Stability of Polar Structure in Filling-Controlled Giant Tetragonal

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Stability of Polar Structure in Filling-Controlled Giant Tetragonal Perovskite Oxide PbVO3 Hajime Yamamoto,*,†,○ Takahiro Ogata,† Yuki Sakai,∥ and Masaki Azuma*,† †

Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥ Kanagawa Institute of Industrial Science and Technology, Ebina, Kanagawa 243-0435, Japan ○

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 02/06/19. For personal use only.

S Supporting Information *

ABSTRACT: The crystal structure and stability of a giant tetragonal phase in electron-doped Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3) and hole-doped Pb1−xNaxVO3 (x = 0.1, 0.2, and 0.3) were studied. Electron doping effectively destabilized the tetragonal structure. The c/a ratio, spontaneous polarization, and tetragonal-to-cubic phase transition pressure systematically decreased with increasing Bi3+ substitution. In contrast, hole doping hardly affected the tetragonal distortion and structural stability. We showed that electron doping is an effective way to control the stability of the tetragonal phase of PbVO3 with a 3d1 electronic configuration.

1. INTRODUCTION Giant tetragonal perovskite oxides are attractive candidates for negative thermal expansion, ferroelectric, and piezoelectric materials. BiCoO3, PbVO3, Bi2ZnTiO6, and Bi2ZnVO6 were found to have a giant tetragonal structure with the space group P4mm and a c/a ratio larger than 1.2.1−6 Their polar tetragonal to paraelectric phase transitions under high-pressure conditions are accompanied by colossal volume contractions.4,6−8 However, such structural phase transitions are not observed in these compounds upon heating at ambient pressure because their ferroelectric Curie temperatures are much higher than their decomposition temperatures. Controlling the stability of the polar structure is key for realizing negative thermal expansion in giant tetragonal perovskites.9 PbVO3 has a tetragonal perovskite structure with the space group P4mm.3 The c/a ratio and the spontaneous polarization (PS) reach 1.23 and 101 μC/cm2 by the point charge model, respectively.4 The crystal structure is shown in Figure 1 (a). Both A-site and B-site cations of the perovskite ABO3 contribute to the enhanced polar structure. The stereochemical activity of the Pb2+ 6s2 lone pair and strong covalency of Pb−O bonds stabilize the tetragonal structure.10,11 The 3d1 electronic configuration of V4+ further enhances the distortion due to the Jahn−Teller (JT) effect.12 The square-pyramidal coordination of the B-site lifts the 3d orbital degeneracy. The t2g orbital splits into stabilized 3dxy and destabilized degenerate 3dyz, 3dzx orbitals and hence the single electron occupies only the former, as shown in Figure 1 (b).13 The 3d1 electronic configuration results in the square-pyramidal JT distortion.12 The electron in the 3dxy orbital is localized owing to the strong Coulombic repulsion; that is, the material is a Mott insulator. PbVO3 © XXXX American Chemical Society

undergoes a phase transition from tetragonal to cubic phases at a high pressure of ∼3 GPa at room temperature, accompanied by an insulator-to-metal transition (IMT) and a colossal unit cell volume collapse of 10.6%.4,8,14,15 Recently, we reported that electron-doped PbVO3, namely Pb1−xBixVO3 (x = 0.20 and 0.30) and Pb0.76La0.04Bi0.20VO3, exhibited a colossal negative thermal expansion with a volume shrinkage of 6− 8%, even around room temperature.9 Here, we report the influence of filling control on the stability of the tetragonal structure in more detail. The structural evolution of electron-doped Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3) and hole-doped Pb1−xNaxVO3 (x = 0.1, 0.2, and 0.3) was studied. Bi3+ and Na+ were substituted because these ions have relatively similar ionic radii, 1.17 Å for Bi3+, and 1.18 Å for Na+, when the coordination number is 8.16 The tolerance factors t are larger than 1 for all the compounds, specifically 1.039 for PbVO3, 1.018 for Pb0.7Bi0.3VO3, and 1.025 for Pb0.7Na0.3VO3.17 Notably, semiconductive (Mott-insulating) behavior was preserved even after electron or hole doping. The electron doping effectively reduced the degree of tetragonal distortion and tetragonal-to-cubic transition pressure, whereas hole doping did not. Electron doping is thus an effective way to control the stability of the polar tetragonal structure in PbVO3 with a 3d1 electronic configuration.

2. EXPERIMENTAL SECTION Polycrystalline samples of Pb1−xBixVO3 (x = 0, 0.1, 0.2, and 0.3) were synthesized from mixtures of PbO, Bi2O3, V2O3, and V2O5 packed in a Received: November 29, 2018

A

DOI: 10.1021/acs.inorgchem.8b03333 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Crystal structure of PbVO3. (b) Schematic diagram of 3d energy levels and the electronic configuration of V4+ in a pyramidal coordination. Observed (red points), calculated (blue line), and difference (green line) SXRD patterns of (c) Pb0.9Na0.1VO3, (d) Pb0.8Na0.2VO3, (e) Pb0.7Na0.3VO3, (f) Pb0.9Bi0.1VO3, (g) Pb0.8Bi0.2VO3, and (h) Pb0.7Bi0.3VO3 at room temperature. The tick marks correspond to the position of Bragg reflections of the P4mm tetragonal phases (top) and impurity phases of β-Pb3V2O8 (middle) and γ-Pb3V2O8 (bottom) for Pb0.9Na0.1VO3, Pb0.9Bi0.1VO3, Pb0.8Bi0.2VO3, Pb0.7Bi0.3VO3, and β-Pb3V2O8 (middle-upper); γ-Pb3V2O8 (middle-lower); and NaV2O5 (bottom) for Pb0.8Na0.2VO3 and Pb0.7Na0.3VO3. The larger views of the plots (including PbVO3) are presented in Figures S1−7.

structures.6,21−24 The broadening was well-described by Stephens’ phenomenological model.21,25 All the compounds have a tetragonal perovskite structure with the space group P4mm. The obtained SXRD patterns agree well with the calculated patterns. The refined structural parameters, with satisfactorily low agreement factors, are summarized in Table 1. The refinement results indicate about 30% of sodium loss. The amount of a secondary phase of NaV2O5 increased with increasing sodium substitution in Pb1−xNaxVO3 samples most probably because of the sodium loss. The bismuth concentration could not be refined owing to the similar scattering factors of X-rays for Bi and Pb. Figures 2 (a)−(d) show the refined lattice parameters, c/a ratios, unit cell volumes, and bond valence sums (BVSs) for the vanadium ion. All figures are plotted assuming the target chemical compositions. The V−O bond lengths and calculated BVSs are summarized in Table 2. The lattice parameters obey Vegard’s law, indicating that Bi or Na was successfully substituted. The BVSs reveal that the valence of vanadium ions changed in proportion to Bi3+ and Na+ substitution. Interestingly, only bismuth substitution effectively suppressed the tetragonal distortion. The c/a ratios systematically decreased from 1.23 for PbVO3 to 1.09 for Pb0.7Bi0.3VO3. The unit cell volumes also shrank with increasing bismuth substitution, even though the B-site cations were reduced to V(4−x)+. From the viewpoint of ferroelectrics, the volume shrinkage is attributed to the reduction of spontaneous polarization.26 In contrast to bismuth substitution, the c/a

Pt capsule at 6 GPa and 1473 K for 1 h by using a cubic anvil cell-type high-pressure apparatus. Pb1−xNaxVO3 (x = 0, 0.1, 0.2, and 0.3) were synthesized from PbO, NaVO3, V2O3, and V2O5 at 6 GPa and 1273 K for 1 h. NaVO3 was prepared from a stoichiometric mixture of Na2CO3 and V2O5 via a solid-state reaction at an air atmosphere at 823 K for 24 h with intermediate grindings. The synthesized NaVO3 was analyzed using powder X-ray diffraction. Synchrotron X-ray diffraction (SXRD) measurements for Pb1−xBixVO3 and Pb1−xNaxVO3 were carried out with the large Debye−Scherrer camera installed on the BL02B2 beamline of SPring-8.18 The wavelength was 0.420172(1) Å. The diffraction data were analyzed with the Rietveld method using the RIETAN-FP program.19 The crystal structures were drawn using the program VESTA.20 Electrical resistivity measurements by a pseudofour-probe method under high pressure (up to 4 GPa) were performed using a data logger (34972A, Agilent) and a cubic anvil cell-type high-pressure apparatus. Molybdenum sheets were used as electrodes.

3. RESULTS AND DISCUSSION Tetragonal perovskites Pb1−xBixVO3 and Pb1−xNaxVO3 (x = 0, 0.1, 0.2, and 0.3) were successfully obtained using highpressure synthesis. Trace amounts of impurities such as βPb3V2O8, γ-Pb3V2O8, NaV2O5, Pb3(CO3)2(OH)2, and some unknown phases were observed. The calculated mole percentage of impurity phases by the Rietveld refinements are listed in Table S1. The SXRD patterns of Pb1−xBixVO3 and Pb1−xNaxVO3 at room temperature are shown in Figures 1 (c)−(h). Anisotropic line broadening was observed for all the samples owing to the microstrain in ferroelectric domain B

DOI: 10.1021/acs.inorgchem.8b03333 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Refined Structural Parameters and Agreement Factors of PbVO3, Pb1−xNaxVO3, and Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3) at Room Temperature Pb1−xNaxVO3a Pb

Na/Bi

V O(1) O(2) a c c/a V formula weight Rwp Rp RB S

z Uiso g z Uiso g z Uiso z Uiso z Uiso (Å) (Å)

(Å2)

(Å2)

(Å2) (Å2) (Å2)

(Å3) (%) (%) (%)

PbVO3a

x = 0.3

x = 0.2

x = 0.1

0 0.0089(2) 0.798(5) 0 0.0089(2) 0.202(5) 0.5611(7) 0.0139(9) 0.220(2) 0.004(2) 0.690(1) 0.004(1) 3.7690(2) 4.6589(3) 1.23609(3) 66.182(2) 268.920 6.607 4.507 1.548 3.390

0 0.0074(1) 0.868(4) 0 0.0074(1) 0.132(4) 0.5636(5) 0.0069(6) 0.221(2) 0.006(1) 0.687(1) 0.006(1) 3.77512(7) 4.6539(1) 1.23278(1) 66.325(2) 281.794 5.638 3.789 2.323 2.478

0 0.0066(1) 0.936(5) 0 0.0066(1) 0.064(5) 0.5664(4) 0.0062(6) 0.221(2) 0.004(1) 0.694(1) 0.004(1) 3.79038(3) 4.66436(5) 1.23058(1) 67.013(1) 294.348 5.729 3.866 2.447 2.922

0 0.0057(1) 1

0.5667(5) 0.0035(6) 0.219(2) 0.005(1) 0.688(1) 0.005(1) 3.80399(3) 4.67567(6) 1.22915(1) 67.658(1) 306.140 6.946 4.387 2.225 3.181

Pb1−xBixVO3a,b x = 0.1

x = 0.2

x = 0.3

0 0.0081(1) 0.90c 0 0.0081(1) 0.10c 0.5674(4) 0.0048(4) 0.202(1) 0.005(1) 0.686(1) 0.005(1) 3.82004(5) 4.57379(8) 1.19732(2) 66.744(2) 306.318 5.491 3.862 1.784 2.719

0 0.0119(1) 0.80c 0 0.0119(1) 0.20c 0.5642(6) 0.0056(6) 0.176(2) 0.011(1) 0.675(1) 0.011(1) 3.84729(4) 4.4076(1) 1.14564(2) 65.240(2) 306.496 6.330 4.686 2.256 3.099

0 0.0153(1) 0.70c 0 0.0153(1) 0.30c 0.5590(7) 0.0076(6) 0.158(2) 0.012(1) 0.668(1) 0.012(1) 3.88036(4) 4.23091(8) 1.09034(2) 63.706(2) 306.674 6.482 4.417 2.469 3.786

a

Space group: P4mm (No. 99), Z = 1, The temperature factors of A-site cations and oxygens, O(1) and O(2), were assumed as the same, respectively. The Rietveld refinements were performed within a 2θ range of 2−40°. bThe values were cited from ref 9. cOccupation factors were fixed to each formula.

ratios were roughly constant for Pb1−xNaxVO3. The lengths of the a- and c-axes slightly shortened. The unit cell volume shrank, reflecting the oxidization of the vanadium ion to V(4+x)+. The size of the V−O framework predominantly determines the cell volume in a perovskite structure.27 To elucidate the coordination environments of the B-site, the composition evolution of the V−O bonds, spontaneous polarization (PS) calculated based on the point charge model, and the displacement of the vanadium atom along the c-axis from the center of the unit cell (δV) are shown in Figures 3 (a)−(c). In Pb1−xBixVO3, the displacement of vanadium atoms systematically decreased; that is, the VO5 pyramid was changed toward a uniform octahedron. Such a change results in narrower energy splitting between 3dxy and 3dyz,zx orbitals. Spontaneous polarization decreased from 102 to 82 μC/cm2 with increasing Bi3+ substitution. Large polar distortion was preserved in Pb1−xNaxVO3. The small atomic displacements of vanadium and oxygen are attributed to the shrinkage of the vanadium ion due to its increased oxidation state. The spontaneous polarizations were also roughly constant. The Pb/A−O bond lengths are summarized in Figure 4 and Table 3. The A-site cations systematically shifted toward the

Figure 2. (a) Lattice parameters a and c, (b) c/a ratio, (c) cell volume, (d) and BVS for the vanadium ion in PbVO3, Pb1−xNaxVO3, and Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3).

Table 2. V−O Bond Lengths and BVSsa for PbVO3, Pb1−xNaxVO3, and Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3) Pb1−xNaxVO3 V−O(1) (Å) V−O(1)′ (Å) V−O(2) (Å) BVS

PbVO3

x = 0.3

x = 0.2

x = 0.1

1.588(10) 3.071(10) 1.979(2) 4.095

1.593(9) 3.060(9) 1.973(2) 4.103

1.612(8) 3.053(8) 1.986(1) 3.942

1.625(9) 3.051(9) 1.984(2) 3.891

Pb1−xBixVO3 x = 0.1

x = 0.2

x = 0.3

1.674(7) 2.900(7) 1.985(1) 3.717

1.710(9) 2.698(9) 1.985(2) 3.633

1.696(9) 2.534(9) 1.994(1) 3.668

Vi = ∑jSij, Sij = exp{(r0 − rij)/0.37}. Values are calculated using r0 = 1.784 for V4+.

a

C

DOI: 10.1021/acs.inorgchem.8b03333 Inorg. Chem. XXXX, XXX, XXX−XXX

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The strong pyramidal distortion at the B-site preserved the displacement of A-site cations. The A- and B-site coordination analyses indicate that the valence of the vanadium ion is a dominant factor in the change of the polar distortion in Pb1−xBixVO3 and Pb1−xNaxVO3. Next, the stability of the tetragonal phase was evaluated via a comparison of tetragonal-to-cubic phase transition pressures. Figures 5 (b)−(h) shows the pressure dependence of the

Figure 3. (a) V−O bond lengths in PbVO3, Pb1−xNaxVO3, and Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3). Inset shows the environment of the V−O polyhedron. (b) Composition evolution of spontaneous polarization (PS) and (c) the atomic displacement of the vanadium atom along the c-axis from the center of the unit cell (δV).

Figure 5. (a) Schematic illustration of the tetragonal-to-cubic transition and energy levels of 3dxy, 3dyz, and 3dzx orbitals in PbVO3. Pressure dependence of room-temperature electrical resistivity of (b) Pb0.9Na0.1VO3, (c) Pb0.8Na0.2VO3, (d) Pb0.7Na0.3VO3, (e) PbVO 3 , (f) Pb 0.9 Bi 0.1 VO 3 , (g) Pb 0.8 Bi 0.2 VO 3 , and (h) Pb0.7Bi0.3VO3. Red and black circles represent ρ−P and −d(log ρ)/ d, respectively. Orange arrows indicate peaks of the −d(log ρ)/dP curves, defined as IMT pressure. Figure 4. (a) Pb/A−O bond lengths in PbVO3, Pb1−xBixVO3, and Pb1−xNaxVO3 (x = 0.1, 0.2, and 0.3). (b) Schematic illustration of Pb/ A−O bonds. Shortest Pb/A−O(2), intermediate Pb/A−O(1), and longest Pb/A−O(2)′ are present.

electrical resistivity of Pb1−xBixVO3 and Pb1−xNaxVO3. The orange arrow indicates the transition pressure defined by the inflection point in each ρ−P curve. The Mott-insulating (semiconductive) property was retained at ambient pressure despite electron or hole doping. The 3d electrons were likely localized due to the strong electron repulsion on the vanadium ion. A drop in the electrical resistivity (IMT) was observed for all the samples. It was observed at ∼3 GPa in PbVO3, which is consistent with previous reports.4,15 In Pb1−xBixVO3, the transition pressures systematically decreased to ∼0.8 GPa (x

center of the Pb/A−O polyhedron in Pb1−xBixVO3 despite the stereochemical activity of Bi3+ and strong covalency of Bi−O bonds, the same as Pb2+.28 This change is likely associated with the decrease in the spontaneous polarization. The distortions of the Pb/A−O polyhedron were almost constant in Pb1−xNaxVO3 despite the nonstereochemical activity of Na+.

Table 3. Pb/A−O Bond Lengths for PbVO3, Pb1−xNaxVO3, and Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3) Pb1−xNaxVO3 Pb/A−O(1) (Å) Pb/A−O(2) (Å) Pb/A−O(2)′ (Å)

PbVO3

x = 0.3

x = 0.2

x = 0.1

2.856(4) 2.373(4) 3.728(5)

2.861(3) 2.384(3) 3.714(4)

2.871(3) 2.373(3) 3.750(4) D

2.879(3) 2.397(4) 3.736(5)

Pb1−xBixVO3 x = 0.1

x = 0.2

x = 0.3

2.854(3) 2.390(3) 3.673(4)

2.829(3) 2.398(4) 3.543(5)

2.824(2) 2.397(3) 3.426(5)

DOI: 10.1021/acs.inorgchem.8b03333 Inorg. Chem. XXXX, XXX, XXX−XXX

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compounds.30 Only the 3dxy orbital is partially occupied in hole-doped Pb1−xNaxVO3 and the pentavalent V ion with a 3d0 electronic configuration favors out-of-center distortion due to a second-order JT effect.31 The stability of the tetragonal phase was thus hardly reduced by hole doping. It is concluded that the valence state of the vanadium ion determines the characteristics of oxygen coordination, which affects the stability and extent of the tetragonal structure. We believe that filling control may also play a key role in realizing negative thermal expansion in other giant tetragonal perovskites such as BiCoO3.

= 0.3). In contrast, the transition pressures were almost constant in Pb1−xNaxVO3. To further discuss the stability of the tetragonal structure, the composition−pressure phase diagram was constructed based on the IMT pressures, as shown in Figure 6 (a). The

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CONCLUSIONS Electron-doped Pb1−xBixVO3 (x = 0.1, 0.2, and 0.3) and holedoped Pb1−xNaxVO3 (x = 0.1, 0.2, and 0.3) were successfully synthesized using a high-pressure method, and the stability of their tetragonal structure was investigated. The c/a ratio and spontaneous polarization systematically decreased with increasing Bi3+ substitution. The tetragonal-to-cubic phase transition occurred at a moderate pressure of 0.8 GPa in Pb0.7Bi0.3VO3 despite a large spontaneous polarization of 82 μC/cm2. In contrast, hole doping did not affect the polar distortion and transition pressure. We showed that the stability of tetragonal phase can be controlled by electron doping.

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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.inorgchem.8b03333. Observed and calculated SXRD patterns of PbVO3, Pb 0 . 9 Na 0 . 1 VO 3 , Pb 0 . 8 Na 0 . 2 VO 3 , Pb 0 . 7 Na 0 . 3 VO 3 , Pb0.9Bi0.1VO3, Pb0.8 Bi 0.2VO3, and Pb0.7 Bi 0.3VO3 at room temperature and calculated mole percentage of impurity phases by the Rietveld refinements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 6. (a) Pressure phase diagram of Pb1−xA′xVO3 (A′ = Na+, Bi3+, x ≤ 0.3) based on the results of the pressure dependence of electrical resistivity measurement at 300 K. Green and white regions represent tetragonal (insulator) and cubic (metal) phases, respectively. Red points correspond to the transition pressure. (b) Schematic diagram of energy levels of 3dxy, 3dyz, and 3dzx orbitals and 3d electronic coordination in PbVO3, electron-doped Pb1−xBixVO3, and hole-doped Pb1−xNaxVO3. Crystal structures are PbVO3, Pb0.7Bi0.3VO3, and Pb0.7Na0.3VO3 viewed along the a-axis. The cutoff distance of the V−O bond is 3 Å.

ORCID

Hajime Yamamoto: 0000-0001-6327-6803 Masaki Azuma: 0000-0002-8378-321X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research, 16H02393 and 18H05208, from the Japan Society for the Promotion of Science (JSPS) and by Kanagawa Institute of Industrial Science and Technology. The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (2017B1697).

IMT systematically shifted to lower pressure with increasing electron doping. Regarding efficiency, it was estimated that 0.1electron doping corresponds to 0.7−0.8 GPa. Notably, Pb0.7Bi0.3VO3 has a transition pressure of ∼0.8 GPa despite its large spontaneous polarization of 82 μC/cm2; this pressure is significantly lower than the polar(tetragonal)-to-nonpolar transition pressure in other compounds such as BiCoO3 (∼3 GPa)7 and PbTiO3 (∼11.2 GPa).29 The electronic configurations of the vanadium ion and the crystal structures are schematically illustrated in Figure 6 (b). The doped electrons occupying 3dyz and 3dzx orbitals reduce the energy gain of the square-pyramidal JT distortion in Pb1−xBixVO3. The trivalent V ion (3d2 electronic configuration) generally favors octahedral coordination in oxide

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DOI: 10.1021/acs.inorgchem.8b03333 Inorg. Chem. XXXX, XXX, XXX−XXX