phases at RT, but the triclinic phase disappeared at 450 K, and an orthorhombic-to-orthorhombic phase transition that was essentially the same as that for the x = 0.25 sample was observed. It is clarified by HAXPES measurement and PDF analysis that the LT orthorhombic phase has (Bi3+0.5Bi5+0.5)1−xPb4+xNi2+O3 charge distribution with short-range ordering of Bi 3+/Bi 5+. The observed orthorhombic-to-orthorhombic transition is most probably a charge transfer transition to (Bi,Pb)3+Ni3+O3. The phase fractions and the unit-cell volumes of the LT and HT phases were determined by Rietveld analysis, and weighted average unit-cell volumes, f LTVLT + f HTVHT (where f LT (HT) represents the phase fraction of the LT (HT) phase and VLT (HT) represents the unit-cell volume of the LT (HT) phase), are plotted as a function of temperature in Figure 7a. All of the

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CONCLUSION The valence distribution and local structure of Bi1−xPbxNiO3 (x = 0.05, 0.10, 0.15, 0.20, and 0.25) were investigated by means of comprehensive studies of Rietveld analysis of synchrotron Xray diffraction (SXRD) data, Ni L-edge X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of the synchrotron X-ray total scattering data. Short-range ordering of Bi3+ and Bi5+ was observed in Bi0.75Pb0.25NiO3. The average crystal structure was an orthorhombic (Pbnm) one with a unique Bi/Pb site, but the HAXPES study and the PDF analysis revealed the presence of Bi3+/Bi5+ short-range ordering. Volume shrinkage upon heating, as observed in Bi1−xLnxNiO3 and BiNi1−xFexO3, occurred at ∼500 K. This was most probably due to the intermetallic charge transfer between Bi5+ and Ni2+, but other explanations might be possible. The present results support the unstable nature of the Bi4+ state and the valence skipper characteristic of Bi ions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01160. XRD patterns of BiNiO3, PbNiO3 and precursor, and recovered sample of Bi0.2Pb0.2NiO3 before and after washing; EDS spectrum of Bi0.8Pb0.2NiO3; fitting results for Bi 4f HAXPES spectra of Bi0.75Pb0.25NiO3, BiNiO3, and BiFeO3; and fitting results for Pb 4f HAXPES spectra of Bi0.75Pb0.25NiO3, PbNiO3, and PbTiO3 (PDF)

Figure 7. (a) Temperature dependence of the weighted average unitcell volume of Bi1−xPbxNiO3. (b) Composition dependence of the average unit-cell volume at RT.

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samples showed NTE at ∼500 K. Contrary to the results for Bi1−xLnxNiO3 and BiNi1−xFexO3, where the temperature at which NTE is observed (TNTE) decreased as x increased, the TNTE in this case was almost independent of x. On the other hand, the transition temperature range increased as x increased. The volume shrinkage of the x = 0.05 sample was almost discontinuous, while the x = 0.25 sample exhibited an NTE with a coefficient of linear thermal expansion (α) equal to −42 ppm/K within the temperature range of 500−550 K. TNTE, the transition temperature range, and the α value of the x = 0.05− 0.25 samples are summarized in Table 3. Figure 7b shows the composition dependence of the average unit-cell volume for Bi1−xPbxNiO3 at RT. The average unit-cell volume at RT decreased as x decreased. Since the valence of Pb was

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Sakai). *E-mail: [email protected] (M. Azuma). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge Prof. Yoshiyuki Inaguma for the provision of PbNiO3 sample and Prof. Takayuki Uozumi for fruitful discussion. This work was partially supported by KAKENHI on Innovative Areas (No. 26106507), 15K14119 E

DOI: 10.1021/acs.chemmater.6b01160 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

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and 16H02393, from the Japan Society for the Promotion of Science (JSPS), and by the Kanagawa Academy of Science and Technology (KAST). The synchrotron-radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI), Japan Atomic Energy Agency (JAEA) and National Institute for Materials Science (NIMS) (Proposal Nos. 2012B3782, 2013B1748, 2013B1753, 2014A1732, 2014A3703, 2014B1731, 2014B3701, 2014A4905, 2015A4909 and 2015B4905). Part of this work was supported by the NIMS microstructural characterization platform as a program of the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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DOI: 10.1021/acs.chemmater.6b01160 Chem. Mater. XXXX, XXX, XXX−XXX