Melting of dxy Orbital Ordering Accompanied by Suppression of Giant

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Melting of d orbital ordering accompanied by suppression of giant tetragonal distortion and insulator to metal transition in Cr substituted PbVO

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Takahiro Ogata, Yuki Sakai, Hajime Yamamoto, Satyanarayan Patel, Peter Keil, Jurij Koruza, Shogo Kawaguchi, Zhao Pan, Takumi Nishikubo, Jürgen Rödel, and Masaki Azuma Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04680 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Chemistry of Materials

Melting of dxy orbital ordering accompanied by suppression of giant tetragonal distortion and insulator to metal transition in Cr substituted PbVO3 Takahiro Ogata,*, † Yuki Sakai,‡ Hajime Yamamoto,†, # Satyanarayan Patel,‖ Peter Keil,‖ Jurij Koruza,‖ Shogo Kawaguchi,§ Zhao Pan,† Takumi Nishikubo,† Jürgen Rödel,‖, ⏊ and Masaki Azuma*, † †Laboratory

for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan ‡Kanagawa

Institute of Industrial Science and Technology (KISTEC), 705-1 Shimoimaizumi, Ebina, Kanagawa 243-

0435, Japan ‖Institute

of Materials Science, Technische Universität Darmstadt, FG Nichtmetallische-Anorganische Werkstoffe, Alarich-Weiss-Strasse 2, D-64287 Darmstadt, Germany §Research

and Utilization Division, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ⏊Tokyo

Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan ABSTRACT: PbVO3 features giant tetragonal distortion (c/a = 1.23) and spontaneous polarization (Ps = 101 μC/cm2) originating from the ordering of dxy orbital of V4+ with single d electrons. A dedicated study of Cr substituted PbVO3 prepared by high-pressure synthesis afforded a well-defined variation in c/a ratio. Ensuing changes in atomic structure were correlated to ferroelastic domain switching and a temperature-induced structural transition to a cubic phase accompanied with an insulator to metal transition and a negative thermal expansion.

1. INTRODUCTION Phase transitions in perovskite-type compounds are generally accompanied by a change in the unit cell volume, with some of them leading to the technologically important phenomenon of negative thermal expansion (NTE).1,2 For example, ferroelectric PbTiO3 with a large tetragonal distortion (c/a = 1.06), stabilized by both hybridizations of Pb (6s)─ O (2p) and Ti (3d)─ O (2p) orbitals,3,4 features NTE resulting from a structural transition from the tetragonal to the cubic phase on heating. The temperature range of NTE is very wide, but the absolute value of linear coefficient of thermal expansion is small (−3.3×10−6 /K from room temperature (RT) to 673 K)5 for practical applications in the field of nanoscale manufacturing and/or optical communication because the origin of this NTE is a phase transition with a volume shrinkage. Tuning NTE properties by chemical modification is extensively investigated for PbTiO3 and other NTE materials. Herefore, various mechanisms were utilized, such as flexible crystal structure, magnetovolumetric effect, and charge transfer, as summarized in previous review articles.1,2

From the application point of view, both a wide temperature range and a large NTE coefficient are required. Typically, both requirements are mutually exclusive and a compromise is required. However, both parameters can be improved by selecting a parent compound with enhanced unit cell volume owing to a large structural distortion which enables a large volume shrinkage during a structural transition, such as the perovskite PbVO3. PbVO3 has a perovskite PbTiO3-type tetragonal crystal structure with a giant lattice distortion (c/a = 1.23 at RT)6 and a calculated spontaneous polarization Ps exceeding 100 μC/cm2.7,8 Note that these values are much larger than in PbTiO3 (c/a = 1.06 and Ps = 57 μC/cm2 at RT).9 The huge tetragonal distortion originates from the d1 electronic configuration of V4+ with the nondegenerate dxy orbital.10 However, the large structural distortion of PbVO3 hinders polarization reversal by an external field and a temperature-induced tetragonal-to-cubic (T-C) phase transition leading to NTE.8 Synchrotron X-ray powder diffraction (SXRD) revealed a pressure-induced T-C transition accompanied with a drop of electrical resistivity and a volume shrinkage by 10.6 % at around 3 GPa.8,11,12 This transition is rationalized by a melting of dxy orbital ordering, changing from the

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semiconducting (Mott insulator) tetragonal phase to the metallic cubic phase with triply degenerated t2g orbitals. It was recently found that electron doping to PbVO3 by substitution of Bi3+ and La3+ for Pb2+ suppresses the huge tetragonal distortion and enables the T-C phase transition to occur on heating.13 Substitution of other tetravalent transition metal ions for V4+ is also expected to destabilize the dxy orbital ordering and hence induce the T-C structural change accompanied with volume shrinkage and a change into metallic conductivity at ambient pressure. Indeed, NTE with maximum volume collapse of 3.6 % at the phase transition was revealed for PbV1−xTixO3.14 Although composition dependent T-C structural change was reported for PbV1−xCrxO3, the temperature evolutions of the structure and unit cell volume have not been studied.15 In the present study, we investigated the structural change, as well as the mechanical and electrical properties of PbV1−xCrxO3. The suppression of tetragonal distortion and the decrease in spontaneous polarization with increasing Cr substitution were confirmed by structural refinements based on synchrotron X-ray powder diffraction. Moreover, the suppressed tetragonal distortion facilitated ferroelastic domain switching and enabled the T-C phase transition, which is accompanied by NTE. The temperature-induced semiconductor to metal transition, confirming the melting of orbital ordering, was also observed for the first time. 2. EXPERIMENTAL SECTION Polycrystalline samples of PbV1−xCrxO3 (x = 0, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40) were prepared from mixtures of Pb (99.9 % RARE METALLIC), PbO (99.99 % RARE METALLIC), V2O5 (99.99 % RARE METALLIC) and Cr2O3 (99.9 % RARE METALLIC). The powders were mixed in an agate mortar and sealed in gold capsules with a diameter of 3.6 mm and a height of 5.0 mm. Then, they were treated at 8 GPa and 1273–1473 K for 30–120 minutes using a cubic-anvil-type high-pressure apparatus. It should be noted that the recovered sample cracked into small lumps because of the structural change with large volume increase during the pressure release. Disk and cylindrical specimens for resistivity and ferroelastic measurements, respectively, were prepared by compressing the crushed powder samples at 3 GPa for 15 min at RT. SXRD data were collected at BL02B2 beamline of SPring8 equipped with MYTHEN solid state detector.16,17 The incident beam was monochromatized to λ ~0.42 Å. The sample was sealed in glass capillary tubes with an inner diameter of 0.1 mm and was rotated during the measurements. The obtained SXRD data were analyzed using Rietan-FP program18 in order to precisely determine the crystallographic parameters and to calculate Ps. Laboratory X-ray powder diffraction (XRD) data were collected with a diffractometer, (D8 ADVANCE, Bruker) using Cu Kα radiation in the 2θ range of 5–80° and step width of 0.020°. Temperature dependent XRD data were obtained every 20 K with a heating rate of 2 K/min. The lattice parameters and the fractions of high-temperature

and low-temperature phases were refined by Rietveld analysis with TOPAS software. DC electrical resistivity of a pelletized polycrystalline sample was measured with the four-point method using a data acquisition unit (Agilent 34972A) within a cryostat (Pascal-101D-HE) from 60 K to 480 K on heating with a heating rate of 1 K/min. Ferroelastic measurements were performed using a screw-driven load frame (Z010, Zwick GmbH & Co. KG, Ulm, Germany) as described in detail in a previous publication.19 Round cylindrical compacts with a diameter of ~ 3 mm and a thickness between 2 and 3 mm were ground by water-cooled machining on a lathe. Plane parallel loading surfaces were ensured using a surface grinder. Centering of the sample in the load frame was afforded by applying a specially designed centering tool. A pre-stress of −5 MPa was applied to the sample to ensure a smooth contact. Compressive uniaxial mechanical stresses were limited to −130 MPa to prevent breaking of the sample and were applied on unpoled PbV0.75Cr0.25O3 with a rate of 1 MPa/s at room temperature. Strain was measured using a linear variable differential transformer (LVDT) with an estimated error of ~5 %, located near the sample. After unloading, ten cycles to -130 MPa were applied to test for cyclic creep after repeated loading. The change in the domain structure before and after loading was studied by SXRD measurement on cylindrical samples in asymmetric diffraction geometry with a twodimensional imaging plate (IP) detector performed at BL02B2 beamline of SPring-8. The incident beam was monochromatized to λ = 0.99860 Å and the beam size was 0.02 mm (vertical) × 1.0 mm (horizontal). After the sample surface was adjusted to be parallel to the beam, SXRD patterns were collected with the samples slightly tilted and oscillated over the range of Δω = 1-6° in order to prevent an effect of a spotty ring due to non-uniform grain size. 3. RESULT AND DISCUSSION Figure 1 depicts the SXRD patterns of as prepared PbV1−xCrxO3 samples (x = 0, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40) at RT. The splitting between 001 and 100 peaks decreases with increasing Cr substitution implying the decrease of c/a ratio. A slight amount of cubic phase emerges at x = 0.25 and coexists with the tetragonal phase up to x = 0.35. Finally, only the cubic phase is observed at x = 0.4. The unit cell volume of cubic x = 0.4 sample is 60.32 Å3, 5.9 % smaller than that of PbCrO3 with Pb2+0.5Pb4+0.5Cr3+O3 charge distribution, indicating that the valence state is Pb2+(V1-xCrx)4+O3. Crystal structures of the tetragonal phase were further investigated by Rietveld analysis as depicted in Figure 2 and the refined structural parameters are summarized in Table S1. The evolution of lattice parameters for the tetragonal phase, c/a ratio, calculated Ps, and δz, which is defined as the distance from each cation to the centroid of the oxygen polyhedron,2,20,21


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Figure 1. Synchrotron X-ray diffraction patterns of PbV1-xCrxO3 (x = 0, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40) at room temperature. Upper left and right are the crystal structures of tetragonal and cubic phase. The asterisk symbol indicates the peak of gold contamination from the capsule.

Figure 2. Result of Rietveld fitting for (a) PbV0.90Cr0.10O3, (b) PbV0.85Cr0.15O3, (c) PbV0.80Cr0.20O3, and (d) PbV0.75Cr0.25O3. Each symbol represents observed (+), calculated (line), and difference between them (bottom line) profiles, respectively. Bragg reflections for the tetragonal phase, cubic phase (only for PbV0.75Cr0.25O3), and gold (contaminated from the capsule used for HP synthesis) are indicated by tick marks.

Figure 3. Cr content dependence of refined a and c lattice parameters, c/a ratio, spontaneous polarization Ps, and cation displacement δz of PbV1-xCrxO3 (x ≤ 0.25) at room temperature. The parameters for PbVO3 were taken from Ref. (8).

are summarized in Figure 3. All these values, except for a axis length, gradually decrease with increasing Cr substitution. Notably, the c/a ratio of 1.07 and the spontaneous polarization of Ps = 53 μC/cm2 for x = 0.25 are comparable to those of PbTiO3 (c/a = 1.06 and Ps = 57 μC/cm2).9 Owing to the suppressed tetragonal distortion indicated above, polarization is expected to be switched by an electric field (ferroelectricity) or a mechanical field (ferroelasticity), as observed for example in lead lanthanum zirconate titanate (PLZT).22 A representative stress-strain curve of the x = 0.25 sample under uniaxial compression is depicted in Figure 4(a). This composition was chosen due to a reduced c/a ratio (c/a = 1.07) as compared to PbVO3 but with a maximized content of tetragonal phase. Domain switching and elastic strain occur concurrently during loading, while a certain degree of back-switching accompanies the reduction of elastic strain during unloading. The obtained remanent strain indicates the switching of domains, which have not been switched back during unloading. This experiment therefore allows to determine maximum strain and remanent strain as schematically illustrated in the inset of Figure 4(a). In this study, the curves exhibit non-linear behavior, characteristic for ferroelastic perovskite materials.19 The total strain (elastic + domain switching strain) increased up to –1.5 % by increasing the stress to −130 MPa. The ferroelastic behavior in tetragonal perovskites is attributed to the switching of 90o ferroelastic



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Figure 5. Synchrotron X-ray diffraction data for cylindrical samples of PbV0.75Cr0.25O3 before and after loading. (h k l) of tetragonal (T) phase are given.

Figure 4. Stress-strain curves of PbV0.75Cr0.25O3 for 1st (a) and 10 cycles (b). The inset of (a) is the schematic view of stress-strain curve, the remanent strain εr, the maximum strain εmax, and the back-switching strain (εrel − εr). The inset figure (b) shows a plot of remanent strain versus the number of loading cycles.

domains. The coercive stress (stress at maximized domain switching per stress increment) could not be unambiguously determined. Considering the large c/a ratio of 1.07 for this composition, the coercive stress is expected to be higher than the maximum stress applied during this measurement. For example, the coercive stress of a composition at the morphotropic phase boundary in the system BiFeO3-PbTiO3 with lanthanum doping (BLFPT) and a c/a ratio of 1.06 had been determined to be –280 MPa.23 During mechanical unloading, the elastic strain is reduced back to zero and some of the domains exhibit back-switching, leading to a remanent strain of 0.33 %. There are only very few publications available, to which this result could be compared. Compositions in the system BLFPT23 and c/a ratio of 1.06 for example display a maximum strain of 0.2 % under the application of −380 MPa. The maximum remanent strain in this system had been found to be 0.35 % at a c/a ratio = 1.045. In general, the maximum remanent strain can be found at intermediate c/a ratios as large c/a ratios maximize the strain per switching event, but render only a low percentage of switchable domains. Subsequent mechanical cyclic loading was performed to validate that PbV1-xCrxO3 displays similar behavior like PZT in terms of

ferroelastic fatigue with an accumulation of domain switching strain.24 Figure 4 (b) demonstrates that a maximum stress of −130 MPa provided cyclic creep with remanent strain increments between 0.02 and 0.005 %. Accordingly, changes in the fractions of 100 and 001 domains were directly observed by SXRD. Figure 5 displays the magnified SXRD patterns for x = 0.25 sample before and after the loading. The intensity of 100 peak increases while 001 peak decreases, which is consistent with the expectation that 001 domains with larger lattice spacing switch to 100 domains by uniaxial compression. The c/a ratio also changes after loading, suggesting that the remaining strain in the compressed sample affected the lattice parameters. Because of the increase in the in-plane lattice parameter owing to the switching of c-domain to adomain after loading, compressive stress remains in the inplane direction, which effectively reduces the c/a ratio. It should be noted that the lattice parameters changed back to the original values when the sample was crushed into powders and the internal strain was released. The volume fraction of 001 domains, v001 is calculated from the integrated peak intensities as follows25 I001 v001 =

I'001 I001 I'001

+2

( ) I100

I'100

where I’hkl and Ihkl stand for the integrated intensity of hkl diffraction peak before and after loading, respectively. v’001 is equal to 1/3 for a random distribution of domains. The change in the volume fraction of 001 domains after loading, η001 is given by 1 η001 = v001 3 The present data yield η001= −0.286, which is significantly larger than the values for typical tetragonal ferroelastic compounds such as the La-doped PZT.25 The splitting between 001 and 100 peaks decreases after loading reflecting the change in the strain from the adjacent grains due to domain switching. It is noted that polarization


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Figure 7. Temperature dependence of electrical resistivity for PbV0.70Cr0.30O3 measured on heating revealing the semiconductor to metal transition owing to the melting of dxy orbital ordering. The dotted line indicates the tetragonal to cubic transition temperature.

Figure 6. Temperature variation of the XRD of (a) PbV0.85Cr0.15O3, (b) PbV0.80Cr0.20O3, (c) PbV0.75Cr0.25O3 and (d) PbV0.70Cr0.30O3 at elevated temperatures. Indices of the tetragonal (T) and cubic (C) phases are given.

measurements were also performed at RT, but no ferroelectric hysteresis loop was obtained because of large leakage current. Figure 6 (a), (b), (c), and (d) display portions of XRD patterns of PbV1-xCrxO3 x = 0.15, 0.20, 0.25, and 0.30 samples at elevated temperatures. The intensity of 001 and 100 peaks of the tetragonal phase decreases and that of 100 peak for 　 the cubic phase appears on heating. The tetragonal peaks disappear on further heating and only the cubic ones remain indicating the T-C phase transition. It should be noted that the transition was irreversible and the cubic phase does not change back to the tetragonal one even by cooling down to 100 K. Electrical resistivity ρ(T) was measured on heating for a compressed pellet of x = 0.30 sample as provided in Figure 7. While below 360 K the resistivity decreases on heating, as seen also for PbVO3,6,12 it increases above 360 K where the phase completely changed to cubic (Figure 6(d)). This indicates that the T- C structural transition is accompanied by a semiconductor-to-metal transition owing to melting of dxy orbital ordering. This is the first direct evidence of temperature induced melting of orbital ordering in PbVO3 derivatives.

Figure 8 (a), (b), and (c) depict the temperature dependence of lattice constants and the fraction of the cubic phase for x = 0.15, 0.20 and 0.25 measured on heating. All lattice constants continuously increase upon heating, indicating a volume expansion of each phase. Using the lattice constants and the phase fraction, weighted average unit cell volume is calculated and plotted in Figure 8 (d), (e), and (f) as a function of temperature. The increase in the fraction of the cubic phase on heating results in NTE. Both, the transition temperature and the volume shrinkage, decrease with increasing Cr concentration as the tetragonal distortion is suppressed. The maximum volume shrinkage of 6.6 % observed between 433–573 K for x = 0.15 sample is considerably larger compared with those of other NTE materials, such as flexible network type ZrW2O8 (0.8 %),26,27 antiperovskite Mn3ZnN (1.3 %) with magnetovolumetric effect,28 perovskite Bi0.95La0.05NiO3 (2.9 %) with charge transfer,29 and modified PbTiO3.2 Finally, we compare the NTE properties of PbV1-xCrxO3 with recently reported PbV1-xTixO3.14 PbV1-xTixO3 exhibits NTE with maximum volume shrinkage of 3.6 % at the low V concentration up to x = 0.30 (c/a = 1.10). Samples with x > 0.40 decompose below the T-C transition temperature like PbVO3. It is well known that ferroelectric and NTE properties of lead-based ferroelectrics are closely correlated.2 Recently, a new concept of spontaneous volume ferroelectrostriction (SVFS) was proposed for ferroelectric materials in order to quantitatively evaluate the contribution of ferroelectricity to NTE.17,19 Generally, the Debye-Grüneisen equation can be adopted to estimate the thermal expansion of solids from phononic contribution. After deduction of components contributed from the anharmonic phonon vibration, the contribution to native thermal expansion from physical properties such as ferroelectric or magnetic orderings can be directly and quantitatively estimated.2 Thermal expansion is expected to be low at low temperature, and reaches to be zero at 0 K.



Figure 8. Temperature dependence of refined lattice parameters and the fraction of cubic phase for (a) PbV0.85Cr0.15O3, (b) PbV0.80Cr0.20O3, and (c) PbV0.75Cr0.25O3 measured on heating. (d), (e), and (f) are the corresponding unit cell volumes as a function of temperature. Volume shrinkage ΔV between NTE and spontaneous volume ferroelectrostriction ωs are provided in each graph.

It approaches to a constant value above the Debye temperature. Here in the present PbV1-xCrxO3, two factors, anharmonic lattice phonon vibration and ferroelectric order, contribute to the unit cell volume. With decreasing temperature, the contribution from the anharmonic lattice phonon vibration decreases. Thus, the nominal unit cell volume (Vnm) estimated by the extrapolation of the data in the paraelectric region should normally contract, which can be described by the Debye-Grüneisen equation. The

details of SVFS can be evaluated from the equation as follows, 𝑉𝑒𝑥𝑝 ― 𝑉𝑛𝑚 𝜔𝑆 = × 100 (%) 𝑉𝑛𝑚 It should be noted that the present ωS of x = 0.15 may be overestimated since the temperature range corresponding to the cubic phase is only within 100 K which leads to an


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Chemistry of Materials Crystallographic parameters of PbV1-xCrxO3 at room temperature obtained by Rietveld refinements and cation compositions of PbV0.80Cr0.20O3 and PbV0.60Cr0.40O3 measured by energy-dispersive X-ray spectroscopy analysis.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected]

Present Addresses #H.Y.:

Figure 9. The plot of ωS verses the Pb displacement δzPb2 for PbVO3 based PbV1-xCrxO3 and PbTiO3 based PbV1-xTixO3 taken from Ref. (14).

ambiguity estimation of Vnm to some extent. As seen in Figure 8 (d)-(f), the values of ωS are 12.2, 8.2, and 4.1 % for x = 0.15, 0.20, 0.25, respectively. This means that Cr substitution weakens the ferroelectrovolume effect, which is consistent with the tendency of suppressed c/a ratio. In comparison, the present ωS of PbV0.85Cr0.15O3 is much larger than the one of PbTiO3-based ferroelectrics such as PbTiO3 (3.1 %)2, 0.5PbTiO3-0.5BiFeO3 (5 %),21 and recently reported PbV0.3Ti0.7O3 (8.5 %)14 indicating a stronger ferroelectrovolume effect. This corresponds well with the larger volume shrinkage of PbV0.85Cr0.15O3 during the ferroelectric-to- paraelectric phase transition. The large ωS stems from the large PS as depicted in Figure 3. To further elucidate the relationship between ωS and PS, the ωS versus the Pb displacement δzPb2 for PbV1-xCrxO3 of this study and the previously reported PbV1-xTixO3 are plotted in Figure 9.14 One can see a good linear relationship between ωS and δzPb defined as follows 𝜔S~𝛼𝛿𝑧2Pb where α is the coupling coefficient between ωS and δzPb. The relationship between ωS and δzPb is a based on large number of experimental results. Analogous to the Landau theory, such a good relationship has been observed in Pbbased ferroelectrics.2,20,21 The present SVFS effect in ferroelectrics is similar to the spontaneous volume magnetostriction effect in magnetics.30 The larger δzPb of PbV0.85Cr0.15O3 compared to PbV0.3Ti0.7O3 leads to a higher ωS, a stronger ferroelectrovolume effect, and therefore a giant NTE. In conclusion, PbV1-xCrxO3 was successfully synthesized and the structural change was investigated. The tetragonal distortion and spontaneous polarization were suppressed by a chemical substitution of Cr for V. Compounds with suppressed tetragonal distortion show ferroelasticity and a temperature-induced T-C structural transition. The phase transition is accompanied by NTE and an electrical transition from semiconductor to metal reflecting the melting of dxy orbital ordering.

ASSOCIATED CONTENT Supporting Information

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan.

ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research 16H02393 and 18H05208 from the Japan Society for the Promotion of Science (JSPS), by the Kanagawa Institute of Industrial Science and Technology, and by the World Research Hub Initiative (WRHI) of Tokyo Institute of Technology. The synchrotron-radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (2017A1388, 2018A1636, 2018A1642). S.P. acknowledges support by an Alexander-von-Humboldt fellowship.

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Melting of dxy Orbital Ordering Accompanied by Suppression of Giant

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