Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually

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Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually High Valence Fe3.5+ Peng Xiong,† Fabio D. Romero,†,‡ Yoshiteru Hosaka,† Haichuan Guo,† Takashi Saito,† Wei-Tin Chen,§ Yu-Chun Chuang,∥ Hwo-Shuenn Sheu,∥ Graham McNally,⊥ J. Paul Attfield,⊥ and Yuichi Shimakawa*,†,# †

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Hakubi Project, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan § Center for Condensed Matter Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ∥ National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu30076, Taiwan ⊥ Centre for Science at Extreme Conditions and School of Chemistry, The University of Edinburgh, Edinburgh EH9 3JZ, U.K. # Integrated Research Consortium on Chemical Sciences, Uji, Kyoto 611-0011, Japan ‡

S Supporting Information *

ABSTRACT: A Sr analogue of Ca0.5Bi0.5FeO3, Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe3.5+ by a single charge disproportionation (CD) transition (Fe3.5+ → 0.75Fe3+ + 0.25Fe5+) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca0.5Bi0.5FeO3. Conduction-band narrowing due to the significant bend in the Fe− O−Fe bond in the rhombohedral R3̅c crystal structure stabilized the chargedisproportionated state at low temperatures. Most importantly, Bi3+ ions in Sr0.5Bi0.5FeO3 do not act as countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi5+ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe3+ spins align antiferromagnetically and one-fourth of the Fe3+ spins are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3+ spins.

I. INTRODUCTION Transition-metal ions with unusually high valence states can be stabilized in some oxides synthesized under strong oxidizing conditions.1−4 Electronic instability due to such unusually high valence states is often relieved via charge transitions, and these transitions are typically accompanied by drastic changes in structural, transport, and magnetic properties.5−8 Material systems containing unusually high valence ions thus provide excellent platforms for investigating complex charge−lattice− spin couplings. Although the typical valence states of Fe ions in oxides are 2+ and 3+, unusually high valence states of Fe ions such as Fe4+ can be stabilized in some oxides and are of particular interest. Fe4+ in the perovskite CaFeO3, for instance, relieves its electronic instability by a very unusual charge change at 290 K: charge disproportionation (CD) to Fe3+ and Fe5+ (2Fe4+ → Fe3+ + Fe5+).5 Because the strong hybridization of low-lying Fe 3d orbitals in Fe4+ with O 2p orbitals produces oxygen p holes (ligand holes, L), the electronic structure of the unusually high valence Fe4+ with the nominal d4 electron configuration can be described as d5L. The CD is thus expressed as localization of the ligand holes as 2d5L → d5 + d5L2.9,10 The localization of © XXXX American Chemical Society

the ligand holes at the Fe−O sites alternately makes a rock-salt type order of the Fe3+O6 and Fe5+O6 octahedra. Fe4+ in the cubic perovskite SrFeO3, on the other hand, behaves differently. The itinerant ligand holes in SrFeO3 are stabilized in the broad conduction bands due to the linear Fe−O−Fe bonds and are tolerant to delocalization.11 As a result, SrFeO3 stays metallic down to low temperatures and does not undergo a charge transition. The perovskite Ca 0.5 Bi 0.5 FeO 3 can be obtained by substituting Bi3+ ions for half of the Ca2+ ions in CaFeO3, and the doped electrons change the unusually high valence Fe4+ to Fe3.5+.12 This electron doping induces charge behaviors different from the CD in CaFeO3, and when it is cooled the compound shows successive transitions due to CD (Ca0.5Bi0.5Fe3.5+O3 → Ca0.5Bi0.5Fe3+0.67Fe4.5+0.33O3) at 250 K and intermetallic charge transfer (CT) between B-site Fe and A-site Bi (0.25Bi3+ + 0.33Fe4.5+ → 0.25Bi5+ + 0.33Fe3+) at 200 K. The Bi ions in Ca0.5Bi0.5FeO3 are the countercations accepting the charge transferred from the unusually high Received: October 31, 2017

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DOI: 10.1021/acs.inorgchem.7b02785 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry valence Fe ions, as in the intermetallic CT in BiNiO3 under pressure (Bi3+Ni3+O3 → Bi3+0.5Bi5+0.5Ni2+O3).13 The CT transition occurring after the CD transition completely relieves the electronic instability of Fe3.5+ in Ca0.5Bi0.5FeO3. Both are accompanied by changes in the structural and magnetic properties, indicating strong couplings of the charge, lattice, and spin in Ca0.5Bi0.5FeO3. Our primary concern is what kind of charge transitions would be induced in Bi-doped SrFeO3. Although Fe4+ in SrFeO3 shows neither CD nor CT transitions, it is of interest how the instability of unusually high valence Fe3.5+ is relieved in the presence of Bi3+ that can be oxidized to Bi5+. In this study we prepared the novel perovskite Sr0.5Bi0.5FeO3 under high-pressure and high-temperature conditions and found that at low temperatures it shows a single CD transition of the Fe3.5+ to Fe3+ and Fe5+ in a 3:1 ratio. The Bi ions in Sr0.5Bi0.5FeO3, unlike those in Ca0.5Bi0.5FeO3, did not induce an intermetallic CT transition. We investigated the compound’s crystal and magnetic structures, and here we discuss the observed CD transition.

II. EXPERIMENTAL SECTION Sr0.5Bi0.5FeO3 was prepared by a solid-state reaction under highpressure and high-temperature conditions. Starting materials (SrCO3, Bi2O3, and Fe2O3) were well mixed and calcined at 850 °C. The resultant polycrystalline sample and oxidizing agent KClO4 were well ground, sealed in a platinum capsule, and treated at 4−6 GPa and 1200 °C for 30 min with a cubic anvil-type high-pressure apparatus. The six anvils, which are made of a superhard sintered tungsten carbide alloy, synchronously compress the sample three-dimensionally through a pyrophyllite medium.14 The sample was then quenched to room temperature before the pressure was released. The residual KCl and KClO4 were removed by washing with distilled water. The crystal and magnetic structures of Sr0.5Bi0.5FeO3 were analyzed by evaluating synchrotron X-ray powder diffraction (SXRD) and neutron powder diffraction (NPD) data. The SXRD data from room temperature to 80 K were collected at a wavelength of 0.82656 Å at the beamline TPS09A at the National Synchrotron Radiation Research Center. The sample was packed into a glass capillary with an inner diameter of 0.1 mm. The obtained data were analyzed by the Rietveld method using the RIETAN-FP program.15 NPD measurements were performed on a sample contained in a vanadium can using the WISH diffractometer at the ISIS neutron source, Rutherford Appleton Laboratory, Didcot, U.K. The diffraction data were analyzed with the GSAS suite programs.16 To investigate the valence and magnetic states of Fe ions in Sr0.5Bi0.5FeO3, 57Fe Mössbauer spectroscopy measurements were carried out in a transmission geometry with a constant-acceleration spectrometer by using a 57Co/Rh radiation source. α-Fe was used to calibrate the velocity scale, the isomer shift (IS), and the hyperfine field (HF). The obtained spectra were fitted to Lorentzian functions by using the standard least-squares method. Magnetic properties of the sample were measured with a SQUID magnetometer (MPMS Quantum Design). Resistivity was measured with a standard four-probe method by using a commercial Physical Property Measurement system (Quantum Design).

Figure 1. Rietveld refinement profiles for Sr0.5Bi0.5FeO3. (a) Roomtemperature SXRD data. The bottom and top vertical bars respectively indicate the diffraction peak positions for Sr0.5Bi0.5FeO3 and impurity Fe2O3. (b) Room-temperature NPD data. The bottom vertical bars indicate the nuclear diffraction of Sr0.5Bi0.5FeO3. The middle and top vertical bars respectively show the nuclear and magnetic reflections of impurity Fe2O3. (c) NPD data collected at 4 K. The bottom and top vertical bars respectively show the nuclear and magnetic reflections of Sr0.5Bi0.5FeO3. The third and second vertical bars respectively show the nuclear and magnetic reflections of impurity Fe2O3.

The refined occupancies for the A-site Sr/Bi were respectively 0.486/0.514(3), indicating random distribution of the ions at the site. The oxygen site occupancy refined with the NPD data was 0.991(2), indicating the oxygen stoichiometry of the compound. The occupancy for oxygen was fixed to 1 in the final refinement. Given the composition of Sr0.5Bi0.5FeO3 with typical-valence Sr2+, Bi3+, and O2− ions, unusually high valence Fe3.5+ is suggested in this compound. The bond valence sum (BVS)17,18 value calculated from the Fe−O bond lengths obtained from the structure refinement results is 3.4+, consistent with the nominal valence state of Fe. The unusually high-valence state of Fe was also confirmed by Mössbauer spectroscopy, as shown in Figure 2. The spectrum at room temperature consists of a singlet with an IS of 0.20 mm s−1, which is between the typical values for Fe3+ (∼0.54 mm s−1)19 and Fe4+ (∼0.07 mm s−1)5 for octahedral oxygen coordination and is also very close to the value for Fe3.5+ observed in Ca0.5Bi0.5FeO3 (Table 2). The present Sr0.5Bi0.5FeO3 containing unusually high-valence Fe3.5+ is thus a Sr analogue of the previously reported Ca0.5Bi0.5FeO3. The compound can also be considered to be Bi3+-doped SrFeO3, where the doped electrons due to the substitution of Bi3+ for Sr2+ produce Fe3.5+ at the B-site. The

III. RESULTS AND DISCUSSION The obtained sample was nearly a single phase (unreacted Fe2O3 < 1%). Room-temperature SXRD peaks could be indexed with a rhombohedral unit cell in the space group R3̅c (No. 167). The crystal structure was analyzed by the Rietveld method with SXRD and NPD data, and the results are shown in Figure 1a,b. The structure parameters obtained from the analysis are given in Table 1. The rhombohedral crystal structure is slightly distorted from that of the cubic perovskite. B

DOI: 10.1021/acs.inorgchem.7b02785 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Refined Structural Parameters of Sr0.5Bi0.5FeO3 at Room Temperaturea atom

site

x

y

z

Uiso (Å2)

cccupancy

BVS

Sr Bi Fe O

6a 6a 6b 18e

0 0 0 0.5446(6)

0 0 0 0

0.25 0.25 0 0.25

0.028(4) 0.028(4) 0.0037(4) 0.0013(8)

0.486(3) 0.514(−) 1.0 1.0

2.3 2.2 3.4

a

In the structure refinement the rhombohedral crystal structure was analyzed with the hexagonal cell. The BVS results calculated from the cation− O bond lengths are also given. Crystal data: space group R3̅c, a = 5.5243(2) Å, c = 13.5306(7) Å, Rwp = 2.7%.

Figure 3. Temperature dependence of the lattice parameters a and c and cell volume V of Sr0.5Bi0.5FeO3 in a hexagonal (rhombohedral) R3̅c lattice.

Figure 2. Mössbauer spectra of Sr0.5Bi0.5FeO3 at room temperature (top panel) and 4 K (bottom panel). The black circles show experimental data, and the solid lines show the fittings.

transition temperature is difficult to determine. When we examine the temperature dependence of electrical resistivity (Figure 4) in detail, however, we see that below 230 K it

Table 2. Mössbauer Parameters for Sr0.5Bi0.5FeO3 at Room Temperature and 4 K temp room temp 4K

component 3.5+

Fe Fe3+ Fe5+

IS (mm s−1)

HF (kOe)

area ratio (%)

0.23 0.43 0.00

50.4 25.6

100 75.1 24.9

substitution also causes rhombohedral structural distortion with a significant bend (165.5(2)°) in the Fe−O−Fe bond from the 180° bonding in cubic SrFeO3. Note that the Fe−O− Fe bond angle is larger than that in orthorhombic Ca0.5Bi0.5FeO3 (155.3°). When the sample was cooled, no apparent structural phase transitions were observed. Temperature dependences of the lattice parameters and the cell volume, plotted in Figure 3, show monotonic decreases down to about 80 K. The observed behavior is in sharp contrast to that due to the successive structural phase transitions caused by the CD at 250 K and CT at 200 K in Ca0.5Bi0.5FeO3. Although no structural transition was observed, an unusual charge change occurred in the present Sr0.5Bi0.5FeO3. The Mössbauer spectrum taken at 4 K (Figure 2) clearly shows a charge-disproportionated state of Fe. The spectrum consists of two sextets, with IS values of 0.43 and 0.00 mm s−1, in a 3:1 area ratio. These sextets are very similar to those observed in the charge-disproportionated state of CaFeO3 and are assigned to Fe3+ and Fe5+. The Mössbauer results therefore reveal that the CD transition Fe3.5+ → 0.75Fe3+ + 0.25Fe5+ takes place at a low temperature in Sr0.5Bi0.5FeO3. Although the chargedisproportionated state in Sr0.5Bi0.5FeO3 is evident, the

Figure 4. Temperature dependence of the electrical resistivity as a plot of log ρ versus 1000/T. The blue dashed line represents a typical semiconductor behavior given by an Arrhenius equation.

deviates from the normal semiconductor behavior. In the temperature dependence of magnetic susceptibility (Figure 5b) an anomaly at about 230 K is seen. The Mössbauer spectrum taken at 220 K looks like a single peak but is much broader than the room-temperature spectrum and actually can be fitted with 3:1 components of paramagnetic Fe3+ and Fe5+ (Figure 6 and Table 3). Because no other anomalies were observed in temperature-dependent experimental data, the CD transition temperature of Sr0.5Bi0.5FeO3 appears to be around 230 K. The absence of significant changes in the unit cell volume at the C

DOI: 10.1021/acs.inorgchem.7b02785 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

CD transition temperature is similar to the transition behaviors observed in CaFeO3 and La1/3Sr2/3FeO3.1,20 It is interesting to note that the Bi3+ substitution does not induce a CT transition in SrFeO3, which stays metallic down to low temperatures without any charge transition, whereas it induces a CT transition in CaFeO3, which originally shows a CD transition. Unlike the Bi ions in Ca0.5Bi0.5FeO3, the Bi ions in Sr0.5Bi0.5FeO3 cannot be countercations accepting the ligand (oxygen) holes from the unusually high valence Fe3.5+ ions with the d5L0.5 electron configuration in the oxygen hole description, resulting in the absence of a CT transition. The Bi3+ substitution causes the conduction-band narrowing due to the significant bend (165.5(2)°) in the Fe−O−Fe bond from the 180° bonding in SrFeO3. Therefore, at low temperatures, the ligand holes of Fe3.5+ are localized at some of the Fe sites making the d5L2 electronic state but not at the Bi sites. A negative chemical pressure applied to the Bi3+ ions (ionic radius 1.03 Å for coordination of six oxygens21,22) by the large cavity of the A-site Sr2+ ions (1.18 Å), which is also suggested by the low BVS for Bi (Table 1), prevents the much smaller Bi5+ ions (0.76 Å) from being stabilized, suppressing the intermetallic CT transition. In this sense, the situation is rather similar to the CD transition in La0.5Sr0.5FeO3.23 Although the Mössbauer spectrum of Sr0.5La0.5FeO3 at low temperature was analyzed with three components, their IS values suggest the essential 3:1 CD of Fe3+ and Fe5+. The Fe−O−Fe bond angle in Sr0.5Bi0.5FeO3 (165.5(2)°) is also close to that in Sr0.5La0.5FeO3 (167.8(2)°). (The crystallographic data of our Sr0.5La0.5FeO3 sample is listed in Table SI in the Supporting Information.) Like La3+ in Sr0.5La0.5FeO3, Bi3+ in Sr0.5Bi0.5FeO3 does not trigger a CT transition relieving the electronic instability of Fe3.5+. In the charge-disproportionated state at low temperature there is no indication of ordering of the 3:1 Fe3+ and Fe5+ ions in either the SXRD or NPD patterns. In the low-temperature (5 K) NPD pattern, however, magnetic superstructure reflections are evidently included. The magnetic order of the Fe spins in the CD state is consistent with the Mössbauer spectroscopy measurement result at 4 K, where both Fe3+ and Fe5+ components consist of sextets with large HFs. Assuming that the magnetic moments of all Fe ions are the same due to the random distribution of the Fe3+ and Fe5+ ions, the observed magnetic reflections are reproduced well with a G-type antiferromagnetic structure of the B-site Fe spins and the magnetic structure analysis gives a refined magnetic moment of 2.01(1) μB (Figure 7a). The refined profile of low-temperature NPD is plotted in Figure 1c. The proposed magnetic structure is shown in Figure 7b. In the G-type antiferromagnetic structure, where the nearestneighbor spins align in an antiparallel way, one-fourth of the Fe3+ spin sites are randomly replaced by Fe5+ spins, whose magnetic moments align ferromagnetically with the neighboring Fe3+ moments, according to the Kanamori−Goodenough rules.24 The average magnetic moment with this spin structure model is (0.75 × 5) − (0.25 × 3) = 3 μB. Considering the strong hybridization of the Fe 3d and O 2p orbitals, which would usually reduce the observed magnetic moment of Fe,20,25 this value is reasonably close to the magnetic moment obtained from the neutron magnetic structure analysis. Although this spin arrangement gives 8 μB per 4 Fe spins, those hypothetical ferromagnetic moments cancel out and no net magnetization would be produced due to the random distribution of the Fe5+ spins. This magnetic structure model is

Figure 5. (a) Temperature dependence of the magnetic susceptibility χ of Sr0.5Bi0.5FeO3 in a 10 kOe external magnetic field. The red and blue curves respectively represent ZFC and FC susceptibility. (b) Differential plots of the magnetic susceptibility data. (c) Isothermal magnetization curves measured at temperatures between 300 and 5 K.

Figure 6. Mössbauer spectra measured at different temperatures. The black circles show experimental data, and the solid lines show the fittings.

Table 3. Mössbauer Parameters for Sr0.5Bi0.5FeO3 at 220 K component 3+

Fe Fe5+

IS (mm s−1)

line width (mm s−1)

area ratio (%)

0.31 −0.05

0.57 0.38

74.2 25.8

D

DOI: 10.1021/acs.inorgchem.7b02785 Inorg. Chem. XXXX, XXX, XXX−XXX

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spins align antiferromagnetically, giving a G-type antiferromagnetic structure, and one-fourth of the Fe3+ spin sites are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3+ spins. The random distribution of the Fe5+ spins is consistent with the linear behavior evident in the isothermal M−H measurements. Unlike Ca0.5Bi0.5FeO3, which shows successive CD and intermetallic CT transitions, the Sr analogue Sr0.5Bi0.5FeO3 shows a single CD transition (Fe3.5+ → 0.75Fe3+ + 0.25Fe5+) relieving the electronic instability due to the unusually high valence of Fe3.5+. A significant bend in the Fe−O−Fe bond in the rhombohedral R3̅c crystal structure makes the conduction bands narrow, stabilizing the charge-disproportionated state at low temperatures. The Bi3+ ions in Sr0.5Bi0.5FeO3 cannot be countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of an intermetallic CT transition. A negative chemical pressure applied to the Bi3+ ions by the large cavity of the A-site Sr2+ ions prevents the much smaller Bi5+ ions from being stabilized, suppressing the intermetallic CT transition.

Figure 7. (a) Schematic drawing of the G-type antiferromagnetic structure obtained from analysis with the low-temperature neutron diffraction data. (b) Proposed magnetic structure of Sr0.5Bi0.5FeO3. The green and blue arrows respectively represent the spins of chargedisproportionated Fe3+ and Fe5+.

also consistent with the linear behavior observed in the isothermal M−H measurements below the CD transition temperatures (Figure 5c). The small refined magnetic moments at Fe spin sites may indicate another magnetic structure model with Fe5+ spin moments oriented in random directions, as they have been proposed to be oriented in CeCu3Fe4O12, which also contains Fe3.5+ at B-sites in the Asite-ordered perovskite structure and shows a similar 3:1 CD of Fe3+ and Fe5+.26 However, the present magnetic property measurement results do not support that model because neither net magnetization in the isothermal M−H measurements nor peak broadening of the Fe5+ Mössbauer spectrum below the CD transition temperature was observed. The magnetic transition temperature due to chargedisproportionated Fe3+ and Fe5+ spins was difficult to determine from the magnetic susceptibility measurements (Figure 4a). Nonetheless, the observed susceptibility below about 150 K shows large hysteresis between the field-cooled (FC) and zero-field-cooled (ZFC) mode measurements. The Mössbauer spectrum taken at 150 K apparently contains magnetically ordered sextet components (Figure 6). Therefore, the antiferromagnetic transition temperature of Sr0.5Bi0.5FeO3 appears to be about 150 K, which is lower than the CD transition temperature. The behavior where the magnetic ordering of Fe3+ and Fe5+ spins occurs and is followed by a CD transition is similar to the behaviors observed in CaFeO3 and Ca2FeMnO627 and is different from the simultaneous charge and spin transitions in CaCu3Fe4O12.8 The behavior is also very different from the successive charge transitions in Ca0.5Bi0.5FeO3 with an idle spin state between the CD and intermetallic CT transition temperature.28

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.inorgchem.7b02785. Rietveld refinement profile and refined structure parameters for Sr0.5La0.5FeO3 obtained from structure analysis with room-temperature XRD data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.S.: [email protected]. ORCID

J. Paul Attfield: 0000-0001-9763-3987 Yuichi Shimakawa: 0000-0003-1019-2512 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at the National Synchrotron Radiation Research Center, Taiwan (Proposal Nos. 2017-1-125-1 and 2017-1-125-2). The preliminary experiments were also conducted with the approval of the Japan Synchrotron Radiation Research Institute (Proposal Nos. 2016A1650 and 2017B1736). The ISIS neutron experiments were performed with the permission of the STFC. This work was partially supported by Grants-inAid for Scientific Research (Nos. 16H00888, 16H02266, and 17K19177) and by a grant for the Integrated Research Consortium on Chemical Sciences from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This work was also supported by the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program (A) Advanced Research Networks. Support was also provided by grants from the EPSRC and the Royal Society.

IV. CONCLUSIONS The novel compound Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions, was synthesized under high-pressure and high-temperature conditions. Rietveld refinements of SXRD and NPD data confirmed that the compound crystallized in a distorted perovskite structure with the rhombohedral space group R3̅c. A Mössbauer spectroscopy study revealed that CD (Fe3.5+ → 0.75Fe3+ + 0.25Fe5+) took place below 230 K. The charge-disproportionated Fe3+ and Fe5+ spins magnetically order at about 150 K, which is below the CD transition temperature. The magnetic structure was solved by a combined analysis of the low-temperature NPD profile and isothermal magnetization data. The nearest-neighbor Fe3+

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02785 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b02785 Inorg. Chem. XXXX, XXX, XXX−XXX